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1. Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline

Author

Morabito, L. K., Jackson, N. J., Mooney, S., Sweijen, F., Badole, S., Kukreti, P., Venkattu, D., Groeneveld, C., Kappes, A., Bonnassieux, E., Drabent, A., Iacobelli, M., Croston, J. H., Best, P. N., Bondi, M., Callingham, J. R., Conway, J. E., Deller, A. T., Hardcastle, M. J., McKean, J. P., Miley, G. K., Moldon, J., Röttgering, H. J. A., Tasse, C., Shimwell, T. W., van Weeren, R. J., Anderson, J. M., Asgekar, A., Avruch, I. M., van Bemmel, I. M., Bentum, M. J., Bonafede, A., Brouw, W. N., Butcher, H. R., Ciardi, B., Corstanje, A., Coolen, A., Damstra, S., de Gasperin, F., Duscha, S., Eislöffel, J., Engels, D., Falcke, H., Garrett, M. A., Griessmeier, J., Gunst, A. W., van Haarlem, M. P., Hoeft, M., van der Horst, A. J., Jütte, E., Kadler, M., Koopmans, L. V. E., Krankowski, A., Mann, G., Nelles, A., Oonk, J. B. R., Orru, E., Paas, H., Pandey, V. N., Pizzo, R. F., Pandey-Pommier, M., Reich, W., Rothkaehl, H., Ruiter, M., Schwarz, D. J., Shulevski, A., Soida, M., Tagger, M., Vocks, C., Wijers, R. A. M. J., Wijnholds, S. J., Wucknitz, O., Zarka, P., and Zucca, P.

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics, Astrophysics - Astrophysics of Galaxies
Abstract

[abridged] The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz, although this is technically and logistically challenging. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LOFAR Two-metre Sky Survey (LoTSS) pointing. We perform in-field delay calibration, solution referencing to other calibrators, self-calibration, and imaging of example directions of interest in the field. For this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5 degrees away, while phase solution transferral works well over 1 degree. We demonstrate a check of the astrometry and flux density scale. Imaging in 17 directions, the restoring beam is typically 0.3" x 0.2" although this varies slightly over the entire 5 square degree field of view. We achieve ~80 to 300 $\mu$Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 $\mu$Jy/bm for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image ~900 sources per LoTSS pointing. This equates to ~3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit., Comment: Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR. 24 pages, 16 figures

Published
2021
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2. Sub-arcsecond imaging with the International LOFAR Telescope: II. Completion of the LOFAR Long-Baseline Calibrator Survey

Author

Jackson, Neal, Badole, Shruti, Morgan, John, Chhetri, Rajan, Prusis, Kaspars, Nikolajevs, Atvars, Morabito, Leah, Brentjens, Michiel, Sweijen, Frits, Iacobelli, Marco, Orrù, Emanuela, Sluman, J., Blaauw, R., Mulder, H., van Dijk, P., Mooney, Sean, Deller, Adam, Moldon, Javier, Callingham, J. R., Harwood, Jeremy, Hardcastle, Martin, Heald, George, Drabent, Alexander, McKean, J. P., Asgekar, A., Avruch, I. M., Bentum, M. J., Bonafede, A., Brouw, W. N., Brüggen, M., Butcher, H. R., Ciardi, B., Coolen, A., Corstanje, A., Damstra, S., Duscha, S., Eislöffel, J., Falcke, H., Garrett, M., de Gasperin, F., Griessmeier, J. -M., Gunst, A. W., van Haarlem, M. P., Hoeft, M., van der Horst, A. J., Jütte, E., Koopmans, L. V. E., Krankowski, A., Maat, P., Mann, G., Miley, G. K., Nelles, A., Norden, M., Paas, M., Pandey, V. N., Pandey-Pommier, M., Pizzo, R. F., Reich, W., Rothkaehl, H., Rowlinson, A., Ruiter, M., Shulevski, A., Schwarz, D. J., Smirnov, O., Tagger, M., Vocks, C., van Weeren, R. J., Wijers, R., Wucknitz, O., Zarka, P., Zensus, J. A., and Zucca, P.

Subjects
Astrophysics - Astrophysics of Galaxies, Astrophysics - Instrumentation and Methods for Astrophysics
Abstract

The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30006 observations of 24713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of $\geq$ 200 km baselines is identified by the detection of compact flux density, for declinations north of 30 degrees and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2 arcsec, for sources observed in the survey. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2 arcsec and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the Murchison Widefield Array (MWA) shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS., Comment: Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR

Published
2021
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3. LOFAR imaging of Cygnus A -- Direct detection of a turnover in the hotspot radio spectra

Author

McKean, J. P., Godfrey, L. E. H., Vegetti, S., Wise, M. W., Morganti, R., Hardcastle, M. J., Rafferty, D., Anderson, J., Avruch, I. M., Beck, R., Bell, M. E., van Bemmel, I., Bentum, M. J., Bernardi, G., Best, P., Blaauw, R., Bonafede, A., Breitling, F., Broderick, J. W., Bruggen, M., Cerrigone, L., Ciardi, B., de Gasperin, F., Deller, A., Duscha, S., Engels, D., Falcke, H., Fallows, R. A., Frieswijk, W., Garrett, M. A., Griessmeier, J. M., van Haarlem, M. P., Heald, G., Hoeft, M., van der Horst, A. J., Iacobelli, M., Intema, H., Juette, E., Karastergiou, A., Kondratiev, V. I., Koopmans, L. V. E., Kuniyoshi, M., Kuper, G., van Leeuwen, J., Maat, P., Mann, G., Markoff, S., McFadden, R., McKay-Bukowski, D., Mulcahy, D. D., Munk, H., Nelles, A., Orru, E., Paas, H., Pandey-Pommier, M., Pietka, M., Pizzo, R., Polatidis, A. G., Reich, W., Rottgering, H. J. A., Rowlinson, A., Scaife, A. M. M., Serylak, M., Shulevski, A., Sluman, J., Smirnov, O., Steinmetz, M., Stewart, A., Swinbank, J., Tagger, M., Thoudam, S., Toribio, M. C., Vermeulen, R., Vocks, C., van Weeren, R. J., Wucknitz, O., Yatawatta, S., and Zarka, P.

Subjects
Astrophysics - Astrophysics of Galaxies
Abstract

The low-frequency radio spectra of the hotspots within powerful radio galaxies can provide valuable information about the physical processes operating at the site of the jet termination. These processes are responsible for the dissipation of jet kinetic energy, particle acceleration, and magnetic-field generation. Here we report new observations of the powerful radio galaxy Cygnus A using the Low Frequency Array (LOFAR) between 109 and 183 MHz, at an angular resolution of ~3.5 arcsec. The radio emission of the lobes is found to have a complex spectral index distribution, with a spectral steepening found towards the centre of the source. For the first time, a turnover in the radio spectrum of the two main hotspots of Cygnus A has been directly observed. By combining our LOFAR imaging with data from the Very Large Array at higher frequencies, we show that the very rapid turnover in the hotspot spectra cannot be explained by a low-energy cut-off in the electron energy distribution, as has been previously suggested. Thermal (free-free) absorption or synchrotron self absorption models are able to describe the low-frequency spectral shape of the hotspots, however, as with previous studies, we find that the implied model parameters are unlikely, and interpreting the spectra of the hotspots remains problematic., Comment: 9 pages, 3 figures, accepted for publication in MNRAS

Published
2021
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4. LOFAR 144-MHz follow-up observations of GW170817

Author

Broderick, J. W., Shimwell, T. W., Gourdji, K., Rowlinson, A., Nissanke, S., Hotokezaka, K., Jonker, P. G., Tasse, C., Hardcastle, M. J., Oonk, J. B. R., Fender, R. P., Wijers, R. A. M. J., Shulevski, A., Stewart, A. J., ter Veen, S., Moss, V. A., van der Wiel, M. H. D., Nichols, D. A., Piette, A., Bell, M. E., Carbone, D., Corbel, S., Eislöffel, J., Grießmeier, J. -M., Keane, E. F., Law, C. J., Muñoz-Darias, T., Pietka, M., Serylak, M., van der Horst, A. J., van Leeuwen, J., Wijnands, R., Zarka, P., Anderson, J. M., Bentum, M. J., Blaauw, R., Brouw, W. N., Brüggen, M., Ciardi, B., de Vos, M., Duscha, S., Fallows, R. A., Franzen, T. M. O., Garrett, M. A., Gunst, A. W., Hoeft, M., Hörandel, J. R., Iacobelli, M., Jütte, E., Koopmans, L. V. E., Krankowski, A., Maat, P., Mann, G., Mulder, H., Nelles, A., Paas, H., Pandey-Pommier, M., Pekal, R., Reich, W., Röttgering, H. J. A., Schwarz, D. J., Smirnov, O., Soida, M., Toribio, M. C., van Haarlem, M. P., van Weeren, R. J., Vocks, C., Wucknitz, O., and Zucca, P.

Subjects
Astrophysics - High Energy Astrophysical Phenomena
Abstract

We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGO-Virgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 13.7 degrees when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On time-scales of 130-138 and 371-374 days after the merger event, we obtain 3$\sigma$ upper limits for the afterglow component of 6.6 and 19.5 mJy beam$^{-1}$, respectively. Using our best upper limit and previously published, contemporaneous higher-frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the two-point spectral index $\alpha^{610}_{144} \gtrsim -2.5$. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations., Comment: 9 pages, 2 figures, accepted for publication in MNRAS

Published
2020
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5. Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies

Author

de Gasperin, F., Vink, J., McKean, J. P., Asgekar, A., Bentum, M. J., Blaauw, R., Bonafede, A., Bruggen, M., Breitling, F., Brouw, W. N., Butcher, H. R., Ciardi, B., Cuciti, V., de Vos, M., Duscha, S., Eisloffel, J., Engels, D., Fallows, R. A., Franzen, T. M. O., Garrett, M. A., Gunst, A. W., Horandel, J., Heald, G., Koopmans, L. V. E., Krankowski, A., Maat, P., Mann, G., Mevius, M., Miley, G., Nelles, A., Norden, M. J., Offringa, A. R., Orru, E., Paas, H., Pandey-Pommier, M., Pizzo, R., Reich, W., Rowlinson, A., Schwarz, D. J., Shulevski, A., Smirnov, O., Soida, M., Tagger, M., Toribio, M. C., van Ardenne, A., van der Horst, A. J., van Haarlem, M. P., van Weeren, R. J., Vocks, C., Wucknitz, O., Zarka, P., and Zucca, P.

Subjects
Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Astrophysics of Galaxies, Astrophysics - Instrumentation and Methods for Astrophysics
Abstract

The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (<100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15 arcsec with a high dynamic range and good image fidelity. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna (LBA) system. We reduced the datasets and obtained an image for each A-team source. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further., Comment: 7 pages, 2 figures, accepted A&A, online data on A&A website

Published
2020
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6. LBCS: the LOFAR Long-Baseline Calibrator Survey

Author

Jackson, N., Tagore, A., Deller, A., Moldón, J., Varenius, E., Morabito, L., Wucknitz, O., Carozzi, T., Conway, J., Drabent, A., Kapinska, A., Orrù, E., Brentjens, M., Blaauw, R., Kuper, G., Sluman, J., Schaap, J., Vermaas, N., Iacobelli, M., Cerrigone, L., Shulevski, A., ter Veen, S., Fallows, R., Pizzo, R., Sipior, M., Anderson, J., Avruch, M., Bell, M., van Bemmel, I., Bentum, M., Best, P., Bonafede, A., Breitling, F., Broderick, J., Brouw, W., Brüggen, M., Ciardi, B., Corstanje, A., de Gasperin, F., de Geus, E., Eislöffel, J., Engels, D., Falcke, H., Garrett, M., Griessmeier, J., Gunst, A., van Haarlem, M., Heald, G., Hoeft, M., Hörandel, J., Horneffer, A., Intema, H., Juette, E., Kuniyoshi, M., van Leeuwen, J., Loose, G., Maat, P., McFadden, R., McKay-Bukowski, D., McKean, J., Mulcahy, D., Munk, H., Pandey-Pommier, M., Polatidis, A., Reich, W., Röttgering, H., Rowlinson, A., Scaife, A., Schwarz, D., Steinmetz, M., Swinbank, J., Thoudam, S., Toribio, M., Vermeulen, R., Vocks, C., van Weeren, R., Wise, M., Yatawatta, S., and Zarka, P.

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics, Astrophysics - Astrophysics of Galaxies
Abstract

(abridged). We outline LBCS (the LOFAR Long-Baseline Calibrator Survey), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000 km. Suitable sources must contain significant correlated flux density (50-100mJy) at frequencies around 110--190~MHz on scales of a few hundred mas. At least for the 200--300-km international baselines, we find around 1 suitable calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include calibrator source lists and fringe-rate and delay maps of wide areas -- typically a few degrees -- around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200--600~km, with good calibrators on the longest baselines appearing only at the rate of 0.5 per square degree. Coherence times decrease from 1--3 minutes on 200-km baselines to about 1 minute on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 minutes, is seen on the DE609 baseline, which at 227km is close to being the shortest. We see median coherence times of between 80 and 110 seconds on the four longest baselines (580--600~km), and about 2 minutes for the other baselines. The success of phase transfer from calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 degree., Comment: Accepted by Astronomy & Astrophysics. Error in figure 6 corrected

Published
2016
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7. A large light-mass component of cosmic rays at 10^{17} - 10^{17.5} eV from radio observations

Author

Buitink, S., Corstanje, A., Falcke, H., Hörandel, J. R., Huege, T., Nelles, A., Rachen, J. P., Rossetto, L., Schellart, P ., Scholten, O., ter Veen, S., Thoudam, S., Trinh, T. N. G., Anderson, J., Asgekar, A., Avruch, I. M., Bell, M. E., Bentum, M. J., Bernardi, G., Best, P., Bonafede, A., Breitling, F., Broderick, J. W., Brouw, W. N., Brüggen, M., Butcher, H. R., Carbone, D., Ciardi, B., Conway, J. E., de Gasperin, F., de Geus, E., Deller, A., Dettmar, R. -J., van Diepen, G., Duscha, S., Eislöffel, J., Engels, D., Enriquez, J. E., Fallows, R. A., Fender, R., Ferrari, C., Frieswijk, W., Garrett, M. A., Griessmeier, J. M., Gunst, A. W., van Haarlem, M. P., Hassall, T. E., Heald, G., Hessels, J. W. T., Hoeft, M., Horneffer, A., Iacobelli, M., Intema, H., Juette, E., Karastergiou, A., Kondratiev, V. I., Kramer, M., Kuniyoshi, M., Kuper, G., van Leeuwen, J., Loose, G. M., Maat, P., Mann, G., Markoff, S., McFadden, R., McKay-Bukowski, D., McKean, J. P., Mevius, M., Mulcahy, D. D., Munk, H., Norden, M. J., Orru, E., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pietka, M., Pizzo, R., Polatidis, A. G., Reich, W., Röttgering, H. J. A., Scaife, A. M. M., Schwarz, D. J., Serylak, M., Sluman, J., Smirnov, O., Stappers, B. W., Steinmetz, M., Stewart, A., Swinbank, J., Tagger, M., Tang, Y., Tasse, C., Toribio, M. C., Vermeulen, R., Vocks, C., Vogt, C., van Weeren, R. J., Wijers, R. A. M. J., Wijnholds, S. J., Wise, M. W., Wucknitz, O., Yatawatta, S., Zarka, P., and Zensus, J. A.

Subjects
Astrophysics - High Energy Astrophysical Phenomena, High Energy Physics - Experiment
Abstract

Cosmic rays are the highest energy particles found in nature. Measurements of the mass composition of cosmic rays between 10^{17} eV and 10^{18} eV are essential to understand whether this energy range is dominated by Galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal comes from accelerators capable of producing cosmic rays of these energies. Cosmic rays initiate cascades of secondary particles (air showers) in the atmosphere and their masses are inferred from measurements of the atmospheric depth of the shower maximum, Xmax, or the composition of shower particles reaching the ground. Current measurements suffer from either low precision, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays is a rapidly developing technique, suitable for determination of Xmax with a duty cycle of in principle nearly 100%. The radiation is generated by the separation of relativistic charged particles in the geomagnetic field and a negative charge excess in the shower front. Here we report radio measurements of Xmax with a mean precision of 16 g/cm^2 between 10^{17}-10^{17.5} eV. Because of the high resolution in $Xmax we can determine the mass spectrum and find a mixed composition, containing a light mass fraction of ~80%. Unless the extragalactic component becomes significant already below 10^{17.5} eV, our measurements indicate an additional Galactic component dominating at this energy range., Comment: 35 pages, 11 figures, updated version: Pierre Auger Observatory data ICRC 2015 added to Fig 2

Published
2016
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8. LOFAR MSSS: Detection of a low-frequency radio transient in 400 hrs of monitoring of the North Celestial Pole

Author

Stewart, A. J., Fender, R. P., Broderick, J. W., Hassall, T. E., Muñoz-Darias, T., Rowlinson, A., Swinbank, J. D., Staley, T. D., Molenaar, G. J., Scheers, B., Grobler, T. L., Pietka, M., Heald, G., McKean, J. P., Bell, M. E., Bonafede, A., Breton, R. P., Carbone, D., Cendes, Y., Clarke, A. O., Corbel, S., de Gasperin, F., Eislöffel, J., Falcke, H., Ferrari, C., Grießmeier, J. -M., Hardcastle, M. J., Heesen, V., Hessels, J. W. T., Horneffer, A., Iacobelli, M., Jonker, P., Karastergiou, A., Kokotanekov, G., Kondratiev, V. I., Kuniyoshi, M., Law, C. J., van Leeuwen, J., Markoff, S., Miller-Jones, J. C. A., Mulcahy, D., Orru, E., Pandey-Pommier, M., Pratley, L., Rol, E., Röttgering, H. J. A., Scaife, A. M. M., Shulevski, A., Sobey, C. A., Stappers, B. W., Tasse, C., van der Horst, A. J., van Velzen, S., van Weeren, R. J., Wijers, R. A. M. J., Wijnands, R., Wise, M., Zarka, P., Alexov, A., Anderson, J., Asgekar, A., Avruch, I. M., Bentum, M. J., Bernardi, G., Best, P., Breitling, F., Brüggen, M., Butcher, H. R., Ciardi, B., Conway, J. E., Corstanje, A., de Geus, E., Deller, A., Duscha, S., Frieswijk, W., Garrett, M. A., Gunst, A. W., van Haarlem, M. P., Hoeft, M., Hörandel, J., Juette, E., Kuper, G., Loose, M., Maat, P., McFadden, R., McKay-Bukowski, D., Moldon, J., Munk, H., Norden, M. J., Paas, H., Polatidis, A. G., Schwarz, D., Sluman, J., Smirnov, O., Steinmetz, M., Thoudam, S., Toribio, M. C., Vermeulen, R., Vocks, C., Wijnholds, S. J., Wucknitz, O., and Yatawatta, S.

Subjects
Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Instrumentation and Methods for Astrophysics
Abstract

We present the results of a four-month campaign searching for low-frequency radio transients near the North Celestial Pole with the Low-Frequency Array (LOFAR), as part of the Multifrequency Snapshot Sky Survey (MSSS). The data were recorded between 2011 December and 2012 April and comprised 2149 11-minute snapshots, each covering 175 deg^2. We have found one convincing candidate astrophysical transient, with a duration of a few minutes and a flux density at 60 MHz of 15-25 Jy. The transient does not repeat and has no obvious optical or high-energy counterpart, as a result of which its nature is unclear. The detection of this event implies a transient rate at 60 MHz of 3.9 (+14.7, -3.7) x 10^-4 day^-1 deg^-2, and a transient surface density of 1.5 x 10^-5 deg^-2, at a 7.9-Jy limiting flux density and ~10-minute time-scale. The campaign data were also searched for transients at a range of other time-scales, from 0.5 to 297 min, which allowed us to place a range of limits on transient rates at 60 MHz as a function of observation duration., Comment: 23 pages, 16 figures, 5 tables, accepted for publication in MNRAS

Published
2015
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9. Calibrating the absolute amplitude scale for air showers measured at LOFAR

Author

Nelles, A., Hörandel, J. R., Karskens, T., Krause, M., Buitink, S., Corstanje, A., Enriquez, J. E., Erdmann, M., Falcke, H., Haungs, A., Hiller, R., Huege, T., Krause, R., Link, K., Norden, M. J., Rachen, J. P., Rossetto, L., Schellart, P., Scholten, O., Schröder, F. G., ter Veen, S., Thoudam, S., Trinh, T. N. G., Weidenhaupt, K., Wijnholds, S. J., Anderson, J., Bähren, L., Bell, M. E., Bentum, M. J., Best, P., Bonafede, A., Bregman, J., Brouw, W. N., Bruüggen, M., Butcher, H. R., Carbone, D., Ciardi, B., de Gasperin, F., Duscha, S., Eislöffel, J., Fallows, R. A., Frieswijk, W., Garrett, M. A., van Haarlem, M. P., Heald, G., Hoeft, M., Horneffer, A., Iacobelli, M., Juette, E., Karastergiou, A., Kohler, J., Kondratiev, V. I., Kuniyoshi, M., Kuper, G., van Leeuwen, J., Maat, P., McFadden, R., McKay-Bukowski, D., Orru, E., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pizzo, R., Polatidis, A. G., Reich, W., Röttgering, H., Schwarz, D., Serylak, M., Sluman, J., Smirnov, O., Tasse, C., Toribio, M. C., Vermeulen, R., van Weeren, R. J., Wijers, R. A. M. J., Wucknitz, O., and Zarka, P.

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics, Astrophysics - High Energy Astrophysical Phenomena
Abstract

Air showers induced by cosmic rays create nanosecond pulses detectable at radio frequencies. These pulses have been measured successfully in the past few years at the LOw Frequency ARray (LOFAR) and are used to study the properties of cosmic rays. For a complete understanding of this phenomenon and the underlying physical processes, an absolute calibration of the detecting antenna system is needed. We present three approaches that were used to check and improve the antenna model of LOFAR and to provide an absolute calibration of the whole system for air shower measurements. Two methods are based on calibrated reference sources and one on a calibration approach using the diffuse radio emission of the Galaxy, optimized for short data-sets. An accuracy of 19% in amplitude is reached. The absolute calibration is also compared to predictions from air shower simulations. These results are used to set an absolute energy scale for air shower measurements and can be used as a basis for an absolute scale for the measurement of astronomical transients with LOFAR., Comment: 34 pages, 10 figures

Published
2015
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10. LOFAR: The LOw-Frequency ARray

Author

van Haarlem, M. P., Wise, M. W., Gunst, A. W., Heald, G., McKean, J. P., Hessels, J. W. T., de Bruyn, A. G., Nijboer, R., Swinbank, J., Fallows, R., Brentjens, M., Nelles, A., Beck, R., Falcke, H., Fender, R., Hörandel, J., Koopmans, L. V. E., Mann, G., Miley, G., Röttgering, H., Stappers, B. W., Wijers, R. A. M. J., Zaroubi, S., Akker, M. van den, Alexov, A., Anderson, J., Anderson, K., van Ardenne, A., Arts, M., Asgekar, A., Avruch, I. M., Batejat, F., Bähren, L., Bell, M. E., Bell, M. R., van Bemmel, I., Bennema, P., Bentum, M. J., Bernardi, G., Best, P., Bîrzan, L., Bonafede, A., Boonstra, A. -J., Braun, R., Bregman, J., Breitling, F., van de Brink, R. H., Broderick, J., Broekema, P. C., Brouw, W. N., Brüggen, M., Butcher, H. R., van Cappellen, W., Ciardi, B., Coenen, T., Conway, J., Coolen, A., Corstanje, A., Damstra, S., Davies, O., Deller, A. T., Dettmar, R. -J., van Diepen, G., Dijkstra, K., Donker, P., Doorduin, A., Dromer, J., Drost, M., van Duin, A., Eislöffel, J., van Enst, J., Ferrari, C., Frieswijk, W., Gankema, H., Garrett, M. A., de Gasperin, F., Gerbers, M., de Geus, E., Grießmeier, J. -M., Grit, T., Gruppen, P., Hamaker, J. P., Hassall, T., Hoeft, M., Holties, H., Horneffer, A., van der Horst, A., van Houwelingen, A., Huijgen, A., Iacobelli, M., Intema, H., Jackson, N., Jelic, V., de Jong, A., Juette, E., Kant, D., Karastergiou, A., Koers, A., Kollen, H., Kondratiev, V. I., Kooistra, E., Koopman, Y., Koster, A., Kuniyoshi, M., Kramer, M., Kuper, G., Lambropoulos, P., Law, C., van Leeuwen, J., Lemaitre, J., Loose, M., Maat, P., Macario, G., Markoff, S., Masters, J., McKay-Bukowski, D., Meijering, H., Meulman, H., Mevius, M., Middelberg, E., Millenaar, R., Miller-Jones, J. C. A., Mohan, R. N., Mol, J. D., Morawietz, J., Morganti, R., Mulcahy, D. D., Mulder, E., Munk, H., Nieuwenhuis, L., van Nieuwpoort, R., Noordam, J. E., Norden, M., Noutsos, A., Offringa, A. R., Olofsson, H., Omar, A., Orrú, E., Overeem, R., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pizzo, R., Polatidis, A., Rafferty, D., Rawlings, S., Reich, W., de Reijer, J. -P., Reitsma, J., Renting, A., Riemers, P., Rol, E., Romein, J. W., Roosjen, J., Ruiter, M., Scaife, A., van der Schaaf, K., Scheers, B., Schellart, P., Schoenmakers, A., Schoonderbeek, G., Serylak, M., Shulevski, A., Sluman, J., Smirnov, O., Sobey, C., Spreeuw, H., Steinmetz, M., Sterks, C. G. M., Stiepel, H. -J., Stuurwold, K., Tagger, M., Tang, Y., Tasse, C., Thomas, I., Thoudam, S., Toribio, M. C., van der Tol, B., Usov, O., van Veelen, M., van der Veen, A. -J., ter Veen, S., Verbiest, J. P. W., Vermeulen, R., Vermaas, N., Vocks, C., Vogt, C., de Vos, M., van der Wal, E., van Weeren, R., Weggemans, H., Weltevrede, P., White, S., Wijnholds, S. J., Wilhelmsson, T., Wucknitz, O., Yatawatta, S., Zarka, P., Zensus, A., and van Zwieten, J.

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics
Abstract

LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory., Comment: 56 pages, 34 figures, accepted for publication by A&A

Published
2013
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11. M87 at metre wavelengths: the LOFAR picture

Author

de Gasperin, F., Orru', E., Murgia, M., Merloni, A., Falcke, H., Beck, R., Beswick, R., Birzan, L., Bonafede, A., Bruggen, M., Brunetti, G., Chyzy, K., Conway, J., Croston, J. H., Ensslin, T., Ferrari, C., Heald, G., Heidenreich, S., Jackson, N., Macario, G., McKean, J., Miley, G., Morganti, R., Offringa, A., Pizzo, R., Rafferty, D., Roettgering, H., Shulevski, A., Steinmetz, M., Tasse, C., van der Tol, S., van Driel, W., van Weeren, R. J., van Zwieten, J. E., Alexov, A., Anderson, J., Asgekar, A., Avruch, M., Bell, M., Bell, M. R., Bentum, M., Bernardi, G., Best, P., Breitling, F., Broderick, J. W., Butcher, A., Ciardi, B., Dettmar, R. J., Eisloeffel, J., Frieswijk, W., Gankema, H., Garrett, M., Gerbers, M., Griessmeier, J. M., Gunst, A. W., Hassall, T. E., Hessels, J., Hoeft, M., Horneffer, A., Karastergiou, A., Koehler, J., Koopman, Y., Kuper, G., Maat, P., Mann, G., Mevius, M., Mulcahy, D. D., Munk, H., Nijboer, R., Kuniyoshi, M., Noordam, J., Paas, H., Pandey, M., Pandey, V. N., Polatidis, A., Reich, W., Schoenmakers, A. . P., Sluman, J., Smirnov, O., Sobey, C., Stappers, B., Swinbank, J., Tagger, M., Tang, Y., van Bemmel, I., van Cappellen, W., van Duin, A. P., van Haarlem, M., van Leeuwen, J., Vermeulen, R., Vocks, C., White, S., Wise, M., Wucknitz, O., and Zarka, P.

Subjects
Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics
Abstract

M87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4x10^9 M_sun, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Here we present the first observations made with the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz. Images of M87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M87 extended radio-halo using these observations together with archival data. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ~10 uG, which increases to ~13 uG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ~40 Myr, which in turn provides a jet kinetic power of 6-10x10^44 erg/s., Comment: 22 pages, 16 figures - Accepted in A&A

Published
2012
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12. The LOFAR radio environment

Author

Offringa, A. R., de Bruyn, A. G., Zaroubi, S., van Diepen, G., Martinez-Ruby, O., Labropoulos, P., Brentjens, M. A., Ciardi, B., Daiboo, S., Harker, G., Jelic, V., Kazemi, S., Koopmans, L. V. E., Mellema, G., Pandey, V. N., Pizzo, R. F., Schaye, J., Vedantham, H., Veligatla, V., Wijnholds, S. J., Yatawatta, S., Zarka, P., Alexov, A., Anderson, J., Asgekar, A., Avruch, M., Beck, R., Bell, M., Bell, M. R., Bentum, M., Bernardi, G., Best, P., Birzan, L., Bonafede, A., Breitling, F., Broderick, J. W., Bruggen, M., Butcher, H., Conway, J., de Vos, M., Dettmar, R. J., Eisloeffel, J., Falcke, H., Fender, R., Frieswijk, W., Gerbers, M., Griessmeier, J. M., Gunst, A. W., Hassall, T. E., Heald, G., Hessels, J., Hoeft, M., Horneffer, A., Karastergiou, A., Kondratiev, V., Koopman, Y., Kuniyoshi, M., Kuper, G., Maat, P., Mann, G., McKean, J., Meulman, H., Mevius, M., Mol, J. D., Nijboer, R., Noordam, J., Norden, M., Paas, H., Pandey, M., Pizzo, R., Polatidis, A., Rafferty, D., Rawlings, S., Reich, W., Rottgering, H. J. A., Schoenmakers, A. P., Sluman, J., Smirnov, O., Sobey, C., Stappers, B., Steinmetz, M., Swinbank, J., Tagger, M., Tang, Y., Tasse, C., van Ardenne, A., van Cappellen, W., van Duin, A. P., van Haarlem, M., van Leeuwen, J., van Weeren, R. J., Vermeulen, R., Vocks, C., Wijers, R. A. M. J., Wise, M., and Wucknitz, O.

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics
Abstract

Aims: This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations. Methods: We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz / 1 s resolution. Results: We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR's nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution. Conclusions: Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills., Comment: Accepted for publication in A&A. 15 figures, 16 pages

Published
2012
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13. Wide-band Simultaneous Observations of Pulsars: Disentangling Dispersion Measure and Profile Variations

Author

Hassall, T. E., Stappers, B. W., Hessels, J. W. T., Kramer, M., Alexov, A., Anderson, K., Coenen, T., Karastergiou, A., Keane, E. F., Kondratiev, V. I., Lazaridis, K., van Leeuwen, J., Noutsos, A., Serylak, M., Sobey, C., Verbiest, J. P. W., Weltevrede, P., Zagkouris, K., Fender, R., Wijers, R. A. M. J., Bahren, L., Bell, M. E., Broderick, J. W., Corbel, S., Daw, E. J., Dhillon, V. S., Eisloffel, J., Falcke, H., Griessmeier, J. -M., Jonker, P., Law, C., Markoff, S., Miller-Jones, J. C. A., Osten, R., Rol, E., Scaife, A. M. M., Scheers, B., Schellart, P., Spreeuw, H., Swinbank, J., ter Veen, S., Wise, M. W., Wijnands, R., Wucknitz, O., Zarka, P., Asgekar, A., Bell, M. R., Bentum, M. J., Bernardi, G., Best, P., Bonafede, A., Boonstra, A. J., Brentjens, M., Brouw, W. N., Bruggen, M., Butcher, H. R., Ciardi, B., Garrett, M. A., Gerbers, M., Gunst, A. W., van Haarlem, M. P., Heald, G., Hoeft, M., Holties, H., de Jong, A., Koopmans, L. V. E., Kuniyoshi, M., Kuper, G., Loose, G. M., Maat, P., Masters, J., McKean, J. P., Meulman, H., Mevius, M., Munk, H., Noordam, J. E., Orru, E., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pizzo, R., Polatidis, A., Reich, W., Rottgering, H., Sluman, J., Steinmetz, M., Sterks, C. G. M., Tagger, M., Tang, Y., Tasse, C., Vermeulen, R., van Weeren, R. J., Wijnholds, S. J., and Yatawatta, S.

Subjects
Astrophysics - High Energy Astrophysical Phenomena
Abstract

Dispersion in the interstellar medium is a well known phenomenon that follows a simple relationship, which has been used to predict the time delay of dispersed radio pulses since the late 1960s. We performed wide-band simultaneous observations of four pulsars with LOFAR (at 40-190 MHz), the 76-m Lovell Telescope (at 1400 MHz) and the Effelsberg 100-m Telescope (at 8000 MHz) to test the accuracy of the dispersion law over a broad frequency range. In this paper we present the results of these observations which show that the dispersion law is accurate to better than 1 part in 100000 across our observing band. We use this fact to constrain some of the properties of the ISM along the line-of-sight and use the lack of any aberration or retardation effects to determine upper limits on emission heights in the pulsar magnetosphere. We also discuss the effect of pulse profile evolution on our observations, and the implications that it could have for precision pulsar timing projects such as the detection of gravitational waves with pulsar timing arrays., Comment: 20 Pages, 14 Figures, Accepted for publication in Astronomy & Astrophysics

Published
2012
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14. Optimized Trigger for Ultra-High-Energy Cosmic-Ray and Neutrino Observations with the Low Frequency Radio Array

Author

Singh, K., Mevius, M., Scholten, O., Anderson, J. M., van Ardenne, A., Arts, M., Avruch, M., Asgekar, A., Bell, M., Bennema, P., Bentum, M., Bernadi, G., Best, P., Boonstra, A. -J., Bregman, J., van de Brink, R., Broekema, C., Brouw, W., Brueggen, M., Buitink, S., Butcher, H., van Cappellen, W., Ciardi, B., Coolen, A., Damstra, S., Dettmar, R., van Diepen, G., Dijkstra, K., Donker, P., Doorduin, A., Drost, M., van Duin, A., Eisloeffel, J., Falcke, H., Garrett, M., Gerbers, M., Griessmeier, J., Grit, T., Gruppen, P., Gunst, A., van Haarlem, M., Hoeft, M., Holties, H., Horandel, J., Horneffer, L. A., Huijgen, A., James, C., de Jong, A., Kant, D., Kooistra, E., Koopman, Y., Koopmans, L., Kuper, G., Lambropoulos, P., van Leeuwen, J., Loose, M., Maat, P., Mallary, C., McFadden, R., Meulman, H., Mol, J. -D., Morawietz, J., Mulder, E., Munk, H., Nieuwenhuis, L., Nijboer, R., Norden, M., Noordam, J., Overeem, R., Paas, H., Pandey, V. N., Pandey-Pommier, M., Pizzo, R., Polatidis, A., Reich, W., de Reijer, J., Renting, A., Riemers, P., Roettgering, H., Romein, J., Roosjen, J., Ruiter, M., Schoenmakers, A., Schoonderbeek, G., Sluman, J., Smirnov, O., Stappers, B., Steinmetz, M., Stiepel, H., Stuurwold, K., Tagger, M., Tang, Y., ter Veen, S., Vermeulen, R., de Vos, M., Vogt, C., van der Wal, E., Weggemans, H., Wijnholds, S., Wise, M., Wucknitz, O., Yattawatta, S., and van Zwieten, J.

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics, Astrophysics - High Energy Astrophysical Phenomena
Abstract

When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to detect these pulses. In this work we propose an efficient trigger implementation for LOFAR optimized for the observation of short radio pulses., Comment: Submitted to Nuclear Instruments and Methods in Physics Research Section A

Published
2011
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15. Observing pulsars and fast transients with LOFAR

Author

Stappers, B. W., Hessels, J. W. T., Alexov, A., Anderson, K., Coenen, T., Hassall, T., Karastergiou, A., Kondratiev, V. I., Kramer, M., van Leeuwen, J., Mol, J. D., Noutsos, A., Romein, J. W ., Weltevrede, P., Fender, R., Wijers, R. A. M. J., Bähren, L., Bell, M. E., Broderick, J., Daw, E. J., Dhillon, V. S., Eislöffel, J., Falcke, H., Griessmeier, J., Law, C., Markoff, S., Miller-Jones, J. C. A., Scheers, B., Spreeuw, H., Swinbank, J., ter Veen, S., Wise, M. W., Wucknitz, O., Zarka, P., Anderson, J., Asgekar, A., Avruch, I. M., Beck, R., Bennema, P., Bentum, M. J., Best, P., Bregman, J., Brentjens, M., van de Brink, R. H., Broekema, P. C., Brouw, W. N., Brüggen, M., de Bruyn, A. G., Butcher, H. R., Ciardi, B., Conway, J., Dettmar, R. -J., van Duin, A., van Enst, J., Garrett, M., Gerbers, M., Grit, T., Gunst, A., van Haarlem, M. P., Hamaker, J. P., Heald, G., Hoeft, M., Holties, H., Horneffer, A., Koopmans, L. V. E., Kuper, G., Loose, M., Maat, P., McKay-Bukowski, D., McKean, J. P., Miley, G., Morganti, R., Nijboer, R., Noordam, J. E., Norden, M., Olofsson, H., Pandey-Pommier, M., Polatidis, A., Reich, W., Röttgering, H., Schoenmakers, A., Sluman, J., Smirnov, O., Steinmetz, M., Sterks, C. G. M., Tagger, M., Tang, Y., Vermeulen, R., Vermaas, N., Vogt, C., de Vos, M., Wijnholds, S. J., Yatawatta, S., and Zensus, A.

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics, Astrophysics - Solar and Stellar Astrophysics
Abstract

Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric "radio window": 10-240MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR., Comment: 35 pages, 19 figures, accepted for publication by A&A

Published
2011
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16. A very brief description of LOFAR - the Low Frequency Array

Author

Falcke, H., van Haarlem, M. P., de Bruyn, A. G., Braun, R., Röttgering, H. J. A., Stappers, B., Boland, W. H. W. M., Butcher, H. R., de Geus, E. J., Koopmans, L., Fender, R., Kuijpers, J., Miley, G. K., Schilizzi, R. T., Vogt, C., Wijers, R. A. M. J., Wise, M., Brouw, W. N., Hamaker, J. P., Noordam, J. E., Oosterloo, T., Bähren, L., Brentjens, M. A., Wijnholds, S. J., Bregman, J. D., van Cappellen, W. A., Gunst, A. W., Kant, G. W., Reitsma, J., van der Schaaf, K., and de Vos, C. M.

Subjects
Astrophysics
Abstract

LOFAR (Low Frequency Array) is an innovative radio telescope optimized for the frequency range 30-240 MHz. The telescope is realized as a phased aperture array without any moving parts. Digital beam forming allows the telescope to point to any part of the sky within a second. Transient buffering makes retrospective imaging of explosive short-term events possible. The scientific focus of LOFAR will initially be on four key science projects (KSPs): 1) detection of the formation of the very first stars and galaxies in the universe during the so-called epoch of reionization by measuring the power spectrum of the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2) low-frequency surveys of the sky with of order $10^8$ expected new sources; 3) all-sky monitoring and detection of transient radio sources such as gamma-ray bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003) allowing for the first time access to particles beyond 10^21 eV (Scholten et al. 2006). Apart from the KSPs open access for smaller projects is also planned. Here we give a brief description of the telescope., Comment: 2 pages, IAU GA 2006, Highlights of Astronomy, Volume 14, K.A. van der Hucht, ed

Published
2006
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17. LOFAR - Opening up a new window on the Universe

Author

Rottgering, H. J. A., Braun, R., Barthel, P. D., van Haarlem, M. P., Miley, G. K., Morganti, R., Snellen, I., Falcke, H., de Bruyn, A. G., Stappers, R. B., Boland, W. H. W. M., Butcher, H. R., de Geus, E. J., Koopmans, L., Fender, R., Kuijpers, J., Schilizzi, R. T., Vogt, C., Wijers, R. A. M. J., Wise, M., Brouw, W. N., Hamaker, J. P., Noordam, J. E., Oosterloo, T., Bahren, L., Brentjens, M. A., Wijnholds, S. J., Bregman, J. D., van Cappellen, W. A., Gunst, A. W., Kant, G. W., Reitsma, J., van der Schaaf, K., and de Vos, C. M.

Subjects
Astrophysics
Abstract

LOFAR, the Low Frequency Array, is a next-generation radio telescope that is being built in Northern Europe and expected to be fully operational at the end of this decade. It will operate at frequencies from 15 to 240 MHz (corresponding to wavelengths of 20 to 1.2 m). Its superb sensitivity, high angular resolution, large field of view and flexible spectroscopic capabilities will represent a dramatic improvement over previous facilities at these wavelengths. As such, LOFAR will carry out a broad range of fundamental astrophysical studies. The design of LOFAR has been driven by four fundamental astrophysical applications: (i) The Epoch of Reionisation, (ii) Extragalactic Surveys and their exploitation to study the formation and evolution of clusters, galaxies and black holes, (iii) Transient Sources and their association with high energy objects such as gamma ray bursts, and (iv) Cosmic Ray showers and their exploitation to study the origin of ultra-high energy cosmic rays. In this conference the foreseen LOFAR work on the epoch of reionisation has been covered by de Bruyn and on cosmic ray showers by Falcke. During this contribution we will first present the LOFAR project with an emphasis on the challenges faced when carrying out sensitive imaging at low radio frequencies. Subsequently, we will discuss LOFAR's capabilities to survey the low-frequency radio sky. Main aims for the planned surveys are studies of z>6 radio galaxies, diffuse emission associated with distant clusters and starbursting galaxies at z>2., Comment: 11 pages, 5 figures, to appear in the proceedings of the conference "Cosmology, galaxy formation and astroparticle physics on the pathway to the SKA", Oxford, April 10-12 2006

Published
2006

18. LOFAR: A new radio telescope for low frequency radio observations: Science and project status

Author

Rottgering, H., de Bruyn, A. G., Fender, R. P., Kuijpers, J., van Haarlem, M. P., Johnston-Hollitt, M., and Miley, G. K

Subjects
Astrophysics
Abstract

LOFAR, the Low Frequency Array, is a large radio telescope consisting about 100 soccer field sized antenna stations spread over a region of 400 km in diameter. It will operate in the frequency range from ~10 to 240 MHz, with a resolution at 240 MHz of better than an arcsecond. Its superb sensitivity will allow for a broad range of astrophysical studies. In this contribution we first discuss four major areas of astrophysical research in which LOFAR will undoubtedly make important contributions: reionisation, distant galaxies and AGNs, transient radio sources and cosmic rays. Subsequently, we will discuss the technical concept of the instrument and the status of the LOFAR project, Comment: 8 pages, 2 figures, to appear in the proceedings of the XXI Texas Symposium on Relativistic Astrophysics held on December 9--13 2002, in Florence, Italy

Published
2003
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19. Needle-like structures discovered on positively charged lightning branches

Author

Hare, B. M., Scholten, O., Dwyer, J., Trinh, T. N. G., Buitink, S., ter Veen, S., Bonardi, A., Corstanje, A., Falcke, H., Hörandel, J. R., Huege, T., Mitra, P., Mulrey, K., Nelles, A., Rachen, J. P., Rossetto, L., Schellart, P., Winchen, T., Anderson, J., Avruch, I. M., Bentum, M. J., Blaauw, R., Broderick, J. W., Brouw, W. N., Brüggen, M., Butcher, H. R., Ciardi, B., Fallows, R. A., de Geus, E., Duscha, S., Eislöffel, J., Garrett, M. A., Grießmeier, J. M., Gunst, A. W., van Haarlem, M. P., Hessels, J. W. T., Hoeft, M., van der Horst, A. J., Iacobelli, M., Koopmans, L. V. E., Krankowski, A., Maat, P., Norden, M. J., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pekal, R., Pizzo, R., Reich, W., Rothkaehl, H., Röttgering, H. J. A., Rowlinson, A., Schwarz, D. J., Shulevski, A., Sluman, J., Smirnov, O., Soida, M., Tagger, M., Toribio, M. C., van Ardenne, A., Wijers, R. A. M. J., van Weeren, R. J., Wucknitz, O., Zarka, P., and Zucca, P.

Published
2019
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20. Projection effects in cluster catalogues

Author

van Haarlem, M. P., Frenk, C. S., and White, S. D. M.

Subjects
Astrophysics
Abstract

We investigate the importance of projection effects in the identification of galaxy clusters in 2D galaxy maps and their effect on the estimation of cluster velocity dispersions. A volume limited galaxy catalogue that was derived from a Standard CDM N-body simulation was used. We select clusters using criteria that match those employed in the construction of real cluster catalogues and find that our mock Abell cluster catalogues are heavily contaminated and incomplete. Over one third (34 per cent) of clusters of richness class R>=1 are miclassifications arising from the projection of one or more clumps onto an intrinsically poor cluster. Conversely, 32 per cent of intrinsically rich clusters are missed from the R>=1 catalogues, mostly because of statistical fluctuations in the background count. Selection by X-ray luminosity rather than optical richness reduces, but does not completely eliminate, these problems. Contamination by unvirialised sub-clumps near a cluster leads to a considerable overestimation of the cluster velocity dispersion, even if the analysis is restricted only to galaxies close to the cluster centre. Thus, the distribution of cluster masses - often used to test cosmological models - is a highly unreliable statistic. However, the median value of the distribution is considerably more robust. Improved estimates of the cluster velocity dispersion distribution require constructing new cluster catalogues in which clusters are defined according to the number of galaxies within a radius about three times smaller than the Abell radius., Comment: 18 pages, LaTeX uses MNRAS style file, 12 postscript figures (some bitmapped to reduce size) included. Accepted for publication in MNRAS

Published
1997
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21. Projection Effects in the Abell Catalogue

Author

van Haarlem, M. P.

Subjects
Astrophysics
Abstract

We investigate the influence that projection effects have on the completeness of the the Abell cluster catalogue in the context of an $\Omega_0$=1 CDM Universe. We construct a galaxy catalogue and identify clusters using an algorithm that mimics Abell's method. Over 30% of Richness Class $\geq$1 clusters have been selected despite not meeting the required intrinsic (3D) richness criterion. Velocity dispersions are shown to have been systematically overestimated., Comment: 6 pages, including 2 figures, to appear in proceedings ``Mapping, Measuring and Modelling the Universe'', Valencia 1995, eds. P. Coles & V. Martinez

Published
1996

22. Sub-arcsecond imaging with the International LOFAR Telescope

Author

Jackson, N., primary, Badole, S., additional, Morgan, J., additional, Chhetri, R., additional, Prūsis, K., additional, Nikolajevs, A., additional, Morabito, L., additional, Brentjens, M., additional, Sweijen, F., additional, Iacobelli, M., additional, Orrù, E., additional, Sluman, J., additional, Blaauw, R., additional, Mulder, H., additional, van Dijk, P., additional, Mooney, S., additional, Deller, A., additional, Moldon, J., additional, Callingham, J. R., additional, Harwood, J., additional, Hardcastle, M., additional, Heald, G., additional, Drabent, A., additional, McKean, J. P., additional, Asgekar, A., additional, Avruch, I. M., additional, Bentum, M. J., additional, Bonafede, A., additional, Brouw, W. N., additional, Brüggen, M., additional, Butcher, H. R., additional, Ciardi, B., additional, Coolen, A., additional, Corstanje, A., additional, Damstra, S., additional, Duscha, S., additional, Eislöffel, J., additional, Falcke, H., additional, Garrett, M., additional, de Gasperin, F., additional, Griessmeier, J.-M., additional, Gunst, A. W., additional, van Haarlem, M. P., additional, Hoeft, M., additional, van der Horst, A. J., additional, Jütte, E., additional, Koopmans, L. V. E., additional, Krankowski, A., additional, Maat, P., additional, Mann, G., additional, Miley, G. K., additional, Nelles, A., additional, Norden, M., additional, Paas, M., additional, Pandey, V. N., additional, Pandey-Pommier, M., additional, Pizzo, R. F., additional, Reich, W., additional, Rothkaehl, H., additional, Rowlinson, A., additional, Ruiter, M., additional, Shulevski, A., additional, Schwarz, D. J., additional, Smirnov, O., additional, Tagger, M., additional, Vocks, C., additional, van Weeren, R. J., additional, Wijers, R., additional, Wucknitz, O., additional, Zarka, P., additional, Zensus, J. A., additional, and Zucca, P., additional

Published
2022
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23. Sub-arcsecond imaging with the International LOFAR Telescope

Author

Morabito, L. K., primary, Jackson, N. J., additional, Mooney, S., additional, Sweijen, F., additional, Badole, S., additional, Kukreti, P., additional, Venkattu, D., additional, Groeneveld, C., additional, Kappes, A., additional, Bonnassieux, E., additional, Drabent, A., additional, Iacobelli, M., additional, Croston, J. H., additional, Best, P. N., additional, Bondi, M., additional, Callingham, J. R., additional, Conway, J. E., additional, Deller, A. T., additional, Hardcastle, M. J., additional, McKean, J. P., additional, Miley, G. K., additional, Moldon, J., additional, Röttgering, H. J. A., additional, Tasse, C., additional, Shimwell, T. W., additional, van Weeren, R. J., additional, Anderson, J. M., additional, Asgekar, A., additional, Avruch, I. M., additional, van Bemmel, I. M., additional, Bentum, M. J., additional, Bonafede, A., additional, Brouw, W. N., additional, Butcher, H. R., additional, Ciardi, B., additional, Corstanje, A., additional, Coolen, A., additional, Damstra, S., additional, de Gasperin, F., additional, Duscha, S., additional, Eislöffel, J., additional, Engels, D., additional, Falcke, H., additional, Garrett, M. A., additional, Griessmeier, J., additional, Gunst, A. W., additional, van Haarlem, M. P., additional, Hoeft, M., additional, van der Horst, A. J., additional, Jütte, E., additional, Kadler, M., additional, Koopmans, L. V. E., additional, Krankowski, A., additional, Mann, G., additional, Nelles, A., additional, Oonk, J. B. R., additional, Orru, E., additional, Paas, H., additional, Pandey, V. N., additional, Pizzo, R. F., additional, Pandey-Pommier, M., additional, Reich, W., additional, Rothkaehl, H., additional, Ruiter, M., additional, Schwarz, D. J., additional, Shulevski, A., additional, Soida, M., additional, Tagger, M., additional, Vocks, C., additional, Wijers, R. A. M. J., additional, Wijnholds, S. J., additional, Wucknitz, O., additional, Zarka, P., additional, and Zucca, P., additional

Published
2022
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24. LOFAR 144-MHz follow-up observations of GW170817

Author

Broderick, J W, primary, Shimwell, T W, primary, Gourdji, K, primary, Rowlinson, A, primary, Nissanke, S, primary, Hotokezaka, K, primary, Jonker, P G, primary, Tasse, C, primary, Hardcastle, M J, primary, Oonk, J B R, primary, Fender, R P, primary, Wijers, R A M J, primary, Shulevski, A, primary, Stewart, A J, primary, ter Veen, S, primary, Moss, V A, primary, van der Wiel, M H D, primary, Nichols, D A, primary, Piette, A, primary, Bell, M E, primary, Carbone, D, primary, Corbel, S, primary, Eislöffel, J, primary, Grießmeier, J-M, primary, Keane, E F, primary, Law, C J, primary, Muñoz-Darias, T, primary, Pietka, M, primary, Serylak, M, primary, van der Horst, A J, primary, van Leeuwen, J, primary, Wijnands, R, primary, Zarka, P, primary, Anderson, J M, primary, Bentum, M J, primary, Blaauw, R, primary, Brouw, W N, primary, Brüggen, M, primary, Ciardi, B, primary, de Vos, M, primary, Duscha, S, primary, Fallows, R A, primary, Franzen, T M O, primary, Garrett, M A, primary, Gunst, A W, primary, Hoeft, M, primary, Hörandel, J R, primary, Iacobelli, M, primary, Jütte, E, primary, Koopmans, L V E, primary, Krankowski, A, primary, Maat, P, primary, Mann, G, primary, Mulder, H, primary, Nelles, A, primary, Paas, H, primary, Pandey-Pommier, M, primary, Pekal, R, primary, Reich, W, primary, Röttgering, H J A, primary, Schwarz, D J, primary, Smirnov, O, primary, Soida, M, primary, Toribio, M C, primary, van Haarlem, M P, primary, van Weeren, R J, primary, Vocks, C, primary, Wucknitz, O, primary, and Zucca, P, primary

Published
2020
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25. Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies

Author

de Gasperin, F., primary, Vink, J., additional, McKean, J. P., additional, Asgekar, A., additional, Avruch, I., additional, Bentum, M. J., additional, Blaauw, R., additional, Bonafede, A., additional, Broderick, J. W., additional, Brüggen, M., additional, Breitling, F., additional, Brouw, W. N., additional, Butcher, H. R., additional, Ciardi, B., additional, Cuciti, V., additional, de Vos, M., additional, Duscha, S., additional, Eislöffel, J., additional, Engels, D., additional, Fallows, R. A., additional, Franzen, T. M. O., additional, Garrett, M. A., additional, Gunst, A. W., additional, Hörandel, J., additional, Heald, G., additional, Hoeft, M., additional, Iacobelli, M., additional, Koopmans, L. V. E., additional, Krankowski, A., additional, Maat, P., additional, Mann, G., additional, Mevius, M., additional, Miley, G., additional, Morganti, R., additional, Nelles, A., additional, Norden, M. J., additional, Offringa, A. R., additional, Orrú, E., additional, Paas, H., additional, Pandey, V. N., additional, Pandey-Pommier, M., additional, Pekal, R., additional, Pizzo, R., additional, Reich, W., additional, Rowlinson, A., additional, Rottgering, H. J. A., additional, Schwarz, D. J., additional, Shulevski, A., additional, Smirnov, O., additional, Sobey, C., additional, Soida, M., additional, Steinmetz, M., additional, Tagger, M., additional, Toribio, M. C., additional, van Ardenne, A., additional, van der Horst, A. J., additional, van Haarlem, M. P., additional, van Weeren, R. J., additional, Vocks, C., additional, Wucknitz, O., additional, Zarka, P., additional, and Zucca, P., additional

Published
2020
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26. Corrigendum: A large light-mass component of cosmic rays at 10171017.5 electronvolts from radio observations

Author

Buitink, S., Corstanje, A., Falcke, H., Hrandel, J. R., Huege, T., Nelles, A., Rachen, J. P., Rossetto, L., Schellart, P., Scholten, O., ter Veen, S., Thoudam, S., Trinh, T. N. G., Anderson, J., Asgekar, A., Avruch, I. M., Bell, M. E., Bentum, M. J., Bernardi, G., Best, P., Bonafede, A., Breitling, F., Broderick, J. W., Brouw, W. N., Brggen, M., Butcher, H. R., Carbone, D., Ciardi, B., Conway, J. E., de Gasperin, F., de Geus, E., Deller, A., Dettmar, R.-J., van Diepen, G., Duscha, S., Eislffel, J., Engels, D., Enriquez, J. E., Fallows, R. A., Fender, R., Ferrari, C., Frieswijk, W., Garrett, M. A., Griemeier, J. M., Gunst, A. W., van Haarlem, M. P., Hassall, T. E., Heald, G., Hessels, J. W. T., Hoeft, M., Horneffer, A., Iacobelli, M., Intema, H., Juette, E., Karastergiou, A., Kondratiev, V. I., Kramer, M., Kuniyoshi, M., Kuper, G., van Leeuwen, J., Loose, G. M., Maat, P., Mann, G., Markoff, S., McFadden, R., McKay-Bukowski, D., McKean, J. P., Mevius, M., Mulcahy, D. D., Munk, H., Norden, M. J., Orru, E., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pietka, M., Pizzo, R., Polatidis, A. G., Reich, W., Rttgering, H. J. A., Scaife, A. M. M., Schwarz, D. J., Serylak, M., Sluman, J., Smirnov, O., Stappers, B. W., Steinmetz, M., Stewart, A., Swinbank, J., Tagger, M., Tang, Y., Tasse, C., Toribio, M. C., Vermeulen, R., Vocks, C., Vogt, C., van Weeren, R. J., Wijers, R. A. M. J., Wijnholds, S. J., Wise, M. W., Wucknitz, O., Yatawatta, S., Zarka, P., and Zensus, J. A.

Abstract

Author(s): S. Buitink; A. Corstanje; H. Falcke; J. R. Hrandel; T. Huege; A. Nelles; J. P. Rachen; L. Rossetto; P. Schellart; O. Scholten; S. ter Veen; S. Thoudam; T. N. [...]

Published
2016
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27. Low-frequency radio absorption in Cassiopeia A

Author

Arias, M., primary, Vink, J., additional, de Gasperin, F., additional, Salas, P., additional, Oonk, J. B. R., additional, van Weeren, R. J., additional, van Amesfoort, A. S., additional, Anderson, J., additional, Beck, R., additional, Bell, M. E., additional, Bentum, M. J., additional, Best, P., additional, Blaauw, R., additional, Breitling, F., additional, Broderick, J. W., additional, Brouw, W. N., additional, Brüggen, M., additional, Butcher, H. R., additional, Ciardi, B., additional, de Geus, E., additional, Deller, A., additional, van Dijk, P. C. G., additional, Duscha, S., additional, Eislöffel, J., additional, Garrett, M. A., additional, Grießmeier, J. M., additional, Gunst, A. W., additional, van Haarlem, M. P., additional, Heald, G., additional, Hessels, J., additional, Hörandel, J., additional, Holties, H. A., additional, van der Horst, A. J., additional, Iacobelli, M., additional, Juette, E., additional, Krankowski, A., additional, van Leeuwen, J., additional, Mann, G., additional, McKay-Bukowski, D., additional, McKean, J. P., additional, Mulder, H., additional, Nelles, A., additional, Orru, E., additional, Paas, H., additional, Pandey-Pommier, M., additional, Pandey, V. N., additional, Pekal, R., additional, Pizzo, R., additional, Polatidis, A. G., additional, Reich, W., additional, Röttgering, H. J. A., additional, Rothkaehl, H., additional, Schwarz, D. J., additional, Smirnov, O., additional, Soida, M., additional, Steinmetz, M., additional, Tagger, M., additional, Thoudam, S., additional, Toribio, M. C., additional, Vocks, C., additional, van der Wiel, M. H. D., additional, Wijers, R. A. M. J., additional, Wucknitz, O., additional, Zarka, P., additional, and Zucca, P., additional

Published
2018
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28. Low-frequency radio absorption in Cassiopeia A

Author

Arias, M., Vink, J., de Gasperin, F., Salas, P., Oonk, J. B. R., van Weeren, R. J., van Amesfoort, A. S., Anderson, J., Beck, R., Bell, M. E., Bentum, M. J., Best, P., Blaauw, R., Breitling, F., Broderick, J. W., Brouw, W. N., Brueggen, M., Butcher, H. R., Ciardi, B., de Geus, E., Deller, A., van Dijk, P. C. G., Duscha, S., Eisloeffel, J., Garrett, M. A., Griessmeier, J. M., Gunst, A. W., van Haarlem, M. P., Heald, G., Hessels, J., Horandel, J., Holties, H. A., van der Horst, A. J., Iacobelli, M., Juette, E., Krankowski, A., van Leeuwen, J., Mann, G., McKay-Bukowski, D., McKean, J. P., Mulder, H., Nelles, A., Orru, E., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pekal, R., Pizzo, R., Polatidis, A. G., Reich, W., Rottgering, H. J. A., Rothkaehl, H., Schwarz, D. J., Smirnov, O., Soida, M., Steinmetz, M., Tagger, M., Thoudam, Satyendra, Toribio, M. C., Vocks, C., van der Wiel, M. H. D., Wijers, R. A. M. J., Wucknitz, O., Zarka, P., Zucca, P., Arias, M., Vink, J., de Gasperin, F., Salas, P., Oonk, J. B. R., van Weeren, R. J., van Amesfoort, A. S., Anderson, J., Beck, R., Bell, M. E., Bentum, M. J., Best, P., Blaauw, R., Breitling, F., Broderick, J. W., Brouw, W. N., Brueggen, M., Butcher, H. R., Ciardi, B., de Geus, E., Deller, A., van Dijk, P. C. G., Duscha, S., Eisloeffel, J., Garrett, M. A., Griessmeier, J. M., Gunst, A. W., van Haarlem, M. P., Heald, G., Hessels, J., Horandel, J., Holties, H. A., van der Horst, A. J., Iacobelli, M., Juette, E., Krankowski, A., van Leeuwen, J., Mann, G., McKay-Bukowski, D., McKean, J. P., Mulder, H., Nelles, A., Orru, E., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pekal, R., Pizzo, R., Polatidis, A. G., Reich, W., Rottgering, H. J. A., Rothkaehl, H., Schwarz, D. J., Smirnov, O., Soida, M., Steinmetz, M., Tagger, M., Thoudam, Satyendra, Toribio, M. C., Vocks, C., van der Wiel, M. H. D., Wijers, R. A. M. J., Wucknitz, O., Zarka, P., and Zucca, P.

Abstract

Context. Cassiopeia A is one of the best-studied supernova remnants. Its bright radio and X-ray emission is due to shocked ejecta. Cas A is rather unique in that the unshocked ejecta can also be studied: through emission in the infrared, the radio-active decay of Ti-44, and the low-frequency free-free absorption caused by cold ionised gas, which is the topic of this paper. Aims. Free-free absorption processes are affected by the mass, geometry, temperature, and ionisation conditions in the absorbing gas. Observations at the lowest radio frequencies can constrain a combination of these properties. Methods. We used Low Frequency Array (LOFAR) Low Band Antenna observations at 30-77 MHz and Very Large Array (VLA) L-band observations at 1-2 GHz to fit for internal absorption as parametrised by the emission measure. We simultaneously fit multiple UV-matched images with a common resolution of 17 '' (this corresponds to 0.25 pc for a source at the distance of Cas A). The ample frequency coverage allows us separate the relative contributions from the absorbing gas, the unabsorbed front of the shell, and the absorbed back of the shell to the emission spectrum. We explored the effects that a temperature lower than the similar to 100-500 K proposed from infrared observations and a high degree of clumping can have on the derived physical properties of the unshocked material, such as its mass and density. We also compiled integrated radio flux density measurements, fit for the absorption processes that occur in the radio band, and considered their effect on the secular decline of the source. Results. We find a mass in the unshocked ejecta of M = 2.95 +/- 0.48 M-circle dot for an assumed gas temperature of T = 100 K. This estimate is reduced for colder gas temperatures and, most significantly, if the ejecta are clumped. We measure the reverse shock to have a radius of 114 '' +/- 6 '' and be centred at 23:23:26, +58:48:54 (J2000). We also find that a decrease in the amount of mass

Published
2018
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29. LOFAR MSSS : detection of a low-frequency radio transient in 400 h of monitoring of the North Celestial Pole

Author

Stewart, A. J., Fender, R. P., Broderick, J. W., Hassall, T. E., Muñoz-Darias, T., Rowlinson, A., Swinbank, J. D., Staley, T. D., Molenaar, G. J., Scheers, B., Grobler, T. L., Pietka, M., Heald, G., McKean, J. P., Bell, M. E., Bonafede, A., Breton, R. P., Carbone, D., Cendes, Y., Clarke, A. O., Corbel, S., de Gasperin, F., Eislöffel, J., Falcke, H., Ferrari, C., Grießmeier, J. -M, Hardcastle, M. J., Heesen, V., Hessels, J. W. T., Horneffer, A., Iacobelli, M., Jonker, P., Karastergiou, A., Kokotanekov, G., Kondratiev, V. I., Kuniyoshi, M., Law, C. J., van Leeuwen, J., Markoff, S., Miller-Jones, J. C. A., Mulcahy, D., Orru, E., Pandey-Pommier, M., Pratley, L., Rol, E., Röttgering, H. J. A., Scaife, A. M. M., Shulevski, A., Sobey, C. A., Stappers, B. W., Tasse, C., van der Horst, A. J., van Velzen, S., van Weeren, R. J., Wijers, R. A. M. J., Wijnands, R., Wise, M., Zarka, P., Alexov, A., Anderson, J., Asgekar, A., Avruch, I. M., Bentum, M. J., Bernardi, G., Best, P., Breitling, F., Brüggen, M., Butcher, H. R., Ciardi, B., Conway, J. E., Corstanje, A., de Geus, E., Deller, A., Duscha, S., Frieswijk, W., Garrett, M. A., Gunst, A. W., van Haarlem, M. P., Hoeft, M., Hörandel, J., Juette, E., Kuper, G., Loose, M., Maat, P., McFadden, R., McKay-Bukowski, D., Moldon, J., Munk, H., Norden, M. J., Paas, H., Polatidis, A. G., Schwarz, D., Sluman, J., Smirnov, O., Steinmetz, M., Thoudam, Satyendra, Toribio, M. C., Vermeulen, R., Vocks, C., Wijnholds, S. J., Wucknitz, O., Yatawatta, S., Stewart, A. J., Fender, R. P., Broderick, J. W., Hassall, T. E., Muñoz-Darias, T., Rowlinson, A., Swinbank, J. D., Staley, T. D., Molenaar, G. J., Scheers, B., Grobler, T. L., Pietka, M., Heald, G., McKean, J. P., Bell, M. E., Bonafede, A., Breton, R. P., Carbone, D., Cendes, Y., Clarke, A. O., Corbel, S., de Gasperin, F., Eislöffel, J., Falcke, H., Ferrari, C., Grießmeier, J. -M, Hardcastle, M. J., Heesen, V., Hessels, J. W. T., Horneffer, A., Iacobelli, M., Jonker, P., Karastergiou, A., Kokotanekov, G., Kondratiev, V. I., Kuniyoshi, M., Law, C. J., van Leeuwen, J., Markoff, S., Miller-Jones, J. C. A., Mulcahy, D., Orru, E., Pandey-Pommier, M., Pratley, L., Rol, E., Röttgering, H. J. A., Scaife, A. M. M., Shulevski, A., Sobey, C. A., Stappers, B. W., Tasse, C., van der Horst, A. J., van Velzen, S., van Weeren, R. J., Wijers, R. A. M. J., Wijnands, R., Wise, M., Zarka, P., Alexov, A., Anderson, J., Asgekar, A., Avruch, I. M., Bentum, M. J., Bernardi, G., Best, P., Breitling, F., Brüggen, M., Butcher, H. R., Ciardi, B., Conway, J. E., Corstanje, A., de Geus, E., Deller, A., Duscha, S., Frieswijk, W., Garrett, M. A., Gunst, A. W., van Haarlem, M. P., Hoeft, M., Hörandel, J., Juette, E., Kuper, G., Loose, M., Maat, P., McFadden, R., McKay-Bukowski, D., Moldon, J., Munk, H., Norden, M. J., Paas, H., Polatidis, A. G., Schwarz, D., Sluman, J., Smirnov, O., Steinmetz, M., Thoudam, Satyendra, Toribio, M. C., Vermeulen, R., Vocks, C., Wijnholds, S. J., Wucknitz, O., and Yatawatta, S.

Abstract

We present the results of a four-month campaign searching for low-frequency radio transients near the North Celestial Pole with the Low-Frequency Array (LOFAR), as part of the Multifrequency Snapshot Sky Survey (MSSS). The data were recorded between 2011 December and 2012 April and comprised 2149 11-min snapshots, each covering 175 deg2. We have found one convincing candidate astrophysical transient, with a duration of a few minutes and a flux density at 60 MHz of 15–25 Jy. The transient does not repeat and has no obvious optical or high-energy counterpart, as a result of which its nature is unclear. The detection of this event implies a transient rate at 60 MHz of 3.9−3.7+14.7×10−4" style="position: relative;" tabindex="0" id="MathJax-Element-1-Frame" class="MathJax">3.9+14.7−3.7×10−4 d−1 deg−2, and a transient surface density of 1.5 × 10−5 deg−2, at a 7.9-Jy limiting flux density and ∼10-min time-scale. The campaign data were also searched for transients at a range of other time-scales, from 0.5 to 297 min, which allowed us to place a range of limits on transient rates at 60 MHz as a function of observation duration.

Published
2016
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30. LBCS: The LOFAR Long-Baseline Calibrator Survey

Author

Jackson, N., primary, Tagore, A., additional, Deller, A., additional, Moldón, J., additional, Varenius, E., additional, Morabito, L., additional, Wucknitz, O., additional, Carozzi, T., additional, Conway, J., additional, Drabent, A., additional, Kapinska, A., additional, Orrù, E., additional, Brentjens, M., additional, Blaauw, R., additional, Kuper, G., additional, Sluman, J., additional, Schaap, J., additional, Vermaas, N., additional, Iacobelli, M., additional, Cerrigone, L., additional, Shulevski, A., additional, ter Veen, S., additional, Fallows, R., additional, Pizzo, R., additional, Sipior, M., additional, Anderson, J., additional, Avruch, I. M., additional, Bell, M. E., additional, van Bemmel, I., additional, Bentum, M. J., additional, Best, P., additional, Bonafede, A., additional, Breitling, F., additional, Broderick, J. W., additional, Brouw, W. N., additional, Brüggen, M., additional, Ciardi, B., additional, Corstanje, A., additional, de Gasperin, F., additional, de Geus, E., additional, Eislöffel, J., additional, Engels, D., additional, Falcke, H., additional, Garrett, M. A., additional, Grießmeier, J. M., additional, Gunst, A. W., additional, van Haarlem, M. P., additional, Heald, G., additional, Hoeft, M., additional, Hörandel, J., additional, Horneffer, A., additional, Intema, H., additional, Juette, E., additional, Kuniyoshi, M., additional, van Leeuwen, J., additional, Loose, G. M., additional, Maat, P., additional, McFadden, R., additional, McKay-Bukowski, D., additional, McKean, J. P., additional, Mulcahy, D. D., additional, Munk, H., additional, Pandey-Pommier, M., additional, Polatidis, A. G., additional, Reich, W., additional, Röttgering, H. J. A., additional, Rowlinson, A., additional, Scaife, A. M. M., additional, Schwarz, D. J., additional, Steinmetz, M., additional, Swinbank, J., additional, Thoudam, S., additional, Toribio, M. C., additional, Vermeulen, R., additional, Vocks, C., additional, van Weeren, R. J., additional, Wise, M. W., additional, Yatawatta, S., additional, and Zarka, P., additional

Published
2016
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31. LOFAR imaging of Cygnus A – direct detection of a turnover in the hotspot radio spectra

Author

McKean, J. P., primary, Godfrey, L. E. H., additional, Vegetti, S., additional, Wise, M. W., additional, Morganti, R., additional, Hardcastle, M. J., additional, Rafferty, D., additional, Anderson, J., additional, Avruch, I. M., additional, Beck, R., additional, Bell, M. E., additional, van Bemmel, I., additional, Bentum, M. J., additional, Bernardi, G., additional, Best, P., additional, Blaauw, R., additional, Bonafede, A., additional, Breitling, F., additional, Broderick, J. W., additional, Brüggen, M., additional, Cerrigone, L., additional, Ciardi, B., additional, de Gasperin, F., additional, Deller, A., additional, Duscha, S., additional, Engels, D., additional, Falcke, H., additional, Fallows, R. A., additional, Frieswijk, W., additional, Garrett, M. A., additional, Grießmeier, J. M., additional, van Haarlem, M. P., additional, Heald, G., additional, Hoeft, M., additional, Horst, A. J. van der, additional, Iacobelli, M., additional, Intema, H., additional, Juette, E., additional, Karastergiou, A., additional, Kondratiev, V. I., additional, Koopmans, L. V. E., additional, Kuniyoshi, M., additional, Kuper, G., additional, van Leeuwen, J., additional, Maat, P., additional, Mann, G., additional, Markoff, S., additional, McFadden, R., additional, McKay-Bukowski, D., additional, Mulcahy, D. D., additional, Munk, H., additional, Nelles, A., additional, Orru, E., additional, Paas, H., additional, Pandey-Pommier, M., additional, Pietka, M., additional, Pizzo, R., additional, Polatidis, A. G., additional, Reich, W., additional, Röttgering, H. J. A., additional, Rowlinson, A., additional, Scaife, A. M. M., additional, Serylak, M., additional, Shulevski, A., additional, Sluman, J., additional, Smirnov, O., additional, Steinmetz, M., additional, Stewart, A., additional, Swinbank, J., additional, Tagger, M., additional, Thoudam, S., additional, Toribio, M. C., additional, Vermeulen, R., additional, Vocks, C., additional, van Weeren, R. J., additional, Wucknitz, O., additional, Yatawatta, S., additional, and Zarka, P., additional

Published
2016
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32. Correction: Corrigendum: A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations

Author

Buitink, S., primary, Corstanje, A., additional, Falcke, H., additional, Hörandel, J. R., additional, Huege, T., additional, Nelles, A., additional, Rachen, J. P., additional, Rossetto, L., additional, Schellart, P., additional, Scholten, O., additional, ter Veen, S., additional, Thoudam, S., additional, Trinh, T. N. G., additional, Anderson, J., additional, Asgekar, A., additional, Avruch, I. M., additional, Bell, M. E., additional, Bentum, M. J., additional, Bernardi, G., additional, Best, P., additional, Bonafede, A., additional, Breitling, F., additional, Broderick, J. W., additional, Brouw, W. N., additional, Brüggen, M., additional, Butcher, H. R., additional, Carbone, D., additional, Ciardi, B., additional, Conway, J. E., additional, de Gasperin, F., additional, de Geus, E., additional, Deller, A., additional, Dettmar, R.-J., additional, van Diepen, G., additional, Duscha, S., additional, Eislöffel, J., additional, Engels, D., additional, Enriquez, J. E., additional, Fallows, R. A., additional, Fender, R., additional, Ferrari, C., additional, Frieswijk, W., additional, Garrett, M. A., additional, Grießmeier, J. M., additional, Gunst, A. W., additional, van Haarlem, M. P., additional, Hassall, T. E., additional, Heald, G., additional, Hessels, J. W. T., additional, Hoeft, M., additional, Horneffer, A., additional, Iacobelli, M., additional, Intema, H., additional, Juette, E., additional, Karastergiou, A., additional, Kondratiev, V. I., additional, Kramer, M., additional, Kuniyoshi, M., additional, Kuper, G., additional, van Leeuwen, J., additional, Loose, G. M., additional, Maat, P., additional, Mann, G., additional, Markoff, S., additional, McFadden, R., additional, McKay-Bukowski, D., additional, McKean, J. P., additional, Mevius, M., additional, Mulcahy, D. D., additional, Munk, H., additional, Norden, M. J., additional, Orru, E., additional, Paas, H., additional, Pandey-Pommier, M., additional, Pandey, V. N., additional, Pietka, M., additional, Pizzo, R., additional, Polatidis, A. G., additional, Reich, W., additional, Röttgering, H. J. A., additional, Scaife, A. M. M., additional, Schwarz, D. J., additional, Serylak, M., additional, Sluman, J., additional, Smirnov, O., additional, Stappers, B. W., additional, Steinmetz, M., additional, Stewart, A., additional, Swinbank, J., additional, Tagger, M., additional, Tang, Y., additional, Tasse, C., additional, Toribio, M. C., additional, Vermeulen, R., additional, Vocks, C., additional, Vogt, C., additional, van Weeren, R. J., additional, Wijers, R. A. M. J., additional, Wijnholds, S. J., additional, Wise, M. W., additional, Wucknitz, O., additional, Yatawatta, S., additional, Zarka, P., additional, and Zensus, J. A., additional

Published
2016
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33. A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations

Author

Buitink, S., primary, Corstanje, A., additional, Falcke, H., additional, Hörandel, J. R., additional, Huege, T., additional, Nelles, A., additional, Rachen, J. P., additional, Rossetto, L., additional, Schellart, P., additional, Scholten, O., additional, ter Veen, S., additional, Thoudam, S., additional, Trinh, T. N. G., additional, Anderson, J., additional, Asgekar, A., additional, Avruch, I. M., additional, Bell, M. E., additional, Bentum, M. J., additional, Bernardi, G., additional, Best, P., additional, Bonafede, A., additional, Breitling, F., additional, Broderick, J. W., additional, Brouw, W. N., additional, Brüggen, M., additional, Butcher, H. R., additional, Carbone, D., additional, Ciardi, B., additional, Conway, J. E., additional, de Gasperin, F., additional, de Geus, E., additional, Deller, A., additional, Dettmar, R.-J., additional, van Diepen, G., additional, Duscha, S., additional, Eislöffel, J., additional, Engels, D., additional, Enriquez, J. E., additional, Fallows, R. A., additional, Fender, R., additional, Ferrari, C., additional, Frieswijk, W., additional, Garrett, M. A., additional, Grießmeier, J. M., additional, Gunst, A. W., additional, van Haarlem, M. P., additional, Hassall, T. E., additional, Heald, G., additional, Hessels, J. W. T., additional, Hoeft, M., additional, Horneffer, A., additional, Iacobelli, M., additional, Intema, H., additional, Juette, E., additional, Karastergiou, A., additional, Kondratiev, V. I., additional, Kramer, M., additional, Kuniyoshi, M., additional, Kuper, G., additional, van Leeuwen, J., additional, Loose, G. M., additional, Maat, P., additional, Mann, G., additional, Markoff, S., additional, McFadden, R., additional, McKay-Bukowski, D., additional, McKean, J. P., additional, Mevius, M., additional, Mulcahy, D. D., additional, Munk, H., additional, Norden, M. J., additional, Orru, E., additional, Paas, H., additional, Pandey-Pommier, M., additional, Pandey, V. N., additional, Pietka, M., additional, Pizzo, R., additional, Polatidis, A. G., additional, Reich, W., additional, Röttgering, H. J. A., additional, Scaife, A. M. M., additional, Schwarz, D. J., additional, Serylak, M., additional, Sluman, J., additional, Smirnov, O., additional, Stappers, B. W., additional, Steinmetz, M., additional, Stewart, A., additional, Swinbank, J., additional, Tagger, M., additional, Tang, Y., additional, Tasse, C., additional, Toribio, M. C., additional, Vermeulen, R., additional, Vocks, C., additional, Vogt, C., additional, van Weeren, R. J., additional, Wijers, R. A. M. J., additional, Wijnholds, S. J., additional, Wise, M. W., additional, Wucknitz, O., additional, Yatawatta, S., additional, Zarka, P., additional, and Zensus, J. A., additional

Published
2016
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35. LOFAR MSSS: detection of a low-frequency radio transient in 400 h of monitoring of the North Celestial Pole

Author

Stewart, A. J., primary, Fender, R. P., additional, Broderick, J. W., additional, Hassall, T. E., additional, Muñoz-Darias, T., additional, Rowlinson, A., additional, Swinbank, J. D., additional, Staley, T. D., additional, Molenaar, G. J., additional, Scheers, B., additional, Grobler, T. L., additional, Pietka, M., additional, Heald, G., additional, McKean, J. P., additional, Bell, M. E., additional, Bonafede, A., additional, Breton, R. P., additional, Carbone, D., additional, Cendes, Y., additional, Clarke, A. O., additional, Corbel, S., additional, de Gasperin, F., additional, Eislöffel, J., additional, Falcke, H., additional, Ferrari, C., additional, Grießmeier, J.-M., additional, Hardcastle, M. J., additional, Heesen, V., additional, Hessels, J. W. T., additional, Horneffer, A., additional, Iacobelli, M., additional, Jonker, P., additional, Karastergiou, A., additional, Kokotanekov, G., additional, Kondratiev, V. I., additional, Kuniyoshi, M., additional, Law, C. J., additional, van Leeuwen, J., additional, Markoff, S., additional, Miller-Jones, J. C. A., additional, Mulcahy, D., additional, Orru, E., additional, Pandey-Pommier, M., additional, Pratley, L., additional, Rol, E., additional, Röttgering, H. J. A., additional, Scaife, A. M. M., additional, Shulevski, A., additional, Sobey, C. A., additional, Stappers, B. W., additional, Tasse, C., additional, van der Horst, A. J., additional, van Velzen, S., additional, van Weeren, R. J., additional, Wijers, R. A. M. J., additional, Wijnands, R., additional, Wise, M., additional, Zarka, P., additional, Alexov, A., additional, Anderson, J., additional, Asgekar, A., additional, Avruch, I. M., additional, Bentum, M. J., additional, Bernardi, G., additional, Best, P., additional, Breitling, F., additional, Brüggen, M., additional, Butcher, H. R., additional, Ciardi, B., additional, Conway, J. E., additional, Corstanje, A., additional, de Geus, E., additional, Deller, A., additional, Duscha, S., additional, Frieswijk, W., additional, Garrett, M. A., additional, Gunst, A. W., additional, van Haarlem, M. P., additional, Hoeft, M., additional, Hörandel, J., additional, Juette, E., additional, Kuper, G., additional, Loose, M., additional, Maat, P., additional, McFadden, R., additional, McKay-Bukowski, D., additional, Moldon, J., additional, Munk, H., additional, Norden, M. J., additional, Paas, H., additional, Polatidis, A. G., additional, Schwarz, D., additional, Sluman, J., additional, Smirnov, O., additional, Steinmetz, M., additional, Thoudam, S., additional, Toribio, M. C., additional, Vermeulen, R., additional, Vocks, C., additional, Wijnholds, S. J., additional, Wucknitz, O., additional, and Yatawatta, S., additional

Published
2015
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36. LOFAR: The LOw-Frequency ARray

Author

van Haarlem, M P, Wise, M J, Gunst, A W, Heald, G, McKean, J P, Hessels, J. W. T., de Bruyn, A.G., Nijboer, R, Swinbank, J, Fallows, R A, Butcher, Harvey, Offringa, Andre, Beck, Rainer, Koopmans, L. V. E., Mann, Gottfried, Anderson, J M, van Ardenne, A, Arts, M, Asgekar, A, Bell, Michael R, van Bemmel, Illse, Bennema, P, Bentum, M J, Bernardi, G, Bonafede, A, Braun, R, Breitling, F, Brouw, W. N., Ciardi, Benedetta, Van Enst, J., Frieswijk, W, Garrett, M A, de Geus, Eugene, Hamaker, Johan P., Hoeft, M, Iacobelli, M., Kondratiev, V. I., Kuniyoshi, M, Kuper, G., van Leeuwen, J, Maat, P, Mol, J. D., Mulcahy, D.D., Munk, H, Van Nieuwpoort, Rob, Norden, M J, Noutsos, A, Pandey-Pommier, M, Pizzo, R, Polatidis, A.G., Rafferty, D A, Reich, W, de Reijer, J.-P., Schoenmakers, Arno, Sluman, J, Sobey, C, Steinmetz, M., Tang, Y, Vermeulen, R.C., Vocks, Christian, de Vos, Marco, van Weeren, R. J., Wijnholds, S J, van Haarlem, M P, Wise, M J, Gunst, A W, Heald, G, McKean, J P, Hessels, J. W. T., de Bruyn, A.G., Nijboer, R, Swinbank, J, Fallows, R A, Butcher, Harvey, Offringa, Andre, Beck, Rainer, Koopmans, L. V. E., Mann, Gottfried, Anderson, J M, van Ardenne, A, Arts, M, Asgekar, A, Bell, Michael R, van Bemmel, Illse, Bennema, P, Bentum, M J, Bernardi, G, Bonafede, A, Braun, R, Breitling, F, Brouw, W. N., Ciardi, Benedetta, Van Enst, J., Frieswijk, W, Garrett, M A, de Geus, Eugene, Hamaker, Johan P., Hoeft, M, Iacobelli, M., Kondratiev, V. I., Kuniyoshi, M, Kuper, G., van Leeuwen, J, Maat, P, Mol, J. D., Mulcahy, D.D., Munk, H, Van Nieuwpoort, Rob, Norden, M J, Noutsos, A, Pandey-Pommier, M, Pizzo, R, Polatidis, A.G., Rafferty, D A, Reich, W, de Reijer, J.-P., Schoenmakers, Arno, Sluman, J, Sobey, C, Steinmetz, M., Tang, Y, Vermeulen, R.C., Vocks, Christian, de Vos, Marco, van Weeren, R. J., and Wijnholds, S J

Abstract

LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and prov

Published
2013

37. First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: The case of Abell 2256

Author

van Weeren, R. J., Rottgering, Hubb, Rafferty, D A, Pizzo, R, Bonafede, A, Bruggen, M, Brunetti, G, Ferrari, C, Orru, E, Heald, G, Butcher, Harvey, McKean, J P, Shulevski, Alexander, van Bemmel, Illse, Miley, George, Hoeft, M, Wise, M J, Anderson, J M, Asgekar, A, Bell, Michael R, Bentum, M J, Bernardi, G, Brouw, W. N., Ciardi, Benedetta, Garrett, M A, Gunst, A W, van Haarlem, M P, Hamaker, Johan P., Hessels, J. W. T., Koopmans, L. V. E., Kuper, G., van Leeuwen, J, Maat, P, Munk, H, Nijboer, R, Pandey-Pommier, M, Polatidis, A.G., Reich, W, Schoenmakers, Arno, Sluman, J, Tang, Y, Vermeulen, R.C., de Vos, Marco, van Weeren, R. J., Rottgering, Hubb, Rafferty, D A, Pizzo, R, Bonafede, A, Bruggen, M, Brunetti, G, Ferrari, C, Orru, E, Heald, G, Butcher, Harvey, McKean, J P, Shulevski, Alexander, van Bemmel, Illse, Miley, George, Hoeft, M, Wise, M J, Anderson, J M, Asgekar, A, Bell, Michael R, Bentum, M J, Bernardi, G, Brouw, W. N., Ciardi, Benedetta, Garrett, M A, Gunst, A W, van Haarlem, M P, Hamaker, Johan P., Hessels, J. W. T., Koopmans, L. V. E., Kuper, G., van Leeuwen, J, Maat, P, Munk, H, Nijboer, R, Pandey-Pommier, M, Polatidis, A.G., Reich, W, Schoenmakers, Arno, Sluman, J, Tang, Y, Vermeulen, R.C., and de Vos, Marco

Abstract

Abell 2256 is one of the best known examples of a galaxy cluster hosting large-scale diffuse radio emission that is unrelated to individual galaxies. It contains both a giant radio halo and a relic, as well as a number of head-tail sources and smaller diffuse steep-spectrum radio sources. The origin of radio halos and relics is still being debated, but over the last years it has become clear that the presence of these radio sources is closely related to galaxy cluster merger events. Here we present the results from the first LOFAR low band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our knowledge, the image presented in this paper at 63 MHz is the deepest ever obtained at frequencies below 100 MHz in general. Both the radio halo and the giant relic are detected in the image at 63 MHz, and the diffuse radio emission remains visible at frequencies as low as 20 MHz. The observations confirm the presence of a previously claimed ultra-steep spectrum source to the west of the cluster center with a spectral index of-2.3 ± 0.4 between 63 and 153 MHz. The steep spectrum suggests that this source is an old part of a head-tail radio source in the cluster. For the radio relic we find an integrated spectral index of-0.81 ± 0.03, after removing the flux contribution from the other sources. This is relatively flat which could indicate that the efficiency of particle acceleration at the shock substantially changed in the last ~0.1 Gyr due to an increase of the shock Mach number. In an alternative scenario, particles are re-accelerated by some mechanism in the downstream region of the shock, resulting in the relatively flat integrated radio spectrum. In the radio halo region we find indications of low-frequency spectral steepening which may suggest that relativistic particles are accelerated in a rather inhomogeneous turbulent region.

Published
2012

38. Optimized trigger for ultra-high-energy cosmic-ray and neutrino observations with the low frequency radio array

Author

Singh, K, Mevius, M, Scholten, O, Anderson, J M, van Ardenne, A, Arts, M, Avruch, M, Askegar, A, Bell, M, Bennema, P, Butcher, Harvey, Asgekar, A, Bentum, M J, Ciardi, Benedetta, Garrett, M A, Gunst, A W, van Haarlem, M P, Hoeft, M, Koopmans, L. V. E., Kuper, G., van Leeuwen, J, Maat, P, Mol, J. D., Munk, H, Nijboer, R, Norden, M J, Pizzo, R, Polatidis, A.G., Reich, W, de Reijer, J.-P., Schoenmakers, Arno, Sluman, J, Tang, Y, Vermeulen, R.C., de Vos, Marco, Wijnholds, S J, Wise, M J, Singh, K, Mevius, M, Scholten, O, Anderson, J M, van Ardenne, A, Arts, M, Avruch, M, Askegar, A, Bell, M, Bennema, P, Butcher, Harvey, Asgekar, A, Bentum, M J, Ciardi, Benedetta, Garrett, M A, Gunst, A W, van Haarlem, M P, Hoeft, M, Koopmans, L. V. E., Kuper, G., van Leeuwen, J, Maat, P, Mol, J. D., Munk, H, Nijboer, R, Norden, M J, Pizzo, R, Polatidis, A.G., Reich, W, de Reijer, J.-P., Schoenmakers, Arno, Sluman, J, Tang, Y, Vermeulen, R.C., de Vos, Marco, Wijnholds, S J, and Wise, M J

Abstract

When an ultra-high energy neutrino or cosmic-ray strikes the Lunar surface a radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to detect these pulses. In this work we propose an efficient trigger implementation for LOFAR optimized for the observation of short radio pulses.

Published
2012

39. LOFAR - Opening up a new window on the Universe

Author

Rottgering, Hubb, Braun, R, Barthel, P.D, van Haarlem, M P, Miley, George, Morganti, Raffaella, Snellen, I, Falcke, H, de Bruyn, A G, Stappers, R B, Butcher, Harvey, Rottgering, Hubb, Braun, R, Barthel, P.D, van Haarlem, M P, Miley, George, Morganti, Raffaella, Snellen, I, Falcke, H, de Bruyn, A G, Stappers, R B, and Butcher, Harvey

Published
2006

40. Wide-band simultaneous observations of pulsars: disentangling dispersion measure and profile variations

Author

Hassall, T. E., primary, Stappers, B. W., additional, Hessels, J. W. T., additional, Kramer, M., additional, Alexov, A., additional, Anderson, K., additional, Coenen, T., additional, Karastergiou, A., additional, Keane, E. F., additional, Kondratiev, V. I., additional, Lazaridis, K., additional, van Leeuwen, J., additional, Noutsos, A., additional, Serylak, M., additional, Sobey, C., additional, Verbiest, J. P. W., additional, Weltevrede, P., additional, Zagkouris, K., additional, Fender, R., additional, Wijers, R. A. M. J., additional, Bähren, L., additional, Bell, M. E., additional, Broderick, J. W., additional, Corbel, S., additional, Daw, E. J., additional, Dhillon, V. S., additional, Eislöffel, J., additional, Falcke, H., additional, Grießmeier, J.-M., additional, Jonker, P., additional, Law, C., additional, Markoff, S., additional, Miller-Jones, J. C. A., additional, Osten, R., additional, Rol, E., additional, Scaife, A. M. M., additional, Scheers, B., additional, Schellart, P., additional, Spreeuw, H., additional, Swinbank, J., additional, ter Veen, S., additional, Wise, M. W., additional, Wijnands, R., additional, Wucknitz, O., additional, Zarka, P., additional, Asgekar, A., additional, Bell, M. R., additional, Bentum, M. J., additional, Bernardi, G., additional, Best, P., additional, Bonafede, A., additional, Boonstra, A. J., additional, Brentjens, M., additional, Brouw, W. N., additional, Brüggen, M., additional, Butcher, H. R., additional, Ciardi, B., additional, Garrett, M. A., additional, Gerbers, M., additional, Gunst, A. W., additional, van Haarlem, M. P., additional, Heald, G., additional, Hoeft, M., additional, Holties, H., additional, de Jong, A., additional, Koopmans, L. V. E., additional, Kuniyoshi, M., additional, Kuper, G., additional, Loose, G. M., additional, Maat, P., additional, Masters, J., additional, McKean, J. P., additional, Meulman, H., additional, Mevius, M., additional, Munk, H., additional, Noordam, J. E., additional, Orrú, E., additional, Paas, H., additional, Pandey-Pommier, M., additional, Pandey, V. N., additional, Pizzo, R., additional, Polatidis, A., additional, Reich, W., additional, Röttgering, H., additional, Sluman, J., additional, Steinmetz, M., additional, Sterks, C. G. M., additional, Tagger, M., additional, Tang, Y., additional, Tasse, C., additional, Vermeulen, R., additional, van Weeren, R. J., additional, Wijnholds, S. J., additional, and Yatawatta, S., additional

Published
2012
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41. First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256

Author

van Weeren, R. J., primary, Röttgering, H. J. A., additional, Rafferty, D. A., additional, Pizzo, R., additional, Bonafede, A., additional, Brüggen, M., additional, Brunetti, G., additional, Ferrari, C., additional, Orrù, E., additional, Heald, G., additional, McKean, J. P., additional, Tasse, C., additional, de Gasperin, F., additional, Bîrzan, L., additional, van Zwieten, J. E., additional, van der Tol, S., additional, Shulevski, A., additional, Jackson, N., additional, Offringa, A. R., additional, Conway, J., additional, Intema, H. T., additional, Clarke, T. E., additional, van Bemmel, I., additional, Miley, G. K., additional, White, G. J., additional, Hoeft, M., additional, Cassano, R., additional, Macario, G., additional, Morganti, R., additional, Wise, M. W., additional, Horellou, C., additional, Valentijn, E. A., additional, Wucknitz, O., additional, Kuijken, K., additional, Enßlin, T. A., additional, Anderson, J., additional, Asgekar, A., additional, Avruch, I. M., additional, Beck, R., additional, Bell, M. E., additional, Bell, M. R., additional, Bentum, M. J., additional, Bernardi, G., additional, Best, P., additional, Boonstra, A.-J., additional, Brentjens, M., additional, van de Brink, R. H., additional, Broderick, J., additional, Brouw, W. N., additional, Butcher, H. R., additional, van Cappellen, W., additional, Ciardi, B., additional, Eislöffel, J., additional, Falcke, H., additional, Fender, R., additional, Garrett, M. A., additional, Gerbers, M., additional, Gunst, A., additional, van Haarlem, M. P., additional, Hamaker, J. P., additional, Hassall, T., additional, Hessels, J. W. T., additional, Koopmans, L. V. E., additional, Kuper, G., additional, van Leeuwen, J., additional, Maat, P., additional, Millenaar, R., additional, Munk, H., additional, Nijboer, R., additional, Noordam, J. E., additional, Pandey, V. N., additional, Pandey-Pommier, M., additional, Polatidis, A., additional, Reich, W., additional, Scaife, A. M. M., additional, Schoenmakers, A., additional, Sluman, J., additional, Stappers, B. W., additional, Steinmetz, M., additional, Swinbank, J., additional, Tagger, M., additional, Tang, Y., additional, Vermeulen, R., additional, and de Vos, M., additional

Published
2012
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42. Observing pulsars and fast transients with LOFAR

Author

Stappers, B. W., primary, Hessels, J. W. T., additional, Alexov, A., additional, Anderson, K., additional, Coenen, T., additional, Hassall, T., additional, Karastergiou, A., additional, Kondratiev, V. I., additional, Kramer, M., additional, van Leeuwen, J., additional, Mol, J. D., additional, Noutsos, A., additional, Romein, J. W., additional, Weltevrede, P., additional, Fender, R., additional, Wijers, R. A. M. J., additional, Bähren, L., additional, Bell, M. E., additional, Broderick, J., additional, Daw, E. J., additional, Dhillon, V. S., additional, Eislöffel, J., additional, Falcke, H., additional, Griessmeier, J., additional, Law, C., additional, Markoff, S., additional, Miller-Jones, J. C. A., additional, Scheers, B., additional, Spreeuw, H., additional, Swinbank, J., additional, ter Veen, S., additional, Wise, M. W., additional, Wucknitz, O., additional, Zarka, P., additional, Anderson, J., additional, Asgekar, A., additional, Avruch, I. M., additional, Beck, R., additional, Bennema, P., additional, Bentum, M. J., additional, Best, P., additional, Bregman, J., additional, Brentjens, M., additional, van de Brink, R. H., additional, Broekema, P. C., additional, Brouw, W. N., additional, Brüggen, M., additional, de Bruyn, A. G., additional, Butcher, H. R., additional, Ciardi, B., additional, Conway, J., additional, Dettmar, R.-J., additional, van Duin, A., additional, van Enst, J., additional, Garrett, M., additional, Gerbers, M., additional, Grit, T., additional, Gunst, A., additional, van Haarlem, M. P., additional, Hamaker, J. P., additional, Heald, G., additional, Hoeft, M., additional, Holties, H., additional, Horneffer, A., additional, Koopmans, L. V. E., additional, Kuper, G., additional, Loose, M., additional, Maat, P., additional, McKay-Bukowski, D., additional, McKean, J. P., additional, Miley, G., additional, Morganti, R., additional, Nijboer, R., additional, Noordam, J. E., additional, Norden, M., additional, Olofsson, H., additional, Pandey-Pommier, M., additional, Polatidis, A., additional, Reich, W., additional, Röttgering, H., additional, Schoenmakers, A., additional, Sluman, J., additional, Smirnov, O., additional, Steinmetz, M., additional, Sterks, C. G. M., additional, Tagger, M., additional, Tang, Y., additional, Vermeulen, R., additional, Vermaas, N., additional, Vogt, C., additional, de Vos, M., additional, Wijnholds, S. J., additional, Yatawatta, S., additional, and Zensus, A., additional

Published
2011
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46. A large light-mass component of cosmic rays at 1017–1017.5electronvolts from radio observations

Author

Buitink, S., Corstanje, A., Falcke, H., Hörandel, J. R., Huege, T., Nelles, A., Rachen, J. P., Rossetto, L., Schellart, P., Scholten, O., ter Veen, S., Thoudam, S., Trinh, T. N. G., Anderson, J., Asgekar, A., Avruch, I. M., Bell, M. E., Bentum, M. J., Bernardi, G., Best, P., Bonafede, A., Breitling, F., Broderick, J. W., Brouw, W. N., Brüggen, M., Butcher, H. R., Carbone, D., Ciardi, B., Conway, J. E., de Gasperin, F., de Geus, E., Deller, A., Dettmar, R.-J., van Diepen, G., Duscha, S., Eislöffel, J., Engels, D., Enriquez, J. E., Fallows, R. A., Fender, R., Ferrari, C., Frieswijk, W., Garrett, M. A., Grießmeier, J. M., Gunst, A. W., van Haarlem, M. P., Hassall, T. E., Heald, G., Hessels, J. W. T., Hoeft, M., Horneffer, A., Iacobelli, M., Intema, H., Juette, E., Karastergiou, A., Kondratiev, V. I., Kramer, M., Kuniyoshi, M., Kuper, G., van Leeuwen, J., Loose, G. M., Maat, P., Mann, G., Markoff, S., McFadden, R., McKay-Bukowski, D., McKean, J. P., Mevius, M., Mulcahy, D. D., Munk, H., Norden, M. J., Orru, E., Paas, H., Pandey-Pommier, M., Pandey, V. N., Pietka, M., Pizzo, R., Polatidis, A. G., Reich, W., Röttgering, H. J. A., Scaife, A. M. M., Schwarz, D. J., Serylak, M., Sluman, J., Smirnov, O., Stappers, B. W., Steinmetz, M., Stewart, A., Swinbank, J., Tagger, M., Tang, Y., Tasse, C., Toribio, M. C., Vermeulen, R., Vocks, C., Vogt, C., van Weeren, R. J., Wijers, R. A. M. J., Wijnholds, S. J., Wise, M. W., Wucknitz, O., Yatawatta, S., Zarka, P., and Zensus, J. A.

Abstract

High-resolution radio measurements of air showers—cascades of secondary particles in the atmosphere initiated by cosmic rays—reveal that cosmic rays with energies of 1017–1017.5electronvolts have a mixed composition, with light elements (protons and helium nuclei) making up 80 per cent of their mass.

Published
2016
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50. Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies

Author

de Gasperin, F., Vink, J., McKean, J. P., Asgekar, A., Bentum, M. J., Blaauw, R., Bonafede, A., Bruggen, M., Breitling, F., Brouw, W. N., Butcher, H. R., Ciardi, B., Cuciti, V., de Vos, M., Duscha, S., Eisloffel, J., Engels, D., Fallows, R. A., Franzen, T. M. O., Garrett, M. A., Gunst, A. W., Horandel, J., Heald, G., Koopmans, L. V. E., Krankowski, A., Maat, P., Mann, G., Mevius, M., Miley, G., Nelles, A., Norden, M. J., Offringa, A. R., Orrú, E., Paas, H., Pandey-Pommier, M., Pizzo, R., Reich, W., Rowlinson, A., Schwarz, D. J., Shulevski, A., Smirnov, O., Soida, M., Tagger, M., Toribio, M. C., van Ardenne, A., van der Horst, A. J., van Haarlem, M. P., van Weeren, R. J., Vocks, C., Wucknitz, O., Zarka, P., and Zucca, P.

Subjects
13. Climate action, techniques: interferometric, radio continuum: general
Abstract

Astronomy and astrophysics 635, A150 (2020). doi:10.1051/0004-6361/201936844, Context. The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (< 100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation.Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15″ with a high dynamic range and good image fidelity.Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source.Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further., Published by EDP Sciences, Les Ulis

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