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The flat-spectrum radio quasar 3C 345 has been showing gamma-ray activity since the mid-2000s along with activity across the electromagnetic spectrum. A gamma-ray burst in 2009 was successfully linked to relativistic outflow in 43 GHz VLBI observations and has since been analyzed also using single dish measurements. A multi-wavelength follow-up VLBI observation to the 2009 flare in conjunction with 43 GHz catalogue data from the VLBA-BU-BLAZAR and BEAM-ME programs are analyzed in this study in the context of the long-term evolution of the source. We aim to probe the innermost few milli-arcseconds of the ultracompact 3C 345 jet. In the process, we analyze the long-term kinematics of the inner jet and discuss the magnetic field morphology at different scales, as well as the origin of the gamma-ray emission. New observations at 23, 43, and 86 GHz took place on ten epochs between 2017 and 2019. We calibrate the 30 data sets using the rPicard pipeline, image them in Difmap and carry out polarization calibration using the GPCAL pipeline. We complement our VLBI data with ancillay VLBI maps at multiple frequencies, as well as 43 GHz observations carried out in the framework of the BEAM-ME and VLBA-BU-BLAZAR monitoring programs. We find multiple distinct component paths in the inner jet, forming a helical geometry. The helix is anchored at a stationary feature some 0.16 mas from the 43 GHz VLBI core and has a timescale of about 8 years. The characteristic bends in the jet morphology are caused by variations in the component ejection angle. We confirm the result of previous studies that the gamma-ray emission is produced (or caused) by relativistic outflow and violent interactions within the jet.

The Large-aperture Experiment to detect the Dark Age (LEDA) was designed to measure the 21-cm signal from neutral hydrogen at Cosmic Dawn, $z \approx $15-30. Using observations made with the $\approx $ 200 m diameter core of the Owens Valley Long Wavelength Array (OVRO-LWA), we present a 2-D cylindrical spatial power spectrum for data at 43.1-53.5 MHz ($z_{\rm median}\approx 28$) incoherently integrated for 4 hours, and an analysis of the array sensitivity. Power from foregrounds is localized to a "wedge" within $k_\perp, k_\parallel$ space. After calibration of visibilities using 5 bright compact sources including VirA, we measure $\Delta^2(k) \approx 2 \times 10^{12}\ \mathrm{mK}^2$ outside the foreground wedge, where an uncontaminated cosmological signal would lie, in principle. The measured $\Delta^2(k)$ is an upper limit that reflects a combination of thermal instrumental and sky noise, and unmodelled systematics that scatter power from the wedge, as will be discussed. By differencing calibrated visibilities for close pairs of frequency channels, we suppress foreground sky structure and systematics, extract thermal noise, and use a mix of coherent and incoherent integration to simulate a noise-dominated power spectrum for a 3000 h observation and $z = $16-37. For suitable calibration quality, the resulting noise level, $\Delta^2(k) \approx 100$ mK$^2$ (k = 0.3 Mpc$^{-1}$), would be sufficient to detect peaks in the 21-cm spatial power spectrum due to early Ly-$\alpha$ and X-ray sources, as predicted for a range of theoretical model parameters., Comment: 16 pages, 11 figures. Accepted for MNRAS; replaced with accepted version

A successful ground array Radio Frequency (RF)-only self-trigger on 10 high-energy cosmic ray events is demonstrated with 256 dual-polarization antennas of the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). This RF-only capability is predicated on novel techniques for Radio Frequency Interference (RFI) identification and mitigation with an analysis efficiency of 45\% for shower-driven events with a Signal-to-noise ratio $\gtrsim$ 5 against the galactic background noise power of individual antennas. This technique enables more efficient detection of cosmic rays over a wider range of zenith angles than possible via triggers from in-situ particle detectors and can be easily adapted by neutrino experiments relying on RF-only detection. This paper discusses the system design, RFI characterization and mitigation techniques, and initial results from 10 cosmic ray events identified within a 40-hour observing window. A design for a future optimized commensal cosmic-ray detector for the OVRO-LWA is presented, as well as recommendations for developing a similar capability for other experiments -- these designs either reduce data-rate or increase sensitivity by an order of magnitude for many configurations of radio instruments.

The 21\,cm transition of neutral hydrogen is opening an observational window into the cosmic dawn of the universe---the epoch of first star formation. We use 28\,hr of data from the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) to place upper limits on the spatial power spectrum of 21\,cm emission at $z \approx 18.4$ ($\Delta_{21} \lesssim 10^4\,\text{mK}$), and within the absorption feature reported by the EDGES experiment (Bowman et al. 2018). In the process we demonstrate the first application of the double Karhunen-Lo\`{e}ve transform for foreground filtering, and diagnose the systematic errors that are currently limiting the measurement. We also provide an updated model for the angular power spectrum of low-frequency foreground emission measured from the northern hemisphere, which can be used to refine sensitivity forecasts for next-generation experiments., Comment: The authors really hoping you're having a fantastic day. You keep being awesome!

Astrometry, the measurement of positions and motions of the stars, is one of the oldest disciplines in Astronomy, extending back at least as far as Hipparchus' discovery of the precession of Earth's axes in 190 BCE by comparing his catalog with those of his predecessors. Astrometry is fundamental to Astronomy, and critical to many aspects of Astrophysics and Geodesy. In order to understand our planet's and solar system's context within their surroundings, we must be able to to define, quantify, study, refine, and maintain an inertial frame of reference relative to which all positions and motions can be unambiguously and self-consistently described. It is only by using this inertial reference frame that we are able to disentangle our observations of the motions of celestial objects from our own complex path around our star, and its path through the galaxy, and the local group. Every aspect of each area outlined in the call for scientific frontiers in astronomy in the era of the 2020-2030 timeframe will depend on the quality of the inertial reference frame. In this white paper, we propose support for development of radio Very Long Baseline Interferometry (VLBI) capabilities, including the Next Generation Very Large Array (ngVLA), a radio astronomy observatory that will not only support development of a next generation reference frame of unprecedented accuracy, but that will also serve as a highly capable astronomical instrument in its own right. Much like its predecessors, the Very Long Baseline Array (VLBA) and other VLBI telescopes, the proposed ngVLA will provide the foundation for the next three decades for the fundamental reference frame, benefitting astronomy, astrophysics, and geodesy alike., Comment: 8 pages, 3 figures, Astro2020 White Paper

Multimessenger observations of the binary neutron star merger GW170817 have enabled the discovery of a diverse array of electromagnetic counterparts to compact binary mergers, including an unambiguous kilonova, a short gamma-ray burst, and a late-time radio jet. Beyond these counterparts, compact binary mergers are additionally predicted to be accompanied by prompt low-frequency radio emission. The successful observation of a prompt radio counterpart would be immensely valuable, but is made difficult by the short delay between the gravitational-wave and prompt electromagnetic signals as well as the poor localization of gravitational-wave sources. Here, we present the first search for prompt radio emission accompanying a gravitational-wave event, targeting the binary black hole merger GW170104 detected by the Advanced LIGO and Virgo gravitational-wave observatories during their second (O2) observing run. Using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), we search a $\sim900\,\mathrm{deg}^2$ region for transient radio emission within approximately one hour of GW170104, obtaining an upper limit of $2.5\times10^{41}\,\mathrm{erg}\,\mathrm{s}^{-1}$ on its equivalent isotropic luminosity between 27-84 MHz. We additionally discuss plans to target binary neutron star mergers in Advanced LIGO and Virgo's upcoming O3 observing run., Comment: 13 pages + appendices, 7 figures, submitted to ApJ

We have conducted the most sensitive low frequency (below 100 MHz) search to date for prompt, low-frequency radio emission associated with short-duration gamma-ray bursts (GRBs), using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). The OVRO-LWA's nearly full-hemisphere field-of-view ($\sim20$,$000$ square degrees) allows us to search for low-frequency (sub-$100$ MHz) counterparts for a large sample of the subset of GRB events for which prompt radio emission has been predicted. Following the detection of short GRB 170112A by Swift, we used all-sky OVRO-LWA images spanning one hour prior to and two hours following the GRB event to search for a transient source coincident with the position of GRB 170112A. We detect no transient source, with our most constraining $1\sigma$ flux density limit of $650~\text{mJy}$ for frequencies spanning $27~\text{MHz}-84~\text{MHz}$. We place constraints on a number of models predicting prompt, low-frequency radio emission accompanying short GRBs and their potential binary neutron star merger progenitors, and place an upper limit of $L_\text{radio}/L_\gamma \lesssim 7\times10^{-16}$ on the fraction of energy released in the prompt radio emission. These observations serve as a pilot effort for a program targeting a wider sample of both short and long GRBs with the OVRO-LWA, including bursts with confirmed redshift measurements which are critical to placing the most constraining limits on prompt radio emission models, as well as a program for the follow-up of gravitational wave compact binary coalescence events detected by advanced LIGO and Virgo., Comment: 14 pages, 5 figures, ApJ submitted

A host of new low-frequency radio telescopes seek to measure the 21-cm transition of neutral hydrogen from the early universe. These telescopes have the potential to directly probe star and galaxy formation at redshifts $20 \gtrsim z \gtrsim 7$, but are limited by the dynamic range they can achieve against foreground sources of low-frequency radio emission. Consequently, there is a growing demand for modern, high-fidelity maps of the sky at frequencies below 200 MHz for use in foreground modeling and removal. We describe a new widefield imaging technique for drift-scanning interferometers, Tikhonov-regularized $m$-mode analysis imaging. This technique constructs images of the entire sky in a single synthesis imaging step with exact treatment of widefield effects. We describe how the CLEAN algorithm can be adapted to deconvolve maps generated by $m$-mode analysis imaging. We demonstrate Tikhonov-regularized $m$-mode analysis imaging using the Owens Valley Long Wavelength Array (OVRO-LWA) by generating 8 new maps of the sky north of $\delta=-30^\circ$ with 15 arcmin angular resolution, at frequencies evenly spaced between 36.528 MHz and 73.152 MHz, and $\sim$800 mJy/beam thermal noise. These maps are a 10-fold improvement in angular resolution over existing full-sky maps at comparable frequencies, which have angular resolutions $\ge 2^\circ$. Each map is constructed exclusively from interferometric observations and does not represent the globally averaged sky brightness. Future improvements will incorporate total power radiometry, improved thermal noise, and improved angular resolution -- due to the planned expansion of the OVRO-LWA to 2.6 km baselines. These maps serve as a first step on the path to the use of more sophisticated foreground filters in 21-cm cosmology incorporating the measured angular and frequency structure of all foreground contaminants., Comment: 27 pages, 18 figures

Radio astronomy observatories with high throughput back end instruments require real-time data processing. While computing hardware continues to advance rapidly, development of real-time processing pipelines remains difficult and time-consuming, which can limit scientific productivity. Motivated by this, we have developed Bifrost: an open-source software framework for rapid pipeline development. Bifrost combines a high-level Python interface with highly efficient reconfigurable data transport and a library of computing blocks for CPU and GPU processing. The framework is generalizable, but initially it emphasizes the needs of high-throughput radio astronomy pipelines, such as the ability to process data buffers as if they were continuous streams, the capacity to partition processing into distinct data sequences (e.g., separate observations), and the ability to extract specific intervals from buffered data. Computing blocks in the library are designed for applications such as interferometry, pulsar dedispersion and timing, and transient search pipelines. We describe the design and implementation of the Bifrost framework and demonstrate its use as the backbone in the correlation and beamforming back end of the Long Wavelength Array station in the Sevilleta National Wildlife Refuge, NM., Comment: 25 pages, 13 figures, submitted to JAI. For the code, see https://github.com/ledatelescope/bifrost

We present a community study exploring the low frequency (5 - 800 MHz) options and opportunities for the ngVLA project and its infrastructure. We describe a Next Generation LOw Band Observatory (ngLOBO) that will provide access to the low frequency sky in a commensal fashion, operating independently from the ngVLA, but leveraging common infrastructure. This approach provides continuous coverage through an aperture array (called ngLOBO-Low) below 150 MHz and by accessing the primary focus of the ngVLA antennas (called ngLOBO-High) above 150 MHz. ngLOBO preconditions include a) non-interference and b) low relative cost (<5%) with respect to ngVLA. ngLOBO has three primary scientific missions: (1) Radio Large Synoptic Survey Telescope (Radio-LSST): one naturally wide beam, commensal with ngVLA, will conduct a continuous synoptic survey of large swaths of the sky for both slow and fast transients; (2) This same commensal beam will provide complementary low frequency images of all ngVLA targets and their environment {\it when such data enhances their value}. (3) Independent beams from the ngLOBO-Low aperture array will conduct research in astrophysics, Earth science and space weather applications, engaging new communities and attracting independent resources. If ngVLA operates down to 2 GHz or lower, ngLOBO data will enhance ngVLA calibration and dynamic scheduling. Finally, non-variable field sources outside the ngVLA field of view can be harvested for serendipitous science, e.g. population studies for thermal and non-thermal continuum sources. The ngVLA will be a superb, high frequency instrument; ngLOBO will allow it to participate in the worldwide renaissance in low frequency science as well., Comment: 44 pages, 16 figures, LWA memo No. 205

We present a survey of the radio sky accessible from the first station of the Long Wavelength Array (LWA1). Images are presented at nine frequencies between 35 and 80 MHz with spatial resolutions ranging from $4.7^\circ$ to $2.0^\circ$, respectively. The maps cover the sky north of a declination of $-40^\circ$ and represent the most modern systematic survey of the diffuse Galactic emission within this frequency range. We also combine our survey with other low frequency sky maps to create an updated model of the low frequency sky. Due to the low frequencies probed by our survey, the updated model better accounts for the effects of free-free absorption from Galactic ionized Hydrogen. A longer term motivation behind this survey is to understand the foreground emission that obscures the redshifted 21 cm transition of neutral hydrogen from the cosmic dark ages ($z>10$) and, at higher frequencies, the epoch of reionization ($z>6$)., Comment: MNRAS accepted: 15 pages, 16 figures

Blazars are highly variable active galactic nuclei which emit radiation at all wavelengths from radio to gamma-rays. Polarized radiation from blazars is one key piece of evidence for synchrotron radiation at low energies and it also varies dramatically. The polarization of blazars is of interest for understanding the origin, confinement, and propagation of jets. However, even though numerous measurements have been performed, the mechanisms behind jet creation, composition and variability are still debated. We performed simultaneous gamma-ray and optical photopolarimetry observations of 45 blazars between Jul. 2008 and Dec. 2014 to investigate the mechanisms of variability and search for a basic relation between the several subclasses of blazars. We identify a correlation between the maximum degree of optical linear polarization and the gamma-ray luminosity or the ratio of gamma-ray to optical fluxes. Since the maximum polarization degree depends on the condition of the magnetic field (chaotic or ordered), this result implies a systematic difference in the intrinsic alignment of magnetic fields in pc-scale relativistic jets between different types blazars (FSRQs vs. BL Lacs), and consequently between different types of radio galaxies (FR Is vs. FR IIs)., Comment: 17 pages, 30 figures, 6 tables. Accepted for publication in Astrophysical Journal

We report the detection of giant pulse emission from PSR B0950+08 in 24 hours of observations made at 39.4 MHz, with a bandwidth of 16 MHz, using the first station of the Long Wavelength Array, LWA1. We detected 119 giant pulses from PSR B0950+08 (at its dispersion measure), which we define as having SNRs at least 10 times larger than for the mean pulse in our data set. These 119 pulses are 0.035% of the total number of pulse periods in the 24 hours of observations. The rate of giant pulses is about 5.0 per hour. The cumulative distribution of pulse strength $S$ is a steep power law, $N(>S)\propto S^{-4.7}$, but much less steep than would be expected if we were observing the tail of a Gaussian distribution of normal pulses. We detected no other transient pulses in a dispersion measure range from 1 to 90 pc cm$^{-3}$, in the beam tracking PSR B0950+08. The giant pulses have a narrower temporal width than the mean pulse (17.8 ms, on average, vs. 30.5 ms). The pulse widths are consistent with a previously observed weak dependence on observing frequency, which may be indicative of a deviation from a Kolmogorov spectrum of electron density irregularities along the line of sight. The rate and strength of these giant pulses is less than has been observed at $\sim$100 MHz. Additionally, the mean (normal) pulse flux density we observed is less than at $\sim$100 MHz. These results suggest this pulsar is weaker and produces less frequent giant pulses at 39 MHz than at 100 MHz., Comment: 27 pages, 12 figures, typos corrected

New observations of Jupiter's decametric radio emissions have been made with the Long Wavelength Array Station 1 (LWA1) which is capable of making high quality observations as low as 11 MHz. Full Stokes parameters were determined for bandwidths of 16 MHz. Here we present the first LWA1 results for the study of six Io-related events at temporal resolutions as fine as 0.25 ms. LWA1 data show excellent spectral detail in Jovian DAM such as simultaneous left hand circular (LHC) and right hand circular (RHC) polarized Io-related arcs and source envelopes, modulation lane features, S-bursts structures, narrow band N-events, and interactions between S-bursts and N-events. The sensitivity of the LWA1 combined with the low radio frequency interference environment allow us to trace the start of the LHC Io-C source region to much earlier CMLIII than typically found in the literature. We find the Io-C starts as early as CMLIII = 230 degrees at frequencies near 11 MHz. This early start of the Io-C emission may be valuable for refining models of the emission mechanism. We also detect modulation lane structures that appear continuous across LHC and RHC emissions, suggesting that both polarizations may originate from the same hemisphere of Jupiter. We present a study of rare S-bursts detected during an Io-D event and show drift rates are consistent with those from other Io-related sources. Finally, S-N burst events are seen in high spectral and temporal resolution and our data strongly support the co-spatial origins of these events., Comment: 17 pages, 18 figures, two tables. Journal of Geophysical Research - Space Physics accepted

We present the results of an all-sky radio survey between 5 and 9 GHz of the fields surrounding all unassociated gamma-ray objects listed in the Fermi Large Area Telescope Second Source Catalog (2FGL). The goal of these observations is to find all new gamma-ray AGN associations with radio sources >10 mJy at 8 GHz. We observed with the Very Large Array and the Australia Telescope Compact Array the areas around unassociated sources, providing localizations of weak radio point sources found in 2FGL fields at arcmin scales. Then we followed-up a subset of those with the Very Long Baseline and the Long Baseline Arrays to confirm detections of radio emission on parsec-scales. We quantified association probabilities based on known statistics of source counts and assuming a uniform distribution of background sources. In total we found 865 radio sources at arcsec scales as candidates for association and detected 95 of 170 selected for follow-up observations at milliarcsecond resolution. Based on this we obtained firm associations for 76 previously unknown gamma-ray AGN. Comparison of these new AGN associations with the predictions from using the WISE color-color diagram shows that half of the associations are missed. We found that 129 out of 588 observed gamma-ray sources at arcmin scales not a single radio continuum source was detected above our sensitivity limit within the 3-sigma gamma-ray localization. These "empty" fields were found to be particularly concentrated at low Galactic latitudes. The nature of these Galactic gamma-ray emitters is not yet determined., Comment: accepted for publication by ApJS, 18 pages, 10 figures, 12 tables; full electronic versions of tables 2-8 are available as ancillary files

We report results of the first phase of observations with the Australia Telescope Compact Array (ATCA) at 5 and 9 GHz of the fields around 411 gamma-ray sources with declinations < +10 deg detected by Fermi but marked as unassociated in the 2FGL catalogue. We have detected 424 sources with flux densities in a range of 2 mJy to 6 Jy that lie within the 99 per cent localisation uncertainty of 283 gamma-ray sources. Of these, 146 objects were detected in both the 5 and 9 GHz bands. We found 84 sources in our sample with a spectral index flatter than -0.5. The majority of detected sources are weaker than 100 mJy and for this reason were not found in previous surveys. Approximately 1/3 of our sample, 128 objects, have the probability of being associated by more than 10 times than the probability of being a background source found in the vicinity of a gamma-ray object by chance. We present the catalogue of positions of these sources, estimates of their flux densities and spectral indices where available., Comment: Published in Monthly Notices of the Royal Astronomical Society. 9 pages, 7 figures, 3 ascii tables. The tables can be found in the tar-ball of this submission. Revised according to reviewers comments. Contents of tables slightly changed after version 1

The 16th magnitude quasar 3C 345 (redshift z=0.5928) shows structural and emission variability on parsec scales around a compact unresolved radio core. For the last three decades it has been closely monitored with very long baseline interferometry (VLBI), yielding a wealth of information about the physics of relativistic outflows and dynamics of the central regions in AGN. We present here preliminary results for the long-term jet evolution, based on the 15 GHz monitoring data collected by the MOJAVE survey and various other groups over the last ~14 years and combined with data from earlier VLBI observations of 3C 345 which started in 1979. We discuss the trajectories, kinematics, and flux density evolution of enhanced emission regions embedded in the jet and present evidence for geometrical (e.g. precession) and physical (e.g. relativistic shocks and plasma instability) factors determining the morphology and dynamics of relativistic flows on parsec scales., Comment: 6 pages, 4 figures, to appear in Proceedings of "Accretion and Ejection in AGN: A Global View", Lake Como, Italy, June 22-26, 2009, ASP Conference Series, Eds: L. Maraschi, G. Ghisellini, R. Della Ceca and F. Tavecchio

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