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With the advent of multi-wavelength electromagnetic observations of neutron stars, spanning many decades in photon energies, from radio wavelengths up to X-rays and $\gamma$-rays, it becomes possible to significantly constrain the geometry and the location of the associated emission regions. In this work, we use results from the modelling of thermal X-ray observations of PSR~J0030+0451 from the NICER mission and phase-aligned radio and $\gamma$-ray pulse profiles to constrain the geometry of an off-centred dipole able to reproduce the light-curves in these respective bands simultaneously. To this aim, we deduce a configuration with a simple dipole off-centred from the location of the centre of the thermal X-ray hot spots and show that the geometry is compatible with independent constraints from radio and $\gamma$-ray pulsations only, leading to a fixed magnetic obliquity of $\alpha \approx 75\deg$ and a line of sight inclination angle of $\zeta \approx 54\deg$. We demonstrate that an off-centred dipole cannot be rejected by accounting for the thermal X-ray pulse profiles. Moreover, the crescent shape of one spot is interpreted as the consequence of a small scale surface dipole on top of the large scale off-centred dipole., Comment: Accepted for publication in A&A

The LOFAR Tied-Array All-Sky Survey (LOTAAS) is the most sensitive untargeted radio pulsar survey performed at low radio frequencies (119--151\,MHz) to date and has discovered 76 new radio pulsars, among which the 23.5-s pulsar J0250+5854, up until recently the slowest-spinning radio pulsar known. Here, we report on the timing solutions of 35 pulsars discovered by LOTAAS, which include a nulling pulsar and a mildly recycled pulsar, and thereby complete the full timing analysis of the LOTAAS pulsar discoveries. We give an overview of the findings from the full LOTAAS sample of 76 pulsars, discussing their pulse profiles, radio spectra and timing parameters. We found that the pulse profiles of some of the pulsars show profile variations in time or frequency and while some pulsars show signs of scattering, a large majority display no pulse broadening. The LOTAAS discoveries have on average steeper radio spectra and have longer spin periods ($1.4\times$) as well as lower spin-down rates ($3.1\times$) compared to the known pulsar population. We discuss the cause of these differences, and attribute them to a combination of selection effects of the LOTAAS survey as well as previous pulsar surveys, though can not rule out that older pulsars tend to have steeper radio spectra., Comment: Accepted to Astronomy & Astrophysics

The new generation of broad-band radio continuum surveys will provide large data sets with polarization information. New algorithms need to be developed to extract reliable catalogs of linearly polarized sources that can be used to characterize those sources and produce a dense rotation measure (RM) grid to probe magneto-ionized structures along the line of sight via Faraday rotation. The aim of the paper is to develop a computationally efficient and rigorously defined source-finding algorithm for linearly polarized sources. We used a calibrated data set from the LOw Frequency ARray (LOFAR) at 150 MHz centered on the nearby galaxy M51 to search for polarized background sources. With a new imaging software, we re-imaged the field at a resolution of 18''x15'' and cataloged a total of about 3000 continuum sources within 2.5 degrees of the center of M51. We made small Stokes Q and U images centered on each source brighter than 100 mJy in total intensity (201 sources) and used RM synthesis to create corresponding Faraday cubes that were analyzed individually. For each source, the noise distribution function was determined from a subset of measurements at high Faraday depths where no polarization is expected; the peaks in polarized intensity in the Faraday spectrum were identified and the p-value of each source was calculated. Finally, the false discovery rate method was applied to the list of p-values to produce a list of polarized sources and quantify the reliability of the detections. We also analyzed sources fainter than 100 mJy but that were reported as polarized in the literature at at least another radio frequency. Of the 201 sources that were searched for polarization, 6 polarized sources were detected confidently (with a false discovery rate of 5 percent). This corresponds to a number density of 1 polarized source per 3.3 square degrees, or 0.3 source/square degree. [Abridged], Comment: 17 pages, 11 figures, accepted for publication in A&A

We analyse new observations with the International Low Frequency Array (LOFAR) telescope, and archival data from the Multi-Element Radio Linked Interferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA). We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33 GHz. We present an image of Arp 220 at 150 MHz with resolution $0.65''\times0.35''$, sensitivity 0.15 mJy beam$^{-1}$, and integrated flux density $394\pm59$ mJy. More than 80% of the detected flux comes from extended ($6''\approx$2.2 kpc) steep spectrum ($\alpha=-0.7$) emission, likely from star formation in the molecular disk surrounding the two nuclei. We find elongated features extending $0.3''$ (110 pc) and $0.9''$ (330 pc) from the eastern and western nucleus respectively, which we interpret as evidence for outflows. The extent of radio emission requires acceleration of cosmic rays far outside the nuclei. We find that a simple three component model can explain most of the observed radio spectrum of the galaxy. When accounting for absorption at 1.4 GHz, Arp 220 follows the FIR/radio correlation with $q=2.36$, and we estimate a star formation rate of 220 M$_\odot\text{yr}^{-1}$. We derive thermal fractions at 1 GHz of less than 1% for the nuclei, which indicates that a major part of the UV-photons are absorbed by dust. International LOFAR observations shows great promise to detect steep spectrum outflows and probe regions of thermal absorption. However, in LIRGs the emission detected at 150 MHz does not necessarily come from the main regions of star formation. This implies that high spatial resolution is crucial for accurate estimates of star formation rates for such galaxies at 150 MHz., Comment: 17 pages, 9 figures

The angular momentum propagated by a beam of radiation has two contributions: spin angular momentum (SAM) and orbital angular momentum (OAM). SAM corresponds to wave polarisation, while OAM-carrying beams are characterized by a phase which is a function of azimuth. We demonstrate experimentally that radio beams propagating OAM can be generated and coherently detected using ordinary electric dipole antennas. The results presented here could pave the way for novel radio OAM applications in technology and science, including radio communication, passive remote sensing, and new types of active (continuous or pulsed transmission) electromagnetic measurements., Comment: 13 pages, 4 figures

I present an exact and explicit solution to the scalar (Stokes flux intensity) radio interferometer imaging equation on a spherical surface which is valid also for non-coplanar interferometer configurations. This imaging equation is comparable to $w$-term imaging algorithms, but by using a spherical rather than a Cartesian formulation this term has no special significance. The solution presented also allows direct identification of the scalar (spin 0 weighted) spherical harmonics on the sky. The method should be of interest for future multi-spacecraft interferometers, wide-field imaging with non-coplanar arrays, and CMB spherical harmonic measurements using interferometers., Comment: (Fixed references missing in previous arxiv version). This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

The nuclear starburst in the nearby galaxy M82 provides an excellent laboratory for understanding the physics of star formation. This galaxy has been extensively observed in the past, revealing tens of radio-bright compact objects embedded in a diffuse free-free absorbing medium. Our understanding of the structure and physics of this medium in M82 can be greatly improved by high-resolution images at low frequencies where the effects of free-free absorption are most prominent. The aims of this study are, firstly, to demonstrate imaging using international baselines of the Low Frequency Array (LOFAR), and secondly, to constrain low-frequency spectra of compact and diffuse emission in the central starburst region of M82 via high-resolution radio imaging at low frequencies. The international LOFAR telescope was used to observe M82 at 110-126MHz and 146-162MHz. Images were obtained using standard techniques from very long baseline interferometry. images were obtained at each frequency range: one only using international baselines, and one only using the longest Dutch (remote) baselines. The 154MHz image obtained using international baselines is a new imaging record in terms of combined image resolution (0.3$"$) and sensitivity ($\sigma$=0.15mJy/beam) at low frequencies ($<327$MHz). We detected 16 objects at 154MHz, six of these also at 118MHz. Four weaker but resolved features are also found: a linear (50pc) filament and three other resolved objects, of which two show a clear shell structure. We do not detect any emission from either supernova 2008iz or from the radio transient source 43.78+59.3. The images obtained using remote baselines show diffuse emission, associated with the outflow in M82, with reduced brightness in the region of the edge-on star-forming disk., Comment: 15 pages, 7 figures

This paper has been withdrawn to allow publication elsewhere., Comment: This paper has been withdrawn

We derive a generalised van Cittert-Zernike (vC-Z) theorem for radio astronomy that is valid for partially polarized sources over an arbitrarily wide field-of-view (FoV). The classical vC-Z theorem is the theoretical foundation of radio astronomical interferometry, and its application is the basis of interferometric imaging. Existing generalised vC-Z theorems in radio astronomy assume, however, either paraxiality (narrow FoV) or scalar (unpolarized) sources. Our theorem uses neither of these assumptions, which are seldom fulfilled in practice in radio astronomy, and treats the full electromagnetic field. To handle wide, partially polarized fields, we extend the two-dimensional electric field (Jones vector) formalism of the standard "Measurement Equation" of radio astronomical interferometry to the full three-dimensional formalism developed in optical coherence theory. The resulting vC-Z theorem enables all-sky imaging in a single telescope pointing, and imaging using not only standard dual-polarized interferometers (that measure 2-D electric fields), but also electric tripoles and electromagnetic vector-sensor interferometers. We show that the standard 2-D Measurement Equation is easily obtained from our formalism in the case of dual-polarized antenna element interferometers. We find, however, that such dual-polarized interferometers can have polarimetric aberrations at the edges of the FoV that are often correctable. Our theorem is particularly relevant to proposed and recently developed wide FoV interferometers such as LOFAR and SKA, for which direction-dependent effects will be important., Comment: To be published in MNRAS

Electromagnetic waves with an azimuthal phase shift are known to have a well defined orbital angular momentum. Different methods that allow for the detection of the angular momentum are proposed. For some, we discuss the required experimental setup and explore the range of applicability.

We show numerically that vector antenna arrays can generate radio beams which exhibit spin and orbital angular momentum characteristics similar to those of helical Laguerre-Gauss laser beams in paraxial optics. For low frequencies (< 1 GHz), digital techniques can be used to coherently measure the instantaneous, local field vectors and to manipulate them in software. This opens up for new types of experiments that go beyond those currently possible to perform in optics, for information-rich radio physics applications such as radio astronomy, and for novel wireless communication concepts., Comment: 4 pages, 5 figures. Changed title, identical to the paper published in PRL

Information energy is shown here to have properties similar to those of dark energy. The energy associated with each information bit of the universe is found to be defined identically to the characteristic energy of a cosmological constant. Two independent methods are used to estimate the universe information content of ~10^91 bits, a value that provides an information energy total comparable to that of the dark energy. Information energy is also found to have a significantly negative equation of state parameter, w < -0.4, and thus exerts a negative pressure, similar to dark energy., Comment: 5 pages, no figures, no tables

The ELVIS instrument was recently proposed by the authors for the Indian Chandrayaan-1 mission to the Moon and is presently under consideration by the Indian Space Research Organisation (ISRO). The scientific objective of ELVIS is to explore the electromagnetic environment of the moon. ELVIS samples the full three-dimensional (3D) electric field vector, E(x,t), up to 18 MHz, with selective Nyqvist frequency bandwidths down to 5 kHz, and one component of the magnetic field vector, B(x,t), from a few Hz up to 100 kHz.As a transient detector, ELVIS is capable of detecting pulses with a minimum pulse width of 5 ns. The instrument comprises three orthogonal electric dipole antennas, one magnetic search coil antenna and a four-channel digital sampling system, utilising flexible digital down conversion and filtering together with state-of-the-art onboard digital signal processing., Comment: 8 pages, 3 figures. Submitted to the DGLR Int. Symposium "To Moon and Beyond", Bremen, Germany, 2005. Companion paper to arXiv:astro-ph/0509210

Irreducible bilinear tensorial concomitants of an arbitrary complex antisymmetric valence-2 tensor are derived in four-dimensional spacetime. In addition these bilinear concomitants are symmetric (or antisymmetric), self-dual (or anti-self-dual), and hermitian forms in the antisymmetric tensor. An important example of an antisymmetric valence-2 tensor, or bivector, is the electromagnetic field strength tensor which ordinarily is taken to be real-valued. In generalizing to complex-valued bivectors, the authors find the hermitian form versions of the well-known electromagnetic scalar invariants and stress-energy-momentum tensor, but also discover several novel tensors of total valence 2 and 4. These tensors have algebraic similarities to the Riemann, Weyl, and Ricci tensors., Comment: 9 pages, 0 figures, submitted to J. Math. Phys

We derive the bias, variance, covariance, and mean square error of the standard lag windowed correlogram estimator both with and without sample mean removal for complex white noise with an arbitrary mean. We find that the arbitrary mean introduces lag dependent covariance between different lags of the correlogram estimates in spite of the lack of covariance in white noise for non-zeros lags. We provide a heuristic rule for when the sample mean should be, and when it should not be, removed if the true mean is not known. The sampling properties derived here are useful is assesing the general statistical performance of autocovariance and autocorrelation estimators in different parameter regimes. Alternatively, the sampling properties could be used as bounds on the detection of a weak signal in general white noise., Comment: 11 pages, 2 figures, To be published in Journal of Time Series Analysis

We derive the second-order sampling properties of certain autocovariance and autocorrelation estimators for sequences of independent and identically distributed samples. Specifically, the estimators we consider are the classic lag windowed correlogram, the correlogram with subtracted sample mean, and the fixed-length summation correlogram. For each correlogram we derive explicit formulas for the bias, covariance, mean square error and consistency for generalised higher-order white noise sequences. In particular, this class of sequences may have non-zero means, be complexed valued and also includes non-analytical noise signals. We find that these commonly used correlograms exhibit lag dependent covariance despite the fact that these processes are white and hence by definition do not depend on lag., Comment: Submitted to Biometrika