Scattering features and variability of the Crab pulsar

We report on Westerbork Synthesis Radio Telescope observations of the Crab pulsar at 350 MHz from 2012 November 24 until 2015 June 21. During this period we consistently observe variations in the pulse profile of the Crab. Both variations in the scattering width of the pulse profile as well as delayed copies, also known as echoes, are seen regularly. These observations support the classification of two types of echoes: those that follow the truncated exponential shape expected for the thin-screen scattering approximation, and echoes that show a smoother, more Gaussian shape. During a sequence of high-cadence observations in 2015, we find that these non-exponential echoes evolve in time by approaching the main pulse and interpulse in phase, overlapping the main pulse and interpulse, and later receding. We find a pulse scatter-broadening time scale, $\tau$, scaling with frequency as $\nu^{\alpha}$, with $\alpha=-3.9\pm0.5$, which is consistent with expected values for thin-screen scattering modelsComment: 10 pages, 7 figure

Detection of radio emission from stars via proper-motion searches

We present a method for identifying radio stellar sources using their proper-motion. We demonstrate this method using the FIRST, VLASS, RACS-low and RACS-mid radio surveys, and astrometric information from Gaia Data Release 3. We find eight stellar radio sources using this method, two of which have not previously been identified in the literature as radio stars. We determine that this method probes distances of ~90pc when we use FIRST and RACS-mid, and ~250pc when we use FIRST and VLASS. We investigate the time baselines required by current and future radio sky surveys to detect the eight sources we found, with the SKA (6.7 GHz) requiring <3 years between observations to find all eight sources. We also identify nine previously known and 43 candidate variable radio stellar sources that are detected in FIRST (1.4 GHz) but are not detected in RACS-mid (1.37 GHz). This shows that many stellar radio sources are variable, and that surveys with multiple epochs can detect a more complete sample of stellar radio sources.Comment: 20 pages, 6 figures, 6 tables, accepted for publication in PAS

MeerTRAP: Twelve Galactic fast transients detected in a real-time, commensal MeerKAT survey

MeerTRAP is a real-time untargeted search project using the MeerKAT telescope to find single pulses from fast radio transients and pulsars. It is performed commensally with the MeerKAT large survey projects (LSPs), using data from up to 64 of MeerKAT's 13.96~m dishes to form hundreds of coherent beams on sky, each of which is processed in real time to search for millisecond-duration pulses. We present the first twelve Galactic sources discovered by MeerTRAP, with DMs in the range of 33--381~pc~cm$^{-3}$. One source may be Galactic or extragalactic depending on the Galactic electron density model assumed. Follow-up observations performed with the MeerKAT, Lovell, and Parkes radio telescopes have detected repeat pulses from seven of the twelve sources. Pulse periods have been determined for four sources. Another four sources could be localised to the arcsecond-level using a novel implementation of the tied-array beam localisation method.Comment: 16 pages, 14 figure

FRB 20210405I: a nearby Fast Radio Burst localised to sub-arcsecond precision with MeerKAT

We present the first sub-arcsecond localised Fast Radio Burst (FRB) detected using MeerKAT. FRB 20210405I was detected in the incoherent beam using the MeerTRAP pipeline on 2021 April 05 with a signal to noise ratio of 140.8 and a dispersion measure of 565.17 pc cm$^{-3}$. It was detected while MeerTRAP was observing commensally with the ThunderKAT large survey project, and was sufficiently bright that we could use the ThunderKAT 8s images to localise the FRB. Two different models of the dispersion measure in the Milky Way and halo suggest that the source is either right at the edge of the Galaxy, or outside. This highlights the uncertainty in the Milky Way dispersion measure models, particularly in the Galactic Plane, and the uncertainty of Milky Way halo models. Further investigation and modelling of these uncertainties will be facilitated by future detections and localisations of nearby FRBs. We use the combined localisation, dispersion measure, scattering, specific luminosity and chance coincidence probability information to find that the origin is most likely extra-galactic and identify the likely host galaxy of the FRB: 2MASS J1701249$-$4932475. Using SALT spectroscopy and archival observations of the field, we find that the host is a disk/spiral galaxy at a redshift of $z=0.066$.Comment: 15 pages, 4 tables, 10 figures. Accepted to MNRA

FRB 20210405I: the first Fast Radio Burst sub-arcsecond localised with MeerKAT

International audienceWe present the first sub-arcsecond localised Fast Radio Burst (FRB) detected using MeerKAT. The FRB, FRB 20210405I, was detected in the incoherent beam using the MeerTRAP pipeline on 2021 April 05 with a signal to noise ratio of 140.8 and a dispersion measure of 565.17 pc cm$^{-3}$. It was detected while MeerTRAP was observing commensally with the ThunderKAT large survey project, and was sufficiently bright that we could use the ThunderKAT 8s images to localise the FRB. Two different models of the dispersion measure in the Milky Way and halo suggest that the source is either right at the edge of the Galaxy, or outside. However, we use the combined localisation, dispersion measure, scattering, specific luminosity and chance coincidence probability information to find that the origin is most likely extragalactic and identify the likely host galaxy of the FRB: 2MASS J1701249$-$4932475. Using SALT spectroscopy and archival observations of the field, we find that the host is a disk/spiral galaxy at a redshift of $z=0.066$

Discovery of a radio emitting neutron star with an ultra-long spin period of 76 seconds

The radio-emitting neutron star population encompasses objects with spin periods ranging from milliseconds to tens of seconds. As they age and spin more slowly, their radio emission is expected to cease. We present the discovery of an ultra-long period radio-emitting neutron star, J0901-4046, with spin properties distinct from the known spin and magnetic-decay powered neutron stars. With a spin-period of 75.88 s, a characteristic age of 5.3 Myr, and a narrow pulse duty-cycle, it is uncertain how radio emission is generated and challenges our current understanding of how these systems evolve. The radio emission has unique spectro-temporal properties such as quasi-periodicity and partial nulling that provide important clues to the emission mechanism. Detecting similar sources is observationally challenging, which implies a larger undetected population. Our discovery establishes the existence of ultra-long period neutron stars, suggesting a possible connection to the evolution of highly magnetized neutron stars, ultra-long period magnetars, and fast radio burstsComment: Published in Nature Astronomy - https://www.nature.com/articles/s41550-022-01688-