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Context. Repeating fast radio bursts (FRBs) present excellent opportunities to identify FRB progenitors and host environments, as well as decipher the underlying emission mechanism. Detailed studies of repeating FRBs might also hold clues to the origin of FRBs as a population. Aims. We aim to detect bursts from the first two repeating FRBs: FRB 121102 (R1) and FRB 180814.J0422+73 (R2), and characterise their repeat statistics. We also want to significantly improve the sky localisation of R2 and identify its host galaxy. Methods. We use the Westerbork Synthesis Radio Telescope to conduct extensive follow-up of these two repeating FRBs. The new phased-array feed system, Apertif, allows covering the entire sky position uncertainty of R2 with fine spatial resolution in a single pointing. The data were searched for bursts around the known dispersion measures of the two sources. We characterise the energy distribution and the clustering of detected R1 bursts. Results. We detected 30 bursts from R1. The non-Poissonian nature is clearly evident from the burst arrival times, consistent with earlier claims. Our measurements indicate a dispersion measure of 563.5(2) pc cm −3 , suggesting a significant increase in DM over the past few years. Assuming a constant position angle across the burst, we place an upper limit of 8% on the linear polarisation fraction for the brightest burst in our sample. We did not detect any bursts from R2. Conclusions. A single power-law might not fit the R1 burst energy distribution across the full energy range or widely separated detections. Our observations provide improved constraints on the clustering of R1 bursts. Our stringent upper limits on the linear polarisation fraction imply a significant depolarisation, either intrinsic to the emission mechanism or caused by the intervening medium, at 1400 MHz that is not observed at higher frequencies. The non-detection of any bursts from R2, despite nearly 300 hrs of observations, implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of both assuming the source is still active. Another possibility is that R2 has turned off completely, either permanently or for an extended period of time.

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XTE J1810−197 (PSR J1809−1943 was the first ever magnetar which was found to emit transient radio emission. It has recently undergone another radio and high-energy outburst. This is only the second radio outburst that has been observed from this source. We observed J1810−197 soon after its recent radio outburst at low radio frequencies using the Giant Metrewave Radio Telescope. We present the 650 MHz flux density evolution of the source in the early phases of the outburst, and its radio spectrum down to frequencies as low as 300 MHz. The magnetar also exhibits radio emission in the form of strong, narrow bursts. We show that the bursts have a characteristic intrinsic width of the order of 0.5−0.7 ms, and discuss their properties in the context of giant pulses and giant micropulses from other pulsars. We also show that the bursts exhibit spectral structures which cannot be explained by interstellar propagation effects. These structures might indicate a phenomenological link with the repeating fast radio bursts which also show interesting, more detailed frequency structures. While the spectral structures are particularly noticeable in the early phases of the outburst, these seem to be less prominent as well as less frequent in the later phases, suggesting an evolution of the underlying cause of these spectral structures.

We report the detection of a bright fast radio burst, FRB 191108, with Apertif on the Westerbork Synthesis Radio Telescope (WSRT). The interferometer allows us to localise the FRB to a narrow 5″ × 7′ ellipse by employing both multibeam information within the Apertif phased-array feed (PAF) beam pattern, and across different tied-array beams. The resulting sight line passes close to Local Group galaxy M33, with an impact parameter of only 18 kpc with respect to the core. It also traverses the much larger circumgalactic medium of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute ∼10% of its dispersion measure of 588 pc cm−3. FRB 191108 has a Faraday rotation measure of +474 ± 3 rad m−2, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetised plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with Δν ≈ 40 MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data.

The discovery and study of pulsars and fast radio bursts (FRBs) in time-domain radio data is often hampered by radio frequency interference (RFI). Some of this terrestrial RFI is impulsive and bright, and relatively easy to identify and remove. Other anthropogenic signals, however, are weaker yet periodic, and their persistence can drown out astrophysical signals. Here we show that Fourierdomain excision of periodic RFI is an effective and powerful step in detecting weak cosmic signals. We find that applying the method significantly increases the signal-to-noise ratio of transient and periodic pulsar signals. In live studies we detected single pulses from pulsars and FRBs that would otherwise have remained buried in background noise. We show the method has no negative effects on pulsar pulse shape, and that it enhances timing campaigns. We demonstrate the method on real-life data from a number of large radio telescopes, and conclude that Fourier-domain RFI excision increases the effective sensitivity to astrophysical sources by a significant fraction, which can be even larger than an order of magnitude in the case of strong RFI. An accelerated implementation of the method runs on standard time-domain radio data formats and is publicly available.

We present the results obtained from imaging observations, and search for radio pulsations towards the magnetar SGR J1935+2154 made using the Giant Metrewave Radio Telescope, and the Ooty Radio Telescope. We present the high resolution radio image of the supernova remnant (SNR) G57.2+0.8, which is positionally associated with SGR J1935+2154. We did not detect significant periodic radio pulsations from the magnetar, with 8σ upper limits on its flux density of 0.4, and 0.2 mJy at 326.5, and 610 MHz, respectively, for an assumed duty cycle of 10%. The corresponding 6σ upper limits at the two frequencies for any burst emission with an assumed width of 10 ms are 0.5 Jy, and 63 mJy, respectively. No continuum radio point source was detected at the position of SGR J1935+2154 with a 3σ upper limit of 1.2 mJy. We also did not detect significant diffuse radio emission in a radius of 70 arc seconds in coincidence with the diffuse X-ray emission reported recently, with a 3σ upper limit of 4.5 mJy.Using the archival HI spectra, we estimate the distance of SNR G57.2+0.8 to be 11.7 ± 2.8 kpc. Based on measured HI column density (N H ) along this line of sight, we argue that the magnetar could be physically associated with SNR G57.2+0.8. Based on present data, we can not rule out either a pulsar wind nebula or a dust scattering halo origin for the diffuse X-ray emission seen around the magnetar. Subject headings: stars: neutron -stars: magnetars -magnetars: individual (SGR J1935+2154), ISM: supernova remnants -supernova remnants: individual (SNR G57.2+0.8)

We report the results of (a) extensive follow-up observations of the gamma-ray pulsar J1732−3131 that has been recently detected at decameter wavelengths, and (b) deep searches for counterparts of 9 other radio-quiet gamma-ray pulsars at 34 MHz, using the Gauribidanur radio telescope. No periodic signal from J1732−3131 could be detected above a detection threshold of 8σ, even with an effective integration time of more than 40 hours. However, the average profile obtained by combining data from several epochs, at a dispersion measure of 15.44 pc cm −3 , is found to be consistent with that from the earlier detection of this pulsar at a confidence level of 99.2%. We present this consistency between the two profiles as an evidence that J1732−3131 is a faint radio pulsar with an average flux density of 200-400 mJy at 34 MHz. Detection sensitivity of our deep searches, despite the extremely bright sky background at such low frequencies, is generally comparable to that of higher frequency searches for these pulsars, when scaled using reasonable assumptions about the underlying pulsar spectrum. We provide details of our deep searches, and put stringent upper limits on the decameter wavelength flux densities of several radio-quiet gamma-ray pulsars.