Dr Kaustubh Rajwade

Abstract:

We report detections of fast radio bursts (FRBs) from the repeating source FRB 20201124A with Apertif/WSRT and GMRT, and measurements of basic burst properties, especially the dispersion measure (DM) and fluence. Based on comparisons of these properties with previously published larger samples, we argue that the excess DM reported earlier for pulses with integrated signal-to-noise ratios ≲1000 is due to incompletely accounting for what is known as the sad trombone effect, even when using structure-maximizing DM algorithms. Our investigations of fluence distributions next lead us to advise against formal power-law fitting; we especially caution against the use of the least-squares method, and we demonstrate the large biases involved. A maximum likelihood estimator (MLE) provides a much more accurate estimate of the power law, and we provide accessible code for direct inclusion in future research. Our GMRT observations were fortuitously scheduled around the end of the Spring 2021 activity window as recorded by FAST. We detected several bursts (one of them very strong) at 400/600 MHz, a few hours after sensitive FAST non-detections already showed the 1.3 GHz FRB emission to have ceased. After FRB 20180916B, this is a second example of a frequency-dependent activity window identified in a repeating FRB source. Since numerous efforts have so far failed to determine a spin period for FRB 20201124A, we conjecture that it is an ultra-long-period magnetar, with a period on the scale of months, and with a very wide, highly irregular duty cycle. Assuming the emission comes from closed field lines, we used radius-to-frequency mapping and polarization information from other studies to constrain the magnetospheric geometry and location of the emission region. Our initial findings are consistent with a possible connection between FRBs and crustal motion events.

Abstract:

Recent observations have revealed rare, previously unknown flashes of cosmic radio waves lasting from milliseconds to minutes, with a periodicity of minutes to an hour. These transient radio signals must originate from sources in the Milky Way and from coherent emission processes in astrophysical plasma. They are theorized to be produced in the extreme and highly magnetized environments around white dwarfs or neutron stars. However, the astrophysical origin of these signals remains contested, and multiple progenitor models may be needed to explain their diverse properties. Here we present the discovery of a transient radio source, ILT J1101 + 5521, whose roughly minute-long pulses arrive with a periodicity of 125.5 min. We find that ILT J1101 + 5521 is an M dwarf–white dwarf binary system with an orbital period that matches the period of the radio pulses, which are observed when the two stars are in conjunction. The binary nature of ILT J1101 + 5521 establishes that some long-period radio transients originate from orbital motion modulating the observed emission, as opposed to an isolated rotating star. We conclude that ILT J1101 + 5521 is probably a polar system where magnetic interaction has synchronized the rotational and orbital periods of the white dwarf. Magnetic interaction and plasma exchange between two stars has been theorized to generate sporadic radio emission, making ILT J1101 + 5521 a potential low-mass analogue to such mechanisms.