Gamma Ray Bursts -- A radio perspective

Gamma-ray bursts (GRBs) are extremely energetic events at cosmological distances. They provide unique laboratory to investigate fundamental physical processes under extreme conditions. Due to extreme luminosities, GRBs are detectable at very high redshifts and potential tracers of cosmic star formation rate at early epoch. While the launch of {\it Swift} and {\it Fermi} has increased our understanding of GRBs tremendously, many new questions have opened up. Radio observations of GRBs uniquely probe the energetics and environments of the explosion. However, currently only 30\% of the bursts are detected in radio bands. Radio observations with upcoming sensitive telescopes will potentially increase the sample size significantly, and allow one to follow the individual bursts for a much longer duration and be able to answer some of the important issues related to true calorimetry, reverse shock emission and environments around the massive stars exploding as GRBs in the early Universe.Comment: To appear in Advances in Astronomy, special issue "Gamma-Ray Burst in Swift/Fermi Era and Beyond

Peculiar spectral property of coherent radio emission from a hot magnetic star: the case of an extreme oblique rotator

We report ultra-wideband (0.4-4.0 GHz) observation of coherent radio emission via electron cyclotron maser emission (ECME) produced by the hot magnetic star HD 142990. With nearly perpendicular rotation and magnetic dipole axes, it represents an extreme case of oblique rotators. The large obliquity is predicted to cause complex distribution of stellar wind plasma in the magnetosphere (Townsend & Owocki 2005). It has been proposed that such a distribution will give rise to non-trivial frequency dependence of ECME (Das et al. 2020d). Indeed we discovered strong frequency dependence of different pulse-properties, such as appearance of secondary pulses, different cut-off frequencies for pulses observed at different rotational phases etc.. But the unique feature that we observed is that while at sub-GHz frequencies, the star appears to produce ECME in the extra-ordinary mode, at GHz frequencies, the mode indicated by the pulse-property is the ordinary mode. By considering the physical condition needed by such a scenario, we conclude that the required transition of the magneto-ionic mode with frequency is unlikely to occur, and the most promising scenario is refraction caused by the complex plasma distribution surrounding the star. This suggests that the conventional way to deduce the magneto-ionic mode based on ECME observed at a given frequency is not a reliable method for stars with large misalignment between their rotation and magnetic axes. We also find that ECME exhibits an upper cut-off at $\lesssim 3.3$ GHz, which is much smaller than the frequency corresponding to the maximum stellar magnetic field strength.Comment: Accepted for publication in Ap