204 research outputs found
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
GHz, which is much smaller than the frequency corresponding to
the maximum stellar magnetic field strength.Comment: Accepted for publication in Ap
- …