Mansi Kasliwal — Page 5

arXiv2009

Discovery of Radio Afterglow from the Most Distant Cosmic Explosion

We report the discovery of radio afterglow emission from the gamma-ray burst GRB 090423, which exploded at a redshift of 8.3, making it the object with the highest known redshift in the Universe. By combining our radio measurements with existing X-ray and infrared observations, we estimate the kinetic energy of the afterglow, the geometry of the outflow and the density of the circumburst medium. Our best fit model is a quasi-spherical, high-energy explosion in a low, constant-density medium. \event had a similar energy release to the other well-studied high redshift GRB 050904 ($z=6.26$), but their circumburst densities differ by two orders of magnitude. We compare the properties of \event with a sample of GRBs at moderate redshifts. We find that the high energy and afterglow properties of \event are not sufficiently different from other GRBs to suggest a different kind of progenitor, such as a Population III star. However, we argue that it is not clear that the afterglow properties alone can provide convincing identification of Population III progenitors. We suggest that the millimeter and centimeter radio detections of \event at early times contained emission from a reverse shock component. This has important implications for the detection of high redshift GRBs by the next generation of radio facilities.

astro-ph.CO↗

arXiv2008

A comprehensive study of GRB 070125, a most energetic gamma ray burst

We present a comprehensive multiwavelength analysis of the bright, long duration gamma-ray burst GRB 070125, comprised of observations in $γ$-ray, X-ray, optical, millimeter and centimeter wavebands. Simultaneous fits to the optical and X-ray light curves favor a break on day 3.78, which we interpret as the jet break from a collimated outflow. Independent fits to optical and X-ray bands give similar results in the optical bands but shift the jet break to around day 10 in the X-ray light curve. We show that for the physical parameters derived for GRB 070125, inverse Compton scattering effects are important throughout the afterglow evolution. While inverse Compton scattering does not affect radio and optical bands, it may be a promising candidate to delay the jet break in the X-ray band. Radio light curves show rapid flux variations, which are interpreted as due to interstellar scintillation, and are used to derive an upper limit of $2.4 \times 10^{17}$ cm on the radius of the fireball in the lateral expansion phase of the jet. Radio light curves and spectra suggest a high synchrotron self absorption frequency indicative of the afterglow shock wave moving in a dense medium. Our broadband modeling favors a constant density profile for the circumburst medium over a wind-like profile ($R^{-2}$). However, keeping in mind the uncertainty of the parameters, it is difficult to unambiguously distinguish between the two density profiles. Our broadband fits suggest that \event is a burst with high radiative efficiency ($> 60 %$).

astro-ph↗