Research Projects

Long Gamma-Ray Bursts (GRBs) originate in the collapse at the end stages of massive stars. Of the two classes of GRBs, they are better-studied thanks to the richness in the data of afterglows associated with them. Afterglows emerge as the jet from the explosion plows through the medium around the collapsed star launching a relativistic shock. Hence afterglows carry signatures of the jet, the surrounding medium, shock in amplification of the downstream magnetic field. In this project, we focus on radio afterglows of long GRBs. This project is a continuation of DST/EMR/2016/007127, on radio afterglows. This time, we go beyond individual GRB studies we conducted in the previous project. Instead, we will be looking at a large sample of GRB radio afterglows. There are claims in the literature that there is a bimodality in radio afterglows, and GRBs can be divided into radio bright and radio dark. Further, the current research suggests that the radio divide has its origin in the type of central engine or in the nature of the ambient medium around the collapsed star. These preliminary results from statistical studies of radio afterglows are promising. According to our current understanding, GRB central engine could either be a highly magnetized neutron star (magnetar), or a black-hole torus system. The former launches a jet dominated by Poynting flux while the latter launches one dominated by kinetic energy in baryonic matter. Massive stars are expected to be surrounded by a medium formed due to the high mass-loss rate they suffer during the end stages. However, afterglow observations do not commonly find the signature of pre-collapse wind. Understanding the radio bimodality may shed more light into central engines and progenitor evolution. On the other hand, whether the bimodality is due to something totally different? These are the questions we plan to explore in this project. Particularly, we would like to focus on a third element having the potential to bring diversity in afterglows - the magnetic field in the shock downstream. The extent of amplification required in GRB shocks is not well understood. There is a large variation in the downstream magnetic field from afterglow to afterglow. Some afterglows appear to be needing little or no amplification beyond shock compression, while the others indicate a good amount of amplification. In addition, relativistic shock acceleration simulations also reveal a more complex magnetic field structure than what is assumed in general in afterglow studies. In this project, we will study a large sample of afterglows, both radio bright and radio dark, and obtain bounds on the explosion energy, ambient medium density, and magnetic field. The posteriors we obtain reveal the distribution of these parameters from burst to burst, and also serve as a platform for further studies.

GRBs, radio continuum observations, Magnetic fields in shocks