Gamma-ray bursts: The mystery being resolved & new prospects

Discovered serendipitously during the cold-war era, gamma-ray bursts (GRBs) are the most powerful cosmological explosions in the Universe. A given burst typically lasts tens of seconds but releases as much energy as our Sun would over its entire lifespan of ten billion years. Once the distance scale to GRBs was established, which only happened twenty-four years after their discovery, thanks to the long-lasting "afterglow" emission in X-rays/optical/radio that follows the much shorter "prompt" GRB, it was realized that the large amounts of energy release meant a cataclysmic event, namely the core-collapse of a massive star or the merger of two neutron stars. The latter option was also a prime candidate for gravitational wave emission. Over the many years, our understanding of the GRB phenomenon has evolved and a basic picture has emerged. GRBs are powered by ultra-relativistic jets launched from an accreting black hole (or perhaps a fast-spinning strongly magnetized neutron star). However, many important details remain unclear, most importantly, what's the emission mechanism that produces the GRB. This talk will present a broad overview of the observed properties of GRBs and try to connect them to our physical understanding of these bursts. Possible emission mechanisms for the prompt GRB will be discussed and results from numerical simulations will be shown. Some time will be devoted to the peculiar afterglow seen from the first-ever multi-messenger (gravitational waves & electromagnetic signals) detection of a binary neutron star merger and important lessons learned from this event will be emphasized. Finally, new prospects of using linear polarization to pin down the prompt GRB emission mechanism and learning about the magnetic field structure in GRB jets will be presented.