Gamma-Ray Bursts --- A Rapidly Developing Field
Huang Yongfeng, Nanjing University
Gamma-ray bursts (GRBs) are short bursts of gamma-rays from the deep sky, with a daily observed rate of one or two events. They were first serendipitously discovered in late 1960s. There are basically two kinds of GRBs, long ones (typically lasting for tens of seconds) and short ones (typically lasting for only a few hundred milliseconds). Amazingly, both long and short GRBs are one-off, i.e. no two GRBs have been observed to happen at the same position on the sky. In almost 30 years after discovery, the distances of GRBs were still completely unknown, due to the poor localization ability of gamma-ray detectors. The breakthrough came in early 1997, when the Italian-Holland BeppoSAX satellite led to the discovery of long-lasting X-ray, optical and radio afterglows for the first time. GRBs are found to happen at the far side of our Universe, typically releasing an isotropic equivalent radiation energy comparable to that emitted by the Milky Way in a few hundred years, and giving birth to fireballs that expand ultra-relativistically with the bulk Lorentz factor that can be larger than one thousand. Today, it has been widely believed that long GRBs should be produced during the collapse of massive stars (larger than 40 solar mass), while short GRBs should originate from the merge of two neutron stars (or a neutron star with a black hole). In both scenarios, the GRB indicates the birth of a new stellar mass black hole. However, many details are still quite unclear. Obviously, the study of GRBs are closely related to many fields, such as stars, galaxies, interstellar medium, cosmology, cosmic rays, special and general relativity, fluid dynamics, nuclear and particle physics, and even the fate of our human being and lives on the earth!