Gamma ray bursts (GRBs) are intense flashes of gamma rays emanating from cosmic sources and lasting for a few seconds to minutes and were first detected serendipitously in 1967 by gamma ray detectors aboard the Vela satellites, and were announced by Klebesadel et al in 1973 the discovery of GRBs.

To understand the nature of GRBs the first and key step is to determine the distance of GRBs from the Earth. The two experiments, namely Burst and Transient Source Explorer (BATSE) on Compton Gamma Ray Observatory (CGRO) and BeppoSAX， play a leading role in answering this key question.

In 1991 the CGRO with BATSE instrument was launched. The BATSE experiment detected thousands of GRBs and showed that their distribution was nearly uniform in the sky. —not biased towards any particular direction in space, such as toward the galactic plane or the galactic center. Because of the flattened shape of the Milky Way Galaxy, sources within our own galaxy would be strongly concentrated in or near the Galactic plane. The absence of any such pattern in the case of GRBs provided strong evidence that gamma-ray bursts must come from beyond the Milky Way, i.e. are located at cosmological distances far beyond the Milky Way galaxy. The BATSE experiment also showed that there are two distinct classes of GRBs.

Several models for the origin of gamma-ray bursts postulated that the initial burst of gamma rays should be followed by slowly fading emission at longer wavelengths created by collisions between the burst ejecta and interstellar gas. Early searches for this “afterglow” were unsuccessful, largely due to the difficulties in observing a burst’s position at longer wavelengths immediately after the initial burst. The Dutch-Italian satellite BeppoSAX, launched in 1996, made the breakthrough. In 1997 an X-ray camera aboard BeppoSAX measured accurate positions of the X-ray afterglows of GRBs that enabled observers using ground-based optical telescopes to make the first identification of a GRB with a distant galaxy and the first identification of a GRB with a supernova explosion, confirming their cosmological origin. Within a few months, the controversy about the distance scale ended: GRBs are extragalactic events originating within faint galaxies at enormous distances.

The cosmological origin of the GRB shows that during the flash, a GRB outshines all the stars and galaxies in the universe. GRBs appear so bright because they emit narrow beams of relativistic particles, and those that are observed happen to have these beams directed toward Earth. There at least two distinct types of GRBs. The long-duration bursts are associated with rare types of supernova explosions and may be caused by the formation of a black hole at the center of a collapsing massive star. The short-duration bursts may be caused by the merger of two neutron stars.

Although the physical mechanisms responsible for the GRB phenomena remain uncertain, there is no doubt that the GRB sources manifest some of the most extreme physical environments in the cosmos. The ongoing study of GRBs is one of the most exciting fields of astrophysics today.

Gerald Fishman was Principal Investigator of the BATSE experiment on CGRO and Enrico Costa led the development of the Dutch-Italian satellite BeppoSAX. For their outstanding achievements, Enrico Costa and Gerald Fishman are awarded the 2011 Shaw Prize in Astronomy.

Astronomy Selection Committee
The Shaw Prize

28 September 2011, Hong Kong