The Gamma-Ray Burst Alert System and the Results of HETE-2

HETE-2 (High Energy Transient Explorer Satellite 2) is a small Explorer-class satellite designed to detect gamma-ray bursts (GRBs), localize them in real time and distribute the burst coordinates to ground observers within minutes of burst detection. As of August 2003, HETE-2 had localized 47 GRBs, and 19 localizations had led to the detection of X-ray, optical or radio afterglows. The prompt position alert enables researchers to probe the nature of so-called "dark bursts" - GRBs for which no optical afterglow has been found despite accurate localizations. Bursts localized by HETE-2 can be observed more promptly, when the afterglow is intrinsically brighter; thus, afterglows can be detected for bursts which would have otherwise been considered as "dark". In one case of a "dark" burst observed by HETE-2, the optical afterglow was found to be intrinsically faint, and its flux declined rapidly. In another case, the optical emission was likely extinguished by the dust in the vicinity of the GRB source. X-ray rich GRBs or X-ray flashes (XRFs) are found to have many properties in common with classical GRBs, suggesting that they are a single phenomenon. However, optical identification of XRFs has been difficult, perhaps because the counterparts are intrinsically faint; prompt localizations from HETE-2 will help observers identify optical counterparts to XRFs. The bright afterglows of GRB 021004 and GRB 030329 were observed in unprecedented detail by telescopes around the world; GRB 030329 was subsequently identified with SN 2003dh, a Type Ic supernova. Consequently, strong evidence for the connection of long GRBs with core-collapsed supernovae was found.