9 October 2003

Speaker: Don Lamb (Univ. of Chicago)

Title: Scientific highlights of the HETE-2 mission

Abstract:

The HETE-2 mission has been highly productive. It has observed more than 250 GRBs so far. It is currently localizing 25 - 30 GRBs per year, and has localized 43 GRBs to date. Twenty-one of these localizations have led to the detection of X-ray, optical, or radio afterglows, and as of now, 11 of the bursts with afterglows have redshift determinations. HETE-2 has also observed more than 45 bursts from soft gamma-ray repeaters, and more than 700 X-ray bursts.

HETE-2 has confirmed the connection between GRBs and Type Ic supernovae, a singular achievement and certainly one of the scientific highlights of the mission so far. It has established that the isotropic-equivalent energies and luminosities of GRBs are strongly correlated with redshift, implying that GRBs and their progenitors evolve strongly with redshift. Both of these results have profound implications for the nature of GRB progenitors and for the use of GRBs as a probe of cosmology and the early universe.

HETE-2 has placed severe constraints on any X-ray or optical afterglow of a short GRB. It has made it possible to explore the previously unknown behavior optical afterglows at very early times, and has opened up the era of high-resolution spectroscopy of GRB optical afterglows.

HETE-2 is also solving the mystery of ``optically dark'' GRBs, and revealing the nature of X-ray flashes (XRFs). It has shown that XRFs may provide unique insights into the structure of GRB jets, the rate of GRBs, and the nature of Type Ic supernovae. Acceptance of the profound implications of the HETE-2 results requires incontrovertible evidence that can only come from further studies of XRFs. HETE-2 is ideally suited to accurately and rapidly localize XRFs, and study their spectra, whereas Swift is not. This constitutes a compelling reason for continuing HETE-2 during the Swift mission.