GRB 010921: Localization and Observations by the [ITAL]High Energy Transient Explorer[/ITAL] Satellite

On 2001 September 21 at 05:15:50.56 UT, the French Gamma Telescope (FREGATE) on the High Energy Transient Explorer ( HETE ) detected a bright gamma-ray burst (GRB). The burst was also seen by the X -detector on the Wide-field X-ray Monitor (WXM) instrument and was therefore well localized in the X -direction; however, the burst was outside the fully coded field of view of the WXM Y -detector, and therefore information on the Y -direction of the burst was limited. Cross-correlation of the HETE and Ulysses time histories yielded an Interplanetary Network (IPN) annulus that crosses the HETE error strip at an ~45° angle. The intersection of the HETE error strip and the IPN annulus produces a diamond-shaped error region for the location of the burst having an area of 310 arcmin 2 . Based on the FREGATE and WXM light curves, the duration of the burst is characterized by t 90 = 34.2 s in the WXM 4-25 keV energy range, and 23.8 and 21.8 s in the FREGATE 6-40 and 32-400 keV energy ranges, respectively. The fluence of the burst in these same energy ranges is 4.8 × 10 -6 , 5.5 × 10 -6 , and 11.4 × 10 -6 ergs cm -2 , respectively. Subsequent optical and radio observations by ground-based observers have identified the afterglow of GRB 010921 and determined an apparent redshift of z = 0.450.