HETE-2 Localization and Observations of the Short, Hard Gamma-Ray Burst GRB020531


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The {it HETE-2} (hereafter HETE) French Gamma Telescope (FREGATE) and the Wide-field X-ray Monitor (WXM) instruments detected a short ($t_{50} = 360$ msec in the FREGATE 85-300 keV energy band), hard gamma-ray burst (GRB) that occurred at 1578.72 SOD (00:26:18.72 UT) on 31 May 2002. The WXM flight localization software produced a valid location in spacecraft (relative) coordinates. However, since no on-board real-time star camera aspect was available, an absolute localization could not be disseminated. A preliminary localization was reported as a GCN Position Notice at 01:54:22 UT, 88 min after the burst. Further ground analysis produced a refined localization, which can be expressed as a 90% confidence rectangle that is 67 arcminutes in RA and 43 arcminutes in Dec (90% confidence region), centered at RA = +15$^{rm h}$ 14$^{rm m}$ 45$^{rm s}$, Dec = -19$^circ$ 21arcmin 35arcsec (J2000). An IPN localization of the burst was disseminated 18 hours after the GRB (Hurley et al. 2002b). A refined IPN localization was disseminated $approx$ 5 days after the burst. This hexagonal-shaped localization error region is centered on RA = 15$^{rm h}$ 15$^{rm m}$ 03.57$^{rm s}$, -19$^circ$ 24arcmin 51.00arcsec (J2000), and has an area of $approx$ 22 square arcminutes (99.7% confidence region). The prompt localization of this short, hard GRB by HETE and the anti-Sun pointing of the HETE instruments, coupled with the refinement of the localization by the IPN, has made possible rapid follow-up observations of the burst at radio, optical, and X-ray wavelengths.

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