ABSTRACT A bright, X-rayrich gamma-ray burst (GRB) was detected by the French Gamma Telescope (FREGATE) and localized with the Wide Field X-ray Monitor (WXM) and Soft X-ray Camera (SXC) instruments on the High Energy Transient Explorer 2 satellite (HETE-2) at 11:18:34.03 UT (40714.03 SOD) on 2002 December 11. The WXM flight software localized the burst to a 14 radius; this was relayed to the astronomical community 22 s after the start of the burst. Ground analysis of WXM and SXC data provided refined localizations; the latter can be described as a circle with a radius of 2 centered at R.A. 08h09m00s, decl. 06°4420 (J2000.0). GRB 021211 consists of a single, FRED-like pulse with a duration t90 2.3 s at high energies (85400 keV), which increases to t90 8.5 s at low energies (210 keV). The peak photon number and photon energy fluxes in the 2400 keV band are (34.0 ± 1.8) photons cm-2 s-1 and (1.68 ± 0.11) × 10-6 ergs cm-2 s-1, respectively. The energy fluences in the 230 and 30400 keV energy bands are SX = (1.36 ± 0.05) × 10-6 ergs cm-2 and S = (2.17 ± 0.15) × 10-6 ergs cm-2, respectively. Thus, GRB 021211 is an X-rayrich GRB (SX/S = 0.63 > 0.32). The average spectrum of the burst is well fitted by a Band function (low-energy power-law index = -0.805; high-energy power-law index = -2.37; and energy of the peak of the spectrum in F, E = 46.8 keV). The nearreal-time optical follow-up of GRB 021211 made possible by HETE-2 led to the detection of an optical afterglow for what otherwise would quite likely have been classified as an "optically dark" GRB, since the optical transient faded rapidly (from R < 14 to R 19) within the first 20 minutes, and was fainter than R 23 within 24 hr after the burst. GRB 021211 demonstrates that some fraction of burst afterglows are optically dark because their optical afterglows at times greater than 1 hr after the burst are very faint, and previously have often escaped detection. Such bursts are "optically dim" rather than truly optically dark. GRB 021211 also shows that even such optically dim bursts can have very bright optical afterglows at times less than 20 minutes after the burst. Subject heading: gamma rays: bursts On-line material: color figures
HETE-2 localization and observation of the bright, X-Ray Rich Gamma-Ray Burst GRB 021211
2003
Abstract
ABSTRACT A bright, X-rayrich gamma-ray burst (GRB) was detected by the French Gamma Telescope (FREGATE) and localized with the Wide Field X-ray Monitor (WXM) and Soft X-ray Camera (SXC) instruments on the High Energy Transient Explorer 2 satellite (HETE-2) at 11:18:34.03 UT (40714.03 SOD) on 2002 December 11. The WXM flight software localized the burst to a 14 radius; this was relayed to the astronomical community 22 s after the start of the burst. Ground analysis of WXM and SXC data provided refined localizations; the latter can be described as a circle with a radius of 2 centered at R.A. 08h09m00s, decl. 06°4420 (J2000.0). GRB 021211 consists of a single, FRED-like pulse with a duration t90 2.3 s at high energies (85400 keV), which increases to t90 8.5 s at low energies (210 keV). The peak photon number and photon energy fluxes in the 2400 keV band are (34.0 ± 1.8) photons cm-2 s-1 and (1.68 ± 0.11) × 10-6 ergs cm-2 s-1, respectively. The energy fluences in the 230 and 30400 keV energy bands are SX = (1.36 ± 0.05) × 10-6 ergs cm-2 and S = (2.17 ± 0.15) × 10-6 ergs cm-2, respectively. Thus, GRB 021211 is an X-rayrich GRB (SX/S = 0.63 > 0.32). The average spectrum of the burst is well fitted by a Band function (low-energy power-law index = -0.805; high-energy power-law index = -2.37; and energy of the peak of the spectrum in F, E = 46.8 keV). The nearreal-time optical follow-up of GRB 021211 made possible by HETE-2 led to the detection of an optical afterglow for what otherwise would quite likely have been classified as an "optically dark" GRB, since the optical transient faded rapidly (from R < 14 to R 19) within the first 20 minutes, and was fainter than R 23 within 24 hr after the burst. GRB 021211 demonstrates that some fraction of burst afterglows are optically dark because their optical afterglows at times greater than 1 hr after the burst are very faint, and previously have often escaped detection. Such bursts are "optically dim" rather than truly optically dark. GRB 021211 also shows that even such optically dim bursts can have very bright optical afterglows at times less than 20 minutes after the burst. Subject heading: gamma rays: bursts On-line material: color figuresI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
