HETE-2 localization and observation of the bright, X-ray-rich gamma-ray burst GRB 021211

A bright, X-ray-rich 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. 08(h)09(m)00(s), decl. 06degrees44'20" (J2000.0). GRB 021211 consists of a single, FRED-like pulse with a duration t(90) approximate to 2.3 s at high energies (85-400 keV), which increases to t(90) approximate to 8.5 s at low energies (2-10 keV). The peak photon number and photon energy fluxes in the 2-400 keV band are (34.0 +/- 1.8) photons cm(-2) s(-1) and (1.68 +/- 0.11) x 10(-6) ergs cm(-2) s(-1), respectively. The energy fluences in the 2-30 and 30-400 keV energy bands are S-X = (1.36 +/- 0.05) x 10-6 ergs cm(-2) and S,, (2.17 +/- 0.15) x 10(-6) ergs cm(-2), respectively. Thus, GRB 021211 is an X-ray-rich GRB (S-X/S-gamma = 0.63 > 0.32). The average spectrum of the burst is well fitted by a Band function (low-energy power-law index alpha = -0.805(-0.105)(+0-112); high-energy power-law index beta = -2.37(-0.31)(+0.18);; and energy of the peak of the spectrum in vF(v), E-peak(obs) = 46.8(-5.1)(+5.8) keV). The near-real-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.