GCN Circular 478
T. Takeshima (USRA/GSFC), C. Markwardt (U.Md./GSFC), F. Marshall (GSFC), T.Giblin & R.M. Kippen (UAH/MSFC) report: RXTE has discovered an X-ray afterglow from the extremely bright gamma-ray burst detected with BATSE on 1999 Dec. 16.672 UT (Kippen et al., GCN Circ. 463, trigger number 7906). Initial results from the RXTE observations have been distributed in two GCN Notices entitled "GCN/RXTE_PCA BURST POSITION NOTICE". This circular summarizes the results of the RXTE observations. The first RXTE observation searched for an X-ray afterglow by scanning the region around the BATSE LOCBURST position with the PCA, which has 1-degree field-of-view. In two of the scans a bright X-ray source was detected, the first detection occurring at 1999 Dec. 16.840 UT or 4.02 hours after the burst trigger, and second occurring about 200 seconds later. During both detections the pointing direction of the PCA was changing most rapidly in R.A. With such a scan pattern the source Declination cannot be determined with as high a precision as the R.A. The best-fit position from this observation was distributed through GCN at 22:49 UT on Dec. 16. RXTE scanned across the same source again at Dec. 17.126 UT or 10.90 hours after the burst. During this observation the PCA scanned primarily in the Declination direction. The best-fit source position for both observations is R.A. = 77.38 +/- 0.04 and Decl. = 11.30 +/- 0.05 (J2000). This position is consistent with the optical transient detected by Uglesich et al (GCN 472). The best-fit source intensities for the both observations were 1.24 +/- 0.04 and 0.25 +/- 0.01, in units of 1e-10 erg/s/cm2 in the 2-10 keV band. Assuming a power law decay, the evolution of the intensity can be predicted (in the same units) by f = 0.065 x t**(-1.64) where t is the time in days after the burst trigger. Preliminary analysis of the source spectrum indicates a power-law shape with a photon index of about 2.1. This message may be cited.