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
Not found
 
 
, 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
Not found
 
 
).

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.

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