TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  500
SUBJECT: Chandra observation of the X-ray afterglow of GRB991216
DATE:    99/12/19 19:30:29 GMT
FROM:    Luigi Piro at IAS/CNR Frascati  <piro@ias.rm.cnr.it>

L. Piro (IAS/CNR, Rome), G. Garmire (Penn State University), M. Garcia and
the CXC team (CfA, Cambridge), F. Marshall & T. Takeshima (GSFC) report on
behalf of a larger collaboration:


The Chandra X-ray Observatory imaged the field of GRB991216 for 10ks
starting on Dec 18.208 (UT), i.e. about 37 hrs after the GRB (Kippen et
al. GCN n. 463).  From a preliminary analysis of the data we found a
single unknown point source, that we name CXO J050931.4+111706, in the
center of the field. The position is RA = 05h09m31.35s, Dec = 11d 17'
05'.7, consistent with the position of optical counterpart (Uglesich etal,
GCN 472). The difference of 1".7 between the two positions is in fact
within the error present at this stage of the analysis. 

The flux observed in the ACIS-S plus High Energy Transmission Grating
zero-order image is equivalent to 2.5 x 10**-12 ergs/cm**2/sec over
2-10keV, assuming a power law spectrum with a slope of 2.1 and the
galactic Nh of 2.0x10**21 cm**-2.  Given the preliminary nature of the
reduction, this is consistent (within 20%) with the extrapolation of the
decay slope observed with XTE of -1.64 (Takeshima etal, GCN 478). We note
that the x-ray decay slope may be steeper than the optical slope (-1.23
+/- 0.05, Garnavich etal GCN 495; -1.17+/-0.10, Jensen etal GCN 498). A
more detailed analysis of the data is in progress.

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