TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1504 SUBJECT: GRB020813: Fading X-ray Afterglow Observed with Chandra DATE: 02/08/19 02:30:09 GMT FROM: George Ricker at MIT GRB020813: Fading X-ray Afterglow Observed with Chandra R. Vanderspek, H. L. Marshall, P. G. Ford, and G. R. Ricker (MIT Center for Space Research) report: Beginning at August 13.990 (t[burst] + 21.02 hr) and continuing until August 14.892 (t[burst] + 42.67 hr), Chandra High Energy Transmission Grating Spectrometer (HETGS) observations were conducted of a field centered on the optical afterglow candidate (Fox et al, GCN1470) for GRB020813, which was accurately localized by both the HETE WXM and SXC instruments (Villasenor et al, GCN1471; Jernigan et al, GCN1494). We have detected a moderately bright, fading X-ray afterglow consistent in position with the optical source (Henden et al, GCN1478). Over the 76.8 ksec of data accumulation (livetime) for the Chandra observations, the mean counting rate was 0.19 counts/s (summed over the dispersed signal from the HEG and the MEG gratings, and including the 0th order flux). The source we detected faded in brightness according to a power law, with a decay time slope of -1.42 +/- 0.05. (This value is remarkably close to the decay slope values near -1.4 being reported for the afterglow of GRB020813 in the optical; e.g. Malesani et al, GCN1500, and references therein.) The source spectrum we derive from our preliminary analysis of the dispersed HETGS counts is characterized as follows: nH = 7 x 10^20 cm^(-2), consistent with the anticipated galactic absorption in the source direction; dN/dE = A * E^ (-gamma) ph cm^(-2) s^(-1) keV^(-1) , over the 0.6-6 keV range, with A = 0.00051, and gamma = 1.75 Thus, the mean flux for the 0.6 to 6 keV band over the duration of the Chandra observation was ~2.2 x 10^(-12) ergs cm^(-2) s^(-1). Our analyses are continuing, and more detailed results will be posted at: http://space.mit.edu/HETE/ We thank Harvey Tananbaum for his generous allocation of Director's Discretion Time to this observation, and the Chandra X-ray Observatory Operations personnel for the impressive promptness with which this observation was planned and carried out. The preliminary results reported here may be cited.