[vsnet-grb-info 352] GRB020813: Fading X-ray Afterglow Observed with Chandra

[GCN Circulars <gcncirc@lheawww.gsfc.nasa.gov>]

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  <grr@space.mit.edu>

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.

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