GCN Circular 1624

M. Sako, F. A. Harrison report on behalf of the Caltech-NRAO collaboration:

We report TOO observations of GRB021004 taken with the High-Energy
Transmission Grating (HETG) on the Chandra X-ray observatory.  Chandra
observed the afterglow for a total exposure of 87 ksec, beginning at
8:55 UT on October 5, 20.5 hours after the burst.  A preliminary
reduction of the data shows a fading X-ray source at the position of
the optical transient.  The count rate decreased with time throughout
the observation with a power law decay time slope of -1.0 +/- 0.2.
The total HEG + MEG countrate is 0.018 counts sec^-1.  The source is
well-characterized by a power-law spectrum with photon index Gamma =
2.1 +/- 0.1, and flux normalization of 1.9 +/- 0.1 x 10^-4 photons
cm^-2 s^-1 keV^-1 at 1 keV.  There are no signs of excess absorption
above the Galactic value.  The corresponding mean 2 - 10 keV X-ray
flux in the observed frame is 4.3 x 10^-13 erg cm^-2 s^-1.  The
0th-order ACIS-S spectrum is consistent with that of the HETG, with an
average count rate of 0.014 counts sec^-1.

No obvious discrete features are detected in the spectrum.  We note
that the signal-to-noise ratio in this spectrum is a factor of two
better than that of the GRB991216 spectrum obtained with the Chandra
HETG, so that a line feature of similar strength to that reported for
GRB991216 should have been detected.

We thank Harvey Tananbaum for approving director's discretionary time
for this observation, and the staff at the Chandra Science Center
for skillfully implementing this rapid TOO observation.