GCN Circular 2808
Subject
GRB041006: Fading X-ray Afterglow Observed with Chandra
Date
2004-10-15T18:16:51Z (20 years ago)
From
Nat Butler at MIT/CSR <nrbutler@space.mit.edu>
GRB041006: Fading X-ray Afterglow Observed with Chandra
N. Butler, R. Vanderspek, H. L. Marshall, P. G. Ford, G. R. Ricker
(MIT), D. Q. Lamb (U.Chicago), and G. P. Garmire (PSU)
report:
Beginning at October 7.213 (t[burst] + 16.80 hr) and continuing until
October 8.286 (t[burst] + 42.57 hr), Chandra Low Energy Transmission
Grating Spectrometer (LETGS) observations were conducted of a field
centered on the optical afterglow candidate (Da Costa et al, GCN2765) for
GRB041006, which was localized by the HETE WXM instrument (Galassi et al,
GCN2770). We have detected a moderately bright, fading X-ray afterglow
consistent in position with the optical source. Over the 86.3 ksec of data
accumulation (livetime) for the Chandra observations, the mean counting rate
was 0.010 counts/s in the dispersed 1st order LETGS spectrum and 0.007
counts/s in the 0ther order. The source we detected faded in brightness
according to a power law, with a decay time slope of -1.0 +/- 0.1. We fit
the 0th order and 1st order data jointly, requiring 20 or more counts per
spectral bin. An absorbed power-law model provides an acceptable fit
(chi^2/nu = 66.2/63), with the following best-fit paramters over the
0.5-6 keV range:
dN/dE = A * exp[-nH*s(E)] * E^ (-gamma) ph cm^(-2) s^(-1) keV^(-1) ,
with:
A = (1.0+/-0.2) x 10^(-4) ,
gamma = 1.9+/-0.2 , and
nH = (1.1+/-0.5) x 10^21 cm^(-2).
The measured value of nH exceeds the anticipated Galactic column density
(nH = 2.9 x 10^20 cm^(-2)) in the source direction. We measure a
mean flux in the 0.5 to 6 keV band over the duration of the Chandra
observation of ~3.7 x 10^(-13) ergs cm^(-2) s^(-1).
Our analyses are continuing, and more detailed results will be posted at:
http://space.mit.edu/HETE/Bursts/GRB041006
We thank Harvey Tananbaum and the Chandra X-ray Observatory Operations
personnel, particularly Jeremy Drake and Brad Wargelin, for the impressive
promptness with which this observation was planned and carried out.
The preliminary results reported here may be cited.