TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2808
SUBJECT: GRB041006: Fading X-ray Afterglow Observed with Chandra
DATE:    04/10/15 18:16:51 GMT
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.