GCN Circular 11244

A. P. Beardmore (U. Leicester) and C. B. Markwardt (NASA/GSFC) report
on behalf of the Swift-XRT team:

We have analysed 16.6 ks of XRT data for GRB 100906A (Markwardt et
al. GCN Circ. 11227
Not found
 
 
), from 70 s to 40.2 ks after the BAT trigger. The
data comprise 228 s in Windowed Timing (WT) mode (the first 9 s were
taken while Swift was slewing) with the remainder in Photon Counting
(PC) mode. The enhanced XRT position for this burst was given by Goad
et al. (GCN. Circ 11243).

The X-ray light curve initially shows a large flare, peaking at a
count rate of ~2000 count s^-1 at T+120s, contemporary with the late
time BAT activity (Barthelmy et al., GCN Circ. 11233
Not found
 
 
). It then
declines rapidly before breaking to a shallower decay slope of alpha =
0.74 +/- 0.04 after T+270s. This is followed by a further break at
T+10854 s to an slope of 1.96 +0.17 -0.13.

A spectrum formed from the PC mode data can be fit by an absorbed
power-law with a photon index of 2.15 +/-0.08. The best-fitting
absorption column is (8.1 +/- 2.5) x 10^21 cm^-2, at a redshift of
1.727 (Tanvir et al. GCN Circ. 11230
Not found
 
 
), in addition to the Galactic
value of 2.2 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to
observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from
this spectrum is 4.1 x 10^-11 (7.1 x 10^-11) erg cm^-2 count^-1.  We
do not report a WT time averaged spectral fit here as this would be
affected by the spectral evolution during the flaring activity,
evident in the hardness ratio.

If the light curve continues to decay at the same rate, the count rate
at T+24 hours will be 5.9 x 10^-3 count s^-1, corresponding to an
observed (unabsorbed) 0.3-10 keV flux of 2.4 x 10^-13 (4.2 x 10^-13)
erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00433509.

This circular is an official product of the Swift-XRT team.