TITLE:   GCN CIRCULAR
NUMBER:  11074
SUBJECT: GRB 100807A: Swift XRT refined analysis
DATE:    10/08/08 00:29:22 GMT
FROM:    Dirk Grupe at PSU/Swift-XRT  <grupe@astro.psu.edu>

D. Grupe (PSU) reports on behalf of the Swift-XRT team:

We have analyzed 9.7 ks of XRT data for GRB 100807A (Grupe  et al.
GCN Circ. 11067), from 125 s to 24882 s after the  BAT trigger. The
data comprise 36 s in Windowed Timing (WT) mode with the remainder
in Photon Counting (PC) mode. The light curve can be modeled
with  a broken power-law model. After an initial flare the light
curve decays with a slope of 6.4+0.6-1.9. The light curve breaks at
173+/-10s followed by a flatter decay slope of 0.82+/-0.08.

A spectrum formed from the WT mode data can be fitted with an
absorbed power-law with a photon spectral index of 2.12 (+/-0.22).
The best-fitting absorption column is consistent with the Galactic
value of 3.3 x 10^21 cm^-2 (Kalberla et al. 2005). A fit to the pc
mode data is also consistent with the Galactic absorption column
density, but the spectrum is slightly flatter with  a photon index
of 1.79+/-0.37. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from the WT spectrum  is 4.38 x 10^-11
(7.54 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index
of 0.8, the count rate at T+48 hours will be in the order of
1.3 x 10^-3 count s^-1, corresponding to an observed (unabsorbed)
0.3-10 keV flux of 5.7 x 10^-14 (9.8 x 10^-14) erg cm^-2 s^-1.
Note that we are currently in the plateau phase which is followed by
a steeper decay slope.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00431128.

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