TITLE:   GCN CIRCULAR
NUMBER:  8299
SUBJECT: GRB 080928: Swift-XRT refined analysis
DATE:    08/09/29 12:56:41 GMT
FROM:    Cristiano Guidorzi at INAF-OAB  <cristiano.guidorzi@brera.inaf.it>

C. Guidorzi, R. Margutti, J. Mao (INAF-OAB) and T. Sakamoto (GSFC/UMBC)
report on behalf of the Swift-XRT team:

We have analysed the first 7 orbits of Swift-XRT data obtained for
GRB 080928 (Sakamoto et al., GCN Circ. 8292), covering 325 s of Windowed
Timing (WT) mode data and 15.3 ks of Photon  Counting (PC) mode data.
The UVOT-enhanced XRT position was given by Osborne et al. in GCN Circ. 
8295.

The light-curve can be modelled by a broken power-law, with a couple
of bright flares peaking at 210 and 360 s with a FWHM duration of 55
and 35 s and a peak count rate of 420 +/- 10 and 75 +- 3 count s^-1,
respectively. The power-law decay index changes from an initial value
of 1.20 +- 0.03 to 1.7 +- 0.2 after a break time of (11 +- 2) ks
(uncertainties at 1 sigma). The first flare coincides with the main
emission seen by BAT (Fenimore et al. GCN Circ. 8297).

A spectrum extracted from WT data around the first flare, from 176 to 250 s,
can be modelled with an absorbed power-law, with Gamma = 1.45 +- 0.06 and
NH = (1.7 +- 0.2)x1e21 cm^-2, in excess of the Galactic column in this
direction of 5.6e20 cm^-2. Another spectrum extracted around the second
flare, from 300 to 470 s, can be fit with the same model, with
Gamma = 1.85 +- 0.06 and NH = (1.1 +- 0.2)x1e21 cm^-2.
A later spectrum extracted from PC data, from 5 to 47 ks, can be fit with
an absorbed power law with Gamma  = 2.2 +- 0.2 and NH = (1.2 +- 0.4)x1e21
cm^-2. The observed (unabsorbed) flux over this time interval is 8.4x10^-12
(1.2x10^-11) erg cm^-2 s^-1.

If the light-curve continues to decay with the current rate, the count
rate 24 hours after the burst is estimated to be 5.6e-3 count s^-1, which
corresponds to an observed flux of 2.5x10^-13 erg cm^-2 s^-1.

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