TITLE:   GCN CIRCULAR
NUMBER:  8506
SUBJECT: GRB 081109A: Swift-XRT refined analysis
DATE:    08/11/09 16:19:07 GMT
FROM:    Andy Beardmore at U Leicester  <apb@star.le.ac.uk>

A.P. Beardmore, K.L. Page (U. Leicester) and S. Immler (CRESST/GSFC/UMD) 
report on behalf of the Swift-XRT team.

The Swift-XRT started observing the field of GRB 081109A (trigger
#334112, Immler et al., GCN 8500) at 07:03:12.2 UT, 65.6 s after
the BAT trigger. The XRT observed the GRB in Windowed Timing (WT) mode 
until 318 s after the trigger and Photon Counting (PC) mode thereafter.

Using 4661 s of XRT Photon Counting mode data and 7 UVOT images, we
find an astrometrically corrected X-ray position (using the XRT-UVOT
alignment and matching to the USNO-B1 catalogue) of
RA, Dec = 330.7905, -54.7118 which is equivalent to :

RA  (J2000):  22h 03m 9.72s
Dec (J2000): -54d 42m 42.5s

with an uncertainty of 1.4 arcsec (90% confidence).  This is 2.98
arcsec from the reported possible NIR counterpart of D'Avanzo (GCN
8501).

The X-ray light curve from the first three orbits can be modelled with
a broken powerlaw with an initial decay slope of 1.78 +/- 0.06, a break 
time of 353 +/- 40 s, followed by a shallower decay slope of 0.99 +/- 0.04.

The spectrum of the WT data from orbit 1 (from T+71.9 s to T+318.4 s, 
where T is the trigger time) can be well fit by an absorbed powerlaw 
with a photon index of 1.72 +/- 0.08 and column density of 
(1.84 +/- 0.22) x 10^21 cm^-2, compared with the Galactic column 
density of 1.6 x 10^20 cm^-2 in the direction of the burst. The observed
0.3-10 keV flux is  (7.4 +/- 0.4) x 10^-10 erg cm^-2 s^-1. The 
corresponding unabsorbed 0.3-10 keV flux is (9.5 +/- 0.5) x 10^-10 
erg cm^-2 s^-1.

A powerlaw fit to a  spectrum  obtained from  PC mode data spanning 
orbits 2 to 3 (T+4.76 ks to T+12.88 ks) is best fit with a photon 
index of   2.20 +/- 0.20 and column density (3.1 +/- 0.7) x 10^21 cm^-2.
The observed 0.3-10 keV flux over this interval is (7.3 +/- 0.7) x 10^-12
erg cm^-2 s^-1. The  count-to-observed-flux conversion factor is 
6.0e-11 erg cm^-2 count^-1.

Assuming the X-ray emission from the burst continues to decline at the
same rate we predict an XRT count rate of 0.018 count s^-1 at T+24 hour, 
or an observed 0.3-10 keV X-ray flux of 1.1 x 10^-12 erg cm^-2 s^-1.

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