TITLE:   GCN CIRCULAR
NUMBER:  8197
SUBJECT: GRB 080906: Swift-XRT Refined Analysis
DATE:    08/09/07 20:20:43 GMT
FROM:    Loredana Vetere at PSU  <vetere@astro.psu.edu>

L. Vetere, J.L. Racusin, C. Pagani (PSU), J.P. Osborne, A.P. Beardmore,
P.A. Evans and M.R. Goad (U. Leicester)  report on behalf of the Swift-XRT team:

The XRT began observing the field of GRB 080906 (trigger=323984; Vetere et al.,
GCN Circ 8186) at 13:34:27.6 UT, 71.3 seconds after the BAT trigger.

Using 1224 s of XRT Photon Counting mode data and 2 UVOT images, we find an
astrometrically corrected X-ray position (using the XRT-UVOT alignment and
matching to the USNO-B1 catalogue): RA, Dec = 228.04211, -80.51787 which is
equivalent to:

 RA (J2000): 15 12 10.11
 Dec (J2000): -80 31 04.3

 with an uncertainty of 1.8 arcsec (90% confidence).

This position may be improved as more data are received. The latest position can
be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is
described
by Goad et al. (2007, A&A, 476, 1401
http://www.swift.ac.uk/xrt_positions/Goad.pdf),
the current algorithm is an extension of this method.

This position is 80 arcsec away from the ground-processed BAT position given in
GCN Circ. 8196 (Stamatikos et al.) inside BAT error circle.


The WT mode lightcurve shows a number of flares, with the strongest peaking
around
186, 598 and 907 s after the trigger. The PC data in later orbits  shows some
flaring activity and decays as a single power-law  with alpha = 1.25 +/- 0.11.

The WT spectrum of the first 80 s (before the first peak) can be fitted with a
power-law of Gamma = 1.82 +/-0.08 and absorption consistent with the Galactic
value
along the line of sight (9.7x10^20 cm^-2). The observed (unabsorbed) flux over
this
time is 1.98 (2.7)x10^-9 erg cm^-2 s^-1. A spectrum extracted from the PC mode
data
(11.6-75.6 ks after the trigger) can be fitted with a power-law of Gamma = 2.3
+0.1/-0.2
and total NH = (1.7 +/- 0.5)x10^22 cm^-2, slightly in excess of the Galactic
value.
The observed (unabsorbed) flux over this time is 1.7 (3.1)x10^-11 erg cm^-2
s^-1.

If the decay continues with alpha = 1.25, the predicted count-rate at 48 hours
is
0.007 count s^-1, corresponding to an observed (unabsorbed) flux of 2.6
(4.8)x10^-13
erg cm^-2 s^-1.


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