TITLE:   GCN CIRCULAR
NUMBER:  8746
SUBJECT: GRB 081228: Swift-XRT Team refined analysis
DATE:    08/12/28 10:16:19 GMT
FROM:    Kim Page at U.of Leicester  <kpa@star.le.ac.uk>

K.L. Page & P.A. Evans (U. Leicester) report on behalf of the Swift-XRT 
team:

We have analysed the first 4.6 ks of XRT data obtained for GRB 081228 
(Page et al., GCN Circ. 8742), all of which was collected in Photon 
Counting mode. We have further refined the position given by Goad et al. 
in GCN Circ. 8744, and now find an astrometrically corrected X-ray 
position (using the XRT-UVOT alignment and matching UVOT field sources to 
the USNO-B1 catalogue) of

RA (J2000):   02 37 50.89
Dec (J2000): +30 51 09.1

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

The data can be fitted with a single power-law with a decay of alpha = 1.4 
+/- 0.1, although there is some evidence for a flattening after about 8 
ks.

The spectrum of the first orbit of data can be modelled with a power-law 
of photon index Gamma = 1.5 +/- 0.4, absorbed by the Galactic column in 
this direction of 8.8x10^20 cm^-2. The upper limit on the total absorbing 
column is 4.7x10^21 cm^-2. The observed (unabsorbed) flux over this time 
(86-117 s after the trigger) is 1.23x10^-10 (1.38x10^-10) erg cm^-2 s^-1, 
corresponding to a counts to observed flux conversion of 5.1x10^-11 erg 
cm^-2 count^-1.

Taking into account the possible flattening of the decay at later times, 
the count rate at 24 hours is predicted to be between 4.5x10^-3 and 
2.7x10^-4 count s^-1 (observed flux between 2.3x10^-13 and 1.4x10^-14 erg 
cm^-2 s^-1).

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