TITLE:   GCN CIRCULAR
NUMBER:  6738
SUBJECT: GRB 070810: Swift-XRT Team Refined Analysis 
DATE:    07/08/10 10:01:17 GMT
FROM:    Kim Page at U.of Leicester  <kpa@star.le.ac.uk>

K.L. Page, P.A. Evans (U. Leicester), M.M Chester (PSU) and J.R. Cummings 
(NASA/UMBC) report on behalf of the Swift-XRT team:

We have analysed the first four orbits of Swift-XRT data obtained for GRB 
070810 (Cummings et al., GCN Circ. 6733), totalling 7.9 ks of Photon 
Counting (PC) mode data. The enhanced XRT position was given by Evans et 
al. in GCN Circ. 6736, and is 1.7 arcsec from the fading UVOT source 
(Chester et al., GCN Circ. 6735).

The X-ray light-curve is almost flat for the first orbit of PC data, with 
alpha = 0.30 +0.18/-0.20. About 1.1 ks after the trigger, the decay 
steepened to a slope of 1.1 +/- 0.2.

Fitting the PC data from the second to fourth orbits (i.e. after the 
break; 3.8-17.8 ks after the trigger), the spectrum can be modelled by an 
absorbed power-law with Gamma = 2.3 +/- 0.3 and NH = (1.3 +/- 0.5)e21 
cm^-2, in excess of the Galactic value of 1.75e20 cm^-2. The 0.3-10 keV 
observed (unabsorbed) flux during this time is 2.17e-12 (3.34e-12) erg 
cm^-2 s^-1.

If the decay continues with a slope of alpha ~ 1.1, the predicted count 
rate at 24 hours is 4.1e-3 count s^-1. Using a flux conversion factor 
(determined from the post-break spectrum described above) of 1 count s^-1 
~ 4e-11 erg cm^-2 s^-1 (observed) and 5.8e-11 erg cm^-2 s^-1 (unabsorbed), 
this corresponds to an observed (unabsorbed) flux of 1.6e-13 (2.4e-13) erg 
cm^-2 s^-1.

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