TITLE:   GCN CIRCULAR
NUMBER:  6725
SUBJECT: GRB 070808: Swift XRT refined analysis
DATE:    07/08/09 06:15:49 GMT
FROM:    Rhaana Starling at U of Leicester  <rlcs1@star.le.ac.uk>

R. Starling (U. Leicester) and J.R. Cummings (NASA/UMBC) report on behalf of 
the Swift XRT team:

We have analysed the first orbit and part of the second orbit of Swift XRT
data for GRB 070808 (trigger=287260; Cummings et al., GCN Circ. 6718),
covering 114-7500 s since BAT trigger.

Using 799 s of overlapping XRT Photon Counting mode and UVOT V-band
data, we find an astrometrically corrected X-ray position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): 
RA, Dec (J2000) = 6.76402, 1.17635 deg, which is equivalent to:
RA (J2000):  00h 27m 3.36s
Dec (J2000): +01d 10' 34.8''
with an uncertainty of 1.9 arcsec (radius, 90% confidence).
This is 4.7 arcsec from the initial XRT position (GCN Circ. 6718) and 16.7 
arcsec from the BAT refined position (Fenimore et al. 2007, GCN Circ. 6724).

The 0.3-10 keV lightcurve shows a smooth and somewhat curved decay, which can
be modelled with a broken power law with alpha_1=3.5 +0.7/-0.2, T_bk=234 
+13/-20 s and alpha_2=1.06 +0.1/-0.08.

The PC mode spectrum from 500-7500 s since trigger can be modelled with an
absorbed power law with photon index Gamma=2.8 +/-0.5, and 
nH=(1.2 +0.5/-0.3)E22 cm^-2 in excess of the Galactic column of 3E20 
cm^-2. The 0.3-10 keV observed (unabsorbed) flux is 2.4E-12 (1.0E-11) 
erg/cm^2/s corresponding to a count rate of 4.5E-2 count/s and a count 
rate to flux conversion of 1 count/s = 5E-11 erg/cm^2/s.

Assuming the X-ray emission continues to decay with alpha=1.06, we predict a
count rate at T+24h of 9E-4 count/s.

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