TITLE:   GCN CIRCULAR
NUMBER:  6388
SUBJECT: GRB 070508: Swift-XRT refined analysis
DATE:    07/05/08 12:41:59 GMT
FROM:    Claudio Pagani at PSU/Swift-XRT  <pagani@astro.psu.edu>

D. Grupe, J. Racusin, & C. Pagani (PSU) report on behalf of the Swift/XRT team

We have analyzed the first three orbits of GRB 070508 (Grupe et al., GCN 6383) 
with total observing times of 1.3 ks in Windowed Timing mode and 1.3 ks in 
Photon Counting mode in the Swift XRT.
The Photon Counting mode image of the third orbit provides a refined XRT 
position at RA, DEC= 312.7993, -78.3848, which is

RA(J2000) = 20h 51m 11.84s
Dec(J2000) = -78d 23' 05.4"

with an error of 5.6" (90% confidence). This position is 2.6" away from the 
preliminary XRT position reported in GCN 6383 and 1.9" from the optical 
position given in GCN 6386 (Berger & Burgasser).

The XRT observations in Windowed Timing more started at the end of a flare and 
both Windowed timing and Photon Counting mode display the rapid decay of the 
afterglow with a decay slope of 1.15+/-0.11 measured from the PC data. The 
prediction 24h after the burst is a 0.3-10.0 keV flux of 3.2e-12 ergs/s/cm2 
or about 0.04 XRT counts/s.

The Windowed Timing mode data can be fitted by a single power law with a 
photon index Gamma=2.05+-0.04 and an absorption column density of 
NH=(3.8+/-0.2)e21cm-2. The Galactic absorption density in the direction of 
the burst is 8.60e20cm-2 (Dickey & Lockman, 1990). The PC data are consistent 
with this result. According to the relation given in Grupe et al. (2007, AJ, 
133, 2216) the excess absorption suggests that the burst has a redshift of 
less than 2.6.

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