GCN Circular 8231

P.A. Evans (U. Leicester) and S.R. Oates (MSSL/UCL)
report on behalf of the Swift-XRT team.

We have analysed 1.7 ks of XRT data on GRB080915 (Oates et al, GCN Circ
8227), beginning  4.9 ks after the GRB trigger. Swift did not slew
immediately to the burst due to an Earth limb constraint.

We find a faint, fading source with a mean count rate ~0.01 counts per
second at a position of RA, Dec=17.9473, -76.0204 degrees, which is
equivalent to:

RA (J2000):  01h 11m 47.35s
Dec (J2000): -76d 01' 13.4"

with an uncertainy of 6.5 arcsec (radius, 90% containment). This
position is 54 arcsec from the BAT position, within the BAT error
circle.

The light curve shows a power-law decay with an index of 0.94
(+1.19/-0.58); the large uncertainty arises because the light curve
contains only 2 points.

A spectrum formed from the PC data can be modelled with an absorbed
power-law. The absorption column is poorly constained, but consistent
with the Galactic value of 6.7 x 10^20 cm^-2 (Kalberla et al. 2005), and
the photon index is 2.3 (+2.6, -1.3).

If the light curve continues to decay at alpha=0.94 we predict a count
rate at T0+24 hrs of 0.0018  counts per second. This corresponds to an
observed (unabsorbed) 0.3-10 keV flux of 6.36 x 10^-14 (1.22 x 10^-13)
erg cm^-2 s^-1.

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