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
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 ), 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.