P.A. Evans (U. Leicester) and S. Immler (CRESST/GSFC/UMD) report
on behalf of the Swift-XRT team:
We have analysed 1.6 ks of XRT data for GRB 080727B (Immler et al., GCN
Circ 8022). The data were taken in two snapshots; the first started 105
s after the trigger and contained 51 s of Windowed Timing mode data
followed by 500 s of Photon Counting (PC) mode data. The second snapshot
began at T0+11 ks, and contained 1.2 ks of data, all in PC mode.
The UVOT-enhanced XRT position has been given by Goad et al. (GCN Circ.
The light curve shows an initial steep decay with a power-law index of
2.47 (+1.38, -0.39), with a break to a shallower decay of 1.05 (+0.20,
-0.13). The break occurred at T0+267 (+89, -100) s.
A spectrum formed from all available PC mode data can be modelled by an
absorbed power law with a photon index on 1.61 (+0.43, -0.38). The best
fitting absorption column is 8.7 (+4.1, -2.9) x 10^21 cm^-2, in excess
of the Galactic value in the direction of the burst (3.6 x 10^21,
Kalberla et al. 2005). The counts to observed 0.3-10 keV flux
conversion factor deduced from this spectrum is 5.0 x 10^-11 erg cm^-2
If the light curve continues to decay with a slope of 1.05, the count
rate at 24 hours post-trigger will be 1.8 x 10^-3, corresponding to an
observed (unabsorbed) flux of 9.0 x 10^-14 (4.2 x 10^-13) erg cm^-2
This circular is an official product of the Swift-XRT team.