GCN Circular 7994

C. Guidorzi, F. Pasotti (INAF-OAB), F. E. Marshall (NASA/GSFC)
report on behalf of the Swift-XRT team:

We have analysed the first three orbits of Swift-XRT data obtained for GRB
080721 (Marshall et al., GCN Circ. 7988
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 ), covering 1.3 ks and 2.8 ks
of Windowed Timing (WT) and Photon Counting (PC) modes data, respectively,
between 113 s and 10.6 ks after the trigger.
Notably, the whole first orbit data are taken in WT mode.
The UVOT-enhanced XRT position was given by Osborne et al. in GCN Circ. 
7993
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 .

The light-curve can be modelled by an initial power law with index
0.75 +/- 0.04 followed by a constant rate step from ~260 to 330 s,
after which the curve resumes a power-law decay with an index of
1.06 +/- 0.01 up to 10.6 ks (chisq/dof = 1292/1207).

A spectrum extracted from the first orbit of data can be modelled with an
absorbed power-law, with Gamma = 1.90 +/- 0.02 and
NH = (1.10 +/- 0.05)x1e21 cm^-2, in excess of the Galactic column
in this direction of 6.9e20 cm^-2. We found no evidence for spectral
evolution during this time interval.
The observed (unabsorbed) flux over this time interval (113-1370 s
post-trigger) is 2.2e-9 (2.7e-9) erg cm^-2 s^-1.

If the light-curve continues to decay with alpha ~1.06, the count rate 24
hours after the burst is estimated to be 0.23 count s^-1, which
corresponds to an observed flux of 9.6e-12 erg cm^-2 s^-1.

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