TITLE:   GCN CIRCULAR
NUMBER:  7994
SUBJECT: GRB 080721: Swift-XRT refined analysis
DATE:    08/07/21 17:16:43 GMT
FROM:    Cristiano Guidorzi at INAF-OAB  <cristiano.guidorzi@brera.inaf.it>

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), 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.

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.