GCN Circular 9857

A. Rowlinson (U. Leicester) and P. Schady (MSSL-UCL) report on behalf of
the Swift-XRT team:

We have analysed 8.4 ks of XRT data for GRB 090831C (Schady et al. GCN
Circ. 9854
Not found
 
 
), from 121 s to 21.8 ks after the BAT trigger. The data are
entirely in Photon Counting (PC) mode. The enhanced XRT position for
this burst was given by Evans et al. (GCN. Circ 9855).

The light curve has two flares at T+186 s and T+440 s, while the
underlying decay can be modelled with a power-law decay with a decay
index of alpha=0.83 (+0.17, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.84 (+0.21, -0.15). The
best-fitting absorption column is 3.254 (+1.001, -0.014) x 10^21 cm^-2,
consistent with the Galactic value of 3.2 x 10^21 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 4.9 x 10^-11 (7.2 x 10^-11) erg
cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
0.83, the count rate at T+24 hours will be 0.0029 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.4 x
10^-13 (2.1 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00361489.

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