GCN Circular 9893

V. D'Elia, G. Stratta, M. Perri (ASDC) report on behalf of
the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 090904B (D'Elia et al. GCN
Circ. 9881
Not found
 
 
), from 159 s to 37.1 ks after the BAT trigger. The data are
entirely in Photon Counting (PC) mode. The enhanced XRT position for
this burst was given by Osborne et al. (GCN. Circ 9987).

The light curve, after an initial rise, peaks at T+260 s, while the 
following
decay can be modelled with a power-law with a decay
index of alpha=1.20+-0.05 (90% confidence level).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.67 (+0.26, -0.24, 90%
confidence level). The best-fitting absorption column is 6.3
(+2.5, -1.1) x 1021 cm-2, in excess with respect to the Galactic value of
3.3 x 1021 cm-2 (Kalberla et al. 2005). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum is 6.8 x 10-11 (1.1 x 10-10) erg cm-2 count-1.

If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.0040 count s-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.7 x
10-13 (4.4 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/00361831.

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