GCN Circular 9790

A. P. Beardmore (U. Leicester) and J. R. Cummings (NASA/UMBC) report
on behalf of the Swift-XRT team:

We have analysed 8.8 ks of XRT data for GRB 090813 (Cummings et
al., GCN Circ. 9781
Not found
 
 
), from 84 s to 19.3 ks after the BAT trigger. The
data span four orbits, comprising 810 s in Windowed Timing (WT) mode
with the remainder in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Osborne et al. (GCN. Circ 9785).

The X-ray light curve initially follows a shallow decay, with a decay 
index of 0.22 +/ 0.10, before steepening to an index of 1.15 +/- 0.03 at 
about 450 s after the trigger.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.10 +/- 0.07. The
best-fitting absorption column is (2.23 +0.20 -0.19) x 10^21 cm^-2, in
excess of the Galactic value of 6.3 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.00 +0.11 -0.10 and
a best-fitting absorption column of (2.3 +/- 0.3) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.3 x 10^-11 (6.4 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
1.15, the estimated count rate at T+24 hours will be 0.016 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of
6.9 x 10^-13 (1.0x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00359884.

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