GCN Circular 9378

B. A. Rowlinson and P. Evans (U Leicester) report on behalf of the
Swift-XRT team:

We have analysed 3.0 ks of XRT data for GRB 090516 (Rowlinson et al. GCN
Circ. 9374
Not found
 
 
), from 170 s to 6.4 ks after the BAT trigger. The data
comprise 327 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Beardmore et al. (GCN. Circ 9376).

The light curve can be modelled with a power-law decay with two flares
superimposed on it. The initial decay index is alpha=3.27 (+0.11,
-0.10). At T+601 (+138, -52) s the decay flattens to an alpha of 0.93
(+0.11, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.09 (+/-0.04). The
best-fitting absorption column is 1.57 (+0.10, -0.09) x 10^21 cm^-2, in
excess of the Galactic value of 4.5 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.09 (+0.13, -0.12)
and a best-fitting absorption column of 9.6 (+2.6, -2.5) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 3.6 x 10^-11 (4.8 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
0.93, the count rate at T+24 hours will be 0.03 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.4 x 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/00352190.

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