GCN Circular 10789

R.L.C. Starling, A.P. Beardmore (U. Leicester) and E. Troja (NASA/GSFC/ORAU) 
report on behalf of the Swift-XRT team:

We have analysed 5.4 ks of XRT data for GRB 100522A (Troja et al. GCN 
Circ. 10784
Not found
 
 
), from 56 s to 13.6 ks after the BAT trigger. The data 
comprise 87 s in Windowed Timing (WT) mode (the first 7 s were taken while 
Swift was slewing) with the remainder in Photon Counting (PC) mode. The 
enhanced XRT position for this burst was given by Osborne et al. (GCN. 
Circ 10787).

The light curve can be modelled with a series of power-law decays. The 
initial decay index is alpha = 4.8 (+/-0.4). At T+130 s the decay flattens 
to an alpha of 0.53 (+0.04, -0.09) before breaking again at T+7830 s to a 
final decay with index alpha = 1.2 (+2.8, -0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed 
power-law with a photon spectral index of Gamma = 2.82 (+0.32, -0.28). The 
best-fitting absorption column is 4.0 (+0.8, -0.7) x 10^21 cm^-2, in 
excess of the Galactic value of 4.0 x 10^20 cm^-2 (Kalberla et al. 2005). 
The PC mode spectrum has a photon index of 2.14 (+0.30, -0.28) and a 
best-fitting absorption column of 4.0 (+1.2, -1.0) x 10^21 cm^-2. The 
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced 
from this spectrum is 4.4 x 10^-11 (8.1 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 1.2, 
the count rate at T+24 hours will be 0.011 count s^-1, corresponding to an 
observed (unabsorbed) 0.3-10 keV flux of 4.8 x 10^-13 (8.9 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/00422783.

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