GCN Circular 10222

K.L. Page (U. Leicester) and J. L. Racusin (NASA/GSFC) report on behalf of 
the Swift-XRT team: 

We have analysed 12 ks of XRT data for GRB 091130B (Racusin et al. GCN 
Circ. 10214
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 ), from 91 s to 29.1 ks after the BAT trigger. The data 
comprise 155 s in Windowed Timing (WT) mode with the remainder in Photon 
Counting (PC) mode. The enhanced XRT position for this burst was given by 
Goad et al. (GCN. Circ 10218).

The light curve can be modelled with a series of power-law decays. The 
initial index is alpha=-0.36 (+0.87, -1.14) - i.e., the light curve rises 
slightly. At T+107 s the light curve starts decaying with an alpha of 3.58 
(+0.24, -0.17). The light curve breaks again at T+172 s to a decay with 
alpha=2.11 (+0.24, -0.06), before a final break at T+4020 s, after which 
the decay index is 0.32 (+0.10, -0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed 
power-law with a photon spectral index of 2.26 (+0.07, -0.06). The 
best-fitting absorption column is 2.32 (+/-0.17) x 10^21 cm^-2, in excess 
of the Galactic value of 8.0 x 10^19 cm^-2 (Kalberla et al. 2005). The PC 
mode spectrum has a photon index of 2.48 (+0.18, -0.19) and a best-fitting 
absorption column of 1.66 (+0.23, -0.43) x 10^21 cm^-2. The counts to 
observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this 
spectrum is 3.2 x 10^-11 (5.8 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 
0.32, the count rate at T+24 hours will be 0.030 count s^-1, corresponding 
to an observed (unabsorbed) 0.3-10 keV flux of 9.6 x 10^-13 (1.7 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/00377487.

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