TITLE:   GCN CIRCULAR
NUMBER:  10288
SUBJECT: GRB 091221: Swift-XRT refined analysis
DATE:    09/12/22 10:50:17 GMT
FROM:    Jirong Mao at INAF-OAB  <jirong.mao@brera.inaf.it>

J. Mao (INAF-OAB) and H. A. Krimm (CRESST/GSFC/USRA) report on behalf of 
the Swift-XRT team:

We have analysed 4.7 ks of XRT data for GRB 091221 (Krimm et al. GCN 
Circ. 10283), from 78 s to
11.6 ks after the BAT trigger. The data comprise 332 s in Windowed 
Timing (WT) mode with the remainder
 in Photon Counting (PC) mode. The UVOT-enhanced XRT position was given 
in Evans et al.
(GCN Circ. 10287).

The light curve shows a flare feature from T0+90 s to T0+200 s, the peak 
value is about 55 count/s at
T0+106 s. After that, the light curve can be modelled with a power-law 
decay with a decay index of
alpha=1.07 ± 0.05.

A spectrum formed from the WT mode data can be fitted with an absorbed 
power-law with a photon spectral
 index of 2.52 (+0.16, -0.17). The best-fitting absorption column is 3.6 
(+/- 0.4) x 1021 cm^-2, in excess of
 the Galactic value of 1.1 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
4.4 x 10^-11 (7.2 x 10^-11) erg cm^-2 count^-1. The spectrum formed from 
the PC mode data can be fitted
with an absorbed power-law with a photon spectral index of 1.68 (+0.45, 
-0.51). The best-fitting absorption
column is about 1.1 x 1021 cm^-2, in consistent with the Galactic value.

If the light curve continues to decay with a power-law decay index of 
1.07, the count rate at T0+24 hours
will be 2.3 x 10^-3 count s^-1, corresponding to an observed 
(unabsorbed) 0.3-10 keV flux of
1.4 x 10^-13 (1.6 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/00380311.

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