[vsnet-grb-info 7406] GRB 090309: Swift-XRT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8970
SUBJECT: GRB 090309: Swift-XRT refined analysis
DATE:    09/03/10 15:43:56 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at star.le.ac.uk>

P. A. Evans (U Leicester) reports on behalf of the Swift-XRT team:

We have analysed 5.5 ks of XRT data for GRB 090309 (Evans et al. GCN 
Circ. 8964), from 86 s to 12.4 ks after the BAT trigger. The data are 
entirely in Photon Counting (PC) mode. The enhanced XRT position for 
this burst was given by Goad et al. (GCN. Circ 8969).

The light curve can be modelled with a power-law decay with a decay 
index of alpha=0.38 (+0.08, -0.09).

A spectrum formed from the PC mode data can be fitted with an absorbed 
power-law with a photon spectral index of 2.2 (+/-0.3). The best-fitting 
absorption column is 2.8 (+1.5, -1.1) x 10^21, in excess of the Galactic 
value of 1.1 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to 
observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from 
this spectrum is 4.0 x 10^-11 (6.8 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 
0.38, the count rate at T+24 hours will be 0.012 count s^-1, 
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.8 x 
10^-13 (8.2 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/00345945.

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