TITLE:   GCN CIRCULAR
NUMBER:  9388
SUBJECT: GRB 090518: Swift-XRT refined analysis
DATE:    09/05/18 09:44:36 GMT
FROM:    Phil Evans at U of Leicester  <pae9@star.le.ac.uk>

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

We have analysed 6.4 ks of XRT data for GRB 090518 (Evans et al. GCN 
Circ. 9386), from 88 s to 13.0 ks after the BAT trigger. The data are 
entirely in Photon Counting (PC) mode. The enhanced XRT position for 
this burst was given by Evans et al. (GCN. Circ 9387).

The light curve can be modelled with a series of power-law decays. The 
initial decay index is alpha=1.5 (+0.4, -0.3). At T+332 s the decay 
flattens to an alpha of 0.29 (+0.16, -0.38) before breaking again at 
T+2743 s to a final decay with index alpha=1.1 (+/-0.3).

A spectrum formed from the PC mode data can be fitted with an absorbed 
power-law with a photon spectral index of 2.1 (+/-0.4). The best-fitting 
absorption column is 6.2 (+2.3, -1.9) x 10^21 cm^-2, in excess of the 
Galactic value of 4.6 x 10^20 cm^-2 (Kalberla et al. 2005). The counts 
to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from 
this spectrum is 4.9 x 10^-11 (9.9 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 
1.1, the count rate at T+24 hours will be 0.0031 count s^-1, 
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x 
10^-13 (3.1 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/00352420.

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