TITLE:   GCN CIRCULAR
NUMBER:  9262
SUBJECT: GRB 090426: Swift-XRT refined analysis
DATE:    09/04/26 20:27:04 GMT
FROM:    Cristiano Guidorzi at Ferrara U,Italy  <guidorzi@fe.infn.it>

C. Guidorzi (Univ. Ferrara), R. Margutti (Univ. Bicocca & INAF-OAB),
J. Mao (INAF-OAB), and J. R. Cummings (NASA/UMBC) report on behalf of
the Swift-XRT team:

We have analysed the first two orbits of Swift-XRT data obtained for
GRB 090426 (Cummings et al. GCN Circ. 9254), covering 3.0 ks of
Photon Counting (PC) mode data, between 88 s and 13.8 ks after the trigger.
The UVOT-enhanced XRT position was given by Osborne et al. in GCN Circ. 
9259.

The light-curve can be modelled by a simple power-law, with
a decaying index of 0.91 +/- 0.06 (chisq/dof=27.1/25).

The spectrum extracted from PC data in the time interval from 88 s
to 5.6 ks can be modelled with an absorbed power-law, with a photon
index of 1.96 +/- 0.10 and a column density consistent with the Galactic
value of 1.5e20 cm^-2 (Kalberla et al. 2005), with a 90% confidence upper
limit of 3.7e20 cm^-2. The observed (unabsorbed) 0.3 -10 keV flux over
this time interval is 1.1e-11 (1.2e-11) erg cm^-2 count^-1.
Uncertainties are given at 90% confidence.

If the light-curve continues to decay with alpha ~0.91, the count rate 24
hours after the burst is estimated to be 3.9e-3 count s^-1, which
corresponds to an observed flux of 1.7e-13 erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00350479 .

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