TITLE:   GCN CIRCULAR
NUMBER:  8956
SUBJECT: GRB 090308: Swift-XRT refined analysis
DATE:    09/03/09 07:33:46 GMT
FROM:    Phil Evans at U of Leicester  <pae9@star.le.ac.uk>

P. A. Evans and K.L. Page (U Leicester) report on behalf of the 
Swift-XRT team:

We have analysed 4.4 ks of XRT data for GRB 090308 (Evans et al. GCN
Circ. 8948), from 130 s to 12.2 ks after the BAT trigger. The data
comprise 55 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. Using 460 s of PC mode data and 1 UVOT image, we
find an enhanced XRT position (using the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue): RA, Dec = 183.50120,
-48.81710 which is equivalent to:

RA (J2000): 12 14 0.28
Dec(J2000): -48 49 01.6

with an uncertainty of 1.6 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.63 (+0.16, -0.13), followed by a break at T+830 s to an
alpha of 1.2 (+0.5, -1.9).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.93 (+0.28, -0.26). The
best-fitting absorption column is 2.86 (+1.01, -0.88) x 10^21, in excess
of the Galactic value of 8.5 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 4.5 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
1.9, the count rate at T+24 hours will be 2.0 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.1 x 
10^-15 (1.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00345777.

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