TITLE:   GCN CIRCULAR
NUMBER:  8991
SUBJECT: GRB 090313: Swift-XRT refined analysis
DATE:    09/03/13 23:00:22 GMT
FROM:    Jirong Mao at INAF-OAB  <jirong.mao@brera.inaf.it>

J. Mao & R. Margutti (INAF-OAB) report on behalf of the Swift-XRT team:

We have analysed 2 ks of XRT data for GRB 090313 (Mao et al. GCN Circ. 
8980),
from 26.8 ks to 28.6 ks after the BAT trigger. All the data were taken in
Photon Counting (PC) mode. Using 2038 s of XRT Photon Counting mode data
and 1 UVOT image, we find an astrometrically corrected X-ray position 
(using
the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue): RA, Dec = 198.40130, 8.09730 which is equivalent to:

RA (J2000):  13 13 36.30
Dec (J2000): +08 05 50.4

with an uncertainty of 1.8 arcsec (radius, 90% confidence).

The optical candidate (Chornock et al. GCN 8979 and Updike et al. GCN 
8983) lies within
the XRT error circle.

The lightcurve can be modeled as a single power-law with an index around 
5.4. Given the present
data set the value of this parameter is affected by a large uncertainty.

The PC mode spectrum can be fit with an absorbed simple power-law, with 
a photon index
of 2.12 ± 0.33 and an absorbing column density of 1.29 ± 0.97 e21 cm-2,  
in excess of the Galactic
value of 2.10e20 cm-2 (Kalberla et al. 2005). The counts to observed 
(unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 5.1e-11 (5.9e-11) erg 
cm-2 count-1.

Given the very uncertain power-law decay index, no firm prediction of 
the lightcurve can be made.

The results of the automatic XRT analysis are available online at
http://www.swift.ac.uk/xrt_products/00346386.

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