TITLE:   GCN CIRCULAR
NUMBER:  9940
SUBJECT: GRB 090926B: Swift XRT refined analysis 
DATE:    09/09/27 04:16:40 GMT
FROM:    Dirk Grupe at PSU/Swift-XRT  <grupe@astro.psu.edu>

D. Grupe (PSU), P.A. Evans and M.R. Goad (U. Leicester) report
on behalf of the Swift-XRT team:

We have analysed  the first two orbits of XRT data of GRB090926B
(Grupe et al. GCN Circ.  9935). The data comprise 307 s in Windowed
  Timing (WT) mode and 1985 s in Photon Counting (PC) mode.


Using 1692 s of XRT Photon Counting mode data and 1 UVOT
images, 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 =
46.30840, -39.00600 which is equivalent to:

RA (J2000):  03 05 14.01
Dec (J2000): -39 00 21.5

with an uncertainty of 1.7" (radius, 90% confidence). This
position is 32.7" from the BAT ground-calculated position given in
Baumgartner et al (GCN circular 9939).

This position may be improved as more data are received.  The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions.
Position enhancement is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

A spectrum formed from the WT mode data can be fitted with an
absorbed  power-law with a photon spectral index of 1.84+/-0.07
with an   absorption column of (1.70+/-0.22)e21 cm^-2 which is in
excess of the Galactic value of 1.91 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum results is a slightly flatter X-ray
spectrum with Gamma = 1.58+0.22-0.20. The absorption column density,
however, is consistent with the WT result. Based on the excess
absorption, according to Grupe et al. (2007, AJ, 133, 2216), the
redshift of this burst is most likely less than 4.0.


The light curve can be modelled with a broken power-law decay with an
index of alpha=2.00+/-0.05, followed by a break at T+712+190/-160 s
to  an alpha of 1.23 (-0.17, +0.07). If the afterglow continues to
dacay with a slope of 1.23, the predicted count rate 24 hours after
the trigger is 3.4e-3 counts s^-1 or 1.6e-13 ergs s^-1 cm^-2.


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