GCN Circular 15474

M. C. Stroh and J. A. Kennea (PSU) report on behalf of the Swift-XRT
team:

We have analysed 7.9 ks of XRT data for the Fermi/LAT-detected burst:
GRB 131108A,  from 25.9 ks to 49.8 ks after the  Fermi/LAT trigger. The
data are entirely in Photon Counting (PC) mode.  An X-ray source is
detected within the Fermi/LAT error circle. Using 4560 s of PC mode
data and 6 UVOT images, we find an enhanced XRT position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue): RA, Dec = 156.50178, +9.66248 which is equivalent to:

RA (J2000): 10h 26m 00.43s
Dec(J2000): +09d 39' 44.9"

with an uncertainty of 1.6 arcsec (radius, 90% confidence). This
position is 14.4 arcmin from the center of the Fermi/LAT error circle
(GCN #15464), consistant with that position, and inside the   
IPN error box (GCN #15466).

The light curve can be modelled with  a power-law decay with a decay
index of alpha=1.9 (+0.7, -0.6).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+0.4, -0.3). The
best-fitting absorption column is  9.9 (+7.6, -6.5) x 10^20 cm^-2, in
excess of the Galactic value of 3.2 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 3.2 x 10^-11 (4.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.9 (+7.6, -6.5) x 10^20 cm^-2
Galactic foreground: 3.2 x 10^20 cm^-2
Excess significance: 1.7 sigma
Photon index:	     2.2 (+0.4, -0.3)

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 5.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.4 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/00020326.

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