GCN Circular 16848

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

We have analysed 2.5 ks of XRT data for the Fermi/LAT-detected burst:
GRB 140928A (Desiante et al.,  GCN Circ. 16847
Not found
 
 
) from 40.7 ks to 52.7 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 2472 s of PC mode data and 2 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 = 43.69878, -55.92883
which is equivalent to:

RA (J2000): 02h 54m 47.71s
Dec(J2000): -55d 55' 43.8"

with an uncertainty of 1.5 arcsec (radius, 90% confidence). This
position is 588 arcsec from the Fermi/LAT position.  The light curve is
consistent with a constant source of mean count rate 1.9e-01 ct/sec. 

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.83 (+0.26, -0.24). The
best-fitting absorption column is  2.4 (+1.0, -0.9) x 10^21 cm^-2, in
excess of the Galactic value of 1.6 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.3 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+1.0, -0.9) x 10^21 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: 4.2 sigma
Photon index:	     1.83 (+0.26, -0.24)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020418.

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