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GCN Circular 12364

Subject
GRB 110918A: Swift-XRT observations
Date
2011-09-20T13:32:36Z (12 years ago)
From
Vanessa Mangano at INAF-IASFPA <vanessa@ifc.inaf.it>
V. Mangano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/IASFPA), P. A. Evans
(U Leicester), H. A. Krimm (GSFC/USRA)	report on behalf of the
Swift-XRT team:

We have analysed 2.5 ks of XRT data for the IPN-detected burst: GRB
110918A (Hurley et al. et al. GCN Circ. 12357), from 107.4 ks to 113.6
ks after the  IPN trigger. The data are entirely in Photon Counting
(PC) mode.  An new X-ray source is detected close to the edge of the
XRT FoV. Using 2498 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 = 32.53860, -27.10610
which is equivalent to:

RA (J2000): 02h 10m 9.27s
Dec(J2000): -27d 06' 22.0"

with an uncertainty of 1.5 arcsec (radius, 90% confidence). This
position is 640 arcsec from the IPN position, and outside the IPN error
box.  The light curve uses only the first 1.5 ks of data, since after
this the source is too near the edge of the field of view for the light
curve extraction tools to work. This light curve is consistent with a
constant source of mean count rate 2.7e-01 ct/sec. A power-law fit
gives an index of -1.320 (+3.525, -0.013). The source count rate is
definitely above the 3 sigma upper limit of 0.08 ct/sec obtained from
the analysis of RASS data.
We believe that such a bright uncatalogued X-ray source close to the
IPN error box is likely the afterglow of the GRB.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.12 (+0.28, -0.23). The
best-fitting absorption column is  1.5 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 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 2.9 x 10^-11 (4.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.5 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 3.9 sigma
Photon index:	     2.12 (+0.28, -0.23)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020186.

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