[vsnet-grb-info 2204] GRB050925: Swift/XRT limits

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4043
SUBJECT: GRB050925: Swift/XRT limits
DATE:    05/09/27 06:48:40 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

A.P. Beardmore (U. Leicester), K.L. Page (U. Leicester), N. Gehrels (GSFC),
J. Greiner (MPE), J. Kennea (PSU),  J. Nousek (PSU), J.P. Osborne (U.
Leicester) and G. Tagliaferri (INAF-OAB) report on behalf of the Swift/XRT
team:

We have analysed 27.8 ks of XRT data for the BAT soft short burst (trigger
156838; GCN 4034, Holland et al.), starting 99.5 s after the BAT trigger.
No fading X-ray source is found within the refined BAT error circle (GCN
4037, Markwardt et al.). However, there exists a source with a constant
count rate of 0.0015+/-0.0004 counts s^-1 at RA(2000) = 20 13 48.0,
Dec(J2000) = +34 19 53.7 (6.4 arcsec radius uncertainty at 90%
containment) within the BAT error circle, coincident with a B=13.0, R=11.6
magnitude USNO object (GSC0267902398 in the Guide Star Catalogue).

The spectrum of this source is best modelled by either a blackbody (with a
temperature of 0.19 +0.10/-0.05 keV, column density < 0.3E22 cm^-2 and an
unabsorbed 0.2-10.0 keV flux of 7.0E-14 ergs cm^-2 s^-1) or a thermal
plasma Mekal model (with a temperature of 0.87 +0.41/-0.44 keV, column
density < 0.2E22 cm^-2, an abundance consistent with solar and an
unabsorbed 0.2-10.0 keV flux of 4.6E-14 ergs cm^-2 s^-1).  We note the
observed column density upper limit to this source is significantly less
than the Galactic value of 1.1E22 cm^-22 (Dickey & Lockman, 1990),
suggesting it is nearby and unlikely to be a GRB.

Except for the XRT source coincident with the USNO object, there are
no sources detected within the BAT error circle.  The 3 sigma upper
limit on the count-rate is 0.0006 counts s^-1, corresponding to an
unabsorbed 0.2-10 keV flux of 3.0E-14 ergs cm^-2 s^-1 assuming a
Crab-like spectrum.