GCN Circular 4043

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
Not found
 
 
, 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
Not found
 
 
, 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.