GCN Circular 15123

K.L. Page, P.A. Evans (U. Leicester), V. D'Elia (ASDC), J. Gelbord (Eureka
Scientific) and J.L. Racusin (NASA/GSFC) report on behalf of the Swift-XRT
team:

Swift perfomed a Target of Opportunity observation of the Fermi GBM and
LAT-detected GRB 130821A (GCN Circ. 15113
Not found
 
 
, 15115
Not found
 
 
), between 100.7 and 136.5
ks after the trigger. Within the LAT error circle (0.1 degrees at 1-sigma;
statistical error only), in 9.9 ks of data there is a weak source with a
count rate of (1.3 +0.4/-0.3) x 10^-3 count s^-1 with a position of RA,
Dec 314.1601, -12.043481, which is equivalent to
RA(J2000)  =  20h 56m 38.44s
Dec(J2000) = -12d 02' 36.53"
(5.5 arcsec uncertainty, 90% containment). We cannot determine whether the
source is fading.

We note, however, that in a ~10 ks observation faint background sources
are expected, and there are two other detections (one uncatalogued, one a
possible quasar) of a similar count rate outside the LAT 1-sigma error
circle. The logN-logS work by Mateos et al. (2008, A&A, 492, 51) indicate
we would expect to find around one source of this brightness within the
~0.03 deg^2 of the LAT error circle, while Elvis et al. (2009, ApJS, 184,
158) provide a slightly higher sky density, suggesting up to 3 sources
might be expected. It is therefore likely that the source detected in this
XRT observation is a serendipitous background object.

Excluding this possible, weak source, the 3-sigma upper limit on the XRT
count rate within the LAT error circle is 1.8 x 10^-3 count s^-1
(equivalent to an observed flux of 7.3 x 10^-14 erg cm^-2 s^-1 over 0.3-10
keV, assuming a typical counts to flux conversion factor of 4 x 10^-11 erg
cm^-2 count^-1).

We also note that, within the XRT field of view, although outside the LAT
1-sigma error circle, there is an X-ray source with a UVOT-enhanced
position of RA, Dec 313.92706, -11.96634, which is equivalent to
RA(J2000)  =  20h 55m 42.49s
Dec(J2000) = -11d 57' 58.8"
(2.3 arcsec uncertainty, 90% containment). This position is 4 arcsec away
from the catalogued XMM Slew Survey object XMMSL1 J205542.2-115756. The
1-sigma uncertainty on the positions in the slew catalogue is 8 arcsec;
thus, this X-ray source is consistent with being XMMSL1 J205542.2-115756
and is unrelated to GRB 130821A.

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