TITLE: GCN CIRCULAR NUMBER: 15123 SUBJECT: GRB 130821A: Swift-XRT observation DATE: 13/08/24 09:04:45 GMT FROM: Kim Page at U.of Leicester 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, 15115), 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.