{
  "bibcode": "2013GCN.15123....1P",
  "body": "K.L. Page, P.A. Evans (U. Leicester), V. D'Elia (ASDC), J. Gelbord (Eureka\nScientific) and J.L. Racusin (NASA/GSFC) report on behalf of the Swift-XRT\nteam:\n\nSwift perfomed a Target of Opportunity observation of the Fermi GBM and\nLAT-detected GRB 130821A (GCN Circ. 15113, 15115), between 100.7 and 136.5\nks after the trigger. Within the LAT error circle (0.1 degrees at 1-sigma;\nstatistical error only), in 9.9 ks of data there is a weak source with a\ncount rate of (1.3 +0.4/-0.3) x 10^-3 count s^-1 with a position of RA,\nDec 314.1601, -12.043481, which is equivalent to\nRA(J2000)  =  20h 56m 38.44s\nDec(J2000) = -12d 02' 36.53\"\n(5.5 arcsec uncertainty, 90% containment). We cannot determine whether the\nsource is fading.\n\nWe note, however, that in a ~10 ks observation faint background sources\nare expected, and there are two other detections (one uncatalogued, one a\npossible quasar) of a similar count rate outside the LAT 1-sigma error\ncircle. The logN-logS work by Mateos et al. (2008, A&A, 492, 51) indicate\nwe would expect to find around one source of this brightness within the\n~0.03 deg^2 of the LAT error circle, while Elvis et al. (2009, ApJS, 184,\n158) provide a slightly higher sky density, suggesting up to 3 sources\nmight be expected. It is therefore likely that the source detected in this\nXRT observation is a serendipitous background object.\n\nExcluding this possible, weak source, the 3-sigma upper limit on the XRT\ncount rate within the LAT error circle is 1.8 x 10^-3 count s^-1\n(equivalent to an observed flux of 7.3 x 10^-14 erg cm^-2 s^-1 over 0.3-10\nkeV, assuming a typical counts to flux conversion factor of 4 x 10^-11 erg\ncm^-2 count^-1).\n\nWe also note that, within the XRT field of view, although outside the LAT\n1-sigma error circle, there is an X-ray source with a UVOT-enhanced\nposition of RA, Dec 313.92706, -11.96634, which is equivalent to\nRA(J2000)  =  20h 55m 42.49s\nDec(J2000) = -11d 57' 58.8\"\n(2.3 arcsec uncertainty, 90% containment). This position is 4 arcsec away\nfrom the catalogued XMM Slew Survey object XMMSL1 J205542.2-115756. The\n1-sigma uncertainty on the positions in the slew catalogue is 8 arcsec;\nthus, this X-ray source is consistent with being XMMSL1 J205542.2-115756\nand is unrelated to GRB 130821A.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 15123,
  "createdOn": 1377335085000,
  "email": "klp5@leicester.ac.uk",
  "subject": "GRB 130821A: Swift-XRT observation",
  "submitter": "Kim Page at U.of Leicester  <klp5@leicester.ac.uk>",
  "eventId": "GRB 130821A"
}