GCN Circular 17866
A.P. Beardmore (U. Leicester), A. Amaral-Rogers (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'a��� (INAF-IASFPA), D.N. Burrows (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Fermi/LAT-detected burst GRB 150523A collecting 1.4 ks of Photon Counting (PC) mode data between T0+33.0 ks and T0+34.3 ks. An uncatalogued X-ray source is detected and is above the RASS limit, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=115.2859, -45.4209 which is equivalent to: RA (J2000): 07:41:8.62 Dec(J2000): -45:25:15.1 with an uncertainty of 3.7 arcsec (radius, 90% confidence). This position is 3.4 arcmin from the Fermi/LAT position. The light curve is consistent with a constant source of mean count rate 1.2e-01 ct/sec. A power-law fit gives an index of -1.41 (+4.59, -0.05). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.71 (+0.29, -0.21). The best-fitting absorption column is consistent with the Galactic value of 3.5 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.5 x 10^-11 (5.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.5 (+/-1.3) x 10^21 cm^-2 Galactic foreground: 3.5 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.71 (+0.29, -0.21) The results of the XRT-team automatic analysis of the likely afterglow are at http://www.swift.ac.uk/ToO_GRBs/00020520/index_1.php. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020520. This circular is an official product of the Swift-XRT team.