TITLE: GCN CIRCULAR NUMBER: 17827 SUBJECT: GRB 150518A: Swift-XRT afterglow detection DATE: 15/05/19 10:18:56 GMT FROM: Claudio Pagani at U of Leicester B. Sbarufatti (INAF-OAB/PSU), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), D.N. Burrows (PSU), J.A. Kennea (PSU) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the MAXI-detected burst GRB 150518A (Kawamuro et al. GCN Circ. 17825) in a series of observations tiled on the sky. The total exposure time is 2.4 ks, distributed over 7 tiles; the maximum exposure at a single sky location was 742 s. The data were collected between T0+10.4 ks and T0+20.2 ks, and are entirely in Photon Counting (PC) mode. 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=234.2006, +16.3291 which is equivalent to: RA (J2000): 15:36:48.14 Dec(J2000): +16:19:44.9 with an uncertainty of 3.8 arcsec (radius, 90% confidence). This position is 8.2 arcmin from the MAXI position. The light curve is consistent with a constant source of mean count rate 4.0e-01 ct/sec. A power-law fit gives an index of 0.4 (+0.7, -0.9). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 3.6 (+1.0, -0.8). The best-fitting absorption column is 8.2 (+4.0, -3.2) x 10^21 cm^-2, in excess of the Galactic value of 3.4 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.2 x 10^-11 (2.1 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 8.2 (+4.0, -3.2) x 10^21 cm^-2 Galactic foreground: 3.4 x 10^20 cm^-2 Excess significance: 4.1 sigma Photon index: 3.6 (+1.0, -0.8) The results of the XRT-team automatic analysis of the likely afterglow are at http://www.swift.ac.uk/xrt_products/TILED_GRB00044/index_1.php. The results of the full analysis of the tiled XRT observations are available at http://www.swift.ac.uk/xrt_products/TILED_GRB00044. This circular is an official product of the Swift-XRT team.