TITLE: GCN CIRCULAR NUMBER: 17917 SUBJECT: GRB 150608A: Swift-XRT afterglow detection DATE: 15/06/08 22:33:22 GMT FROM: Maria Grazia Bernardini at INAF/Brera M.G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), A. Melandri (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), A. Amaral-Rogers (U. Leicester), J.P. Osborne (U. Leicester), A. D'ai (INAF-IASFPA), A. Maselli (INAF-IASFPA), D.N. Burrows (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 150608A (Arimoto et al. GCN Circ. 17912) in a series of observations tiled on the sky. The total exposure time is 3.1 ks, distributed over 7 tiles; the maximum exposure at a single sky location was 955 s. The observations started at T0+8.0 ks, and are entirely in Photon Counting (PC) mode. An uncatalogued X-ray source is detected in the data collected between T0+9.1 ks and T0+14.7 ks. This source is above the RASS limit and is therefore likely the GRB afterglow. The position of this source is RA, Dec=13.1663, -0.6442 which is equivalent to: RA (J2000): 00:52:39.90 Dec(J2000): -00:38:39.0 with an uncertainty of 3.6 arcsec (radius, 90% confidence). This position is 29.1 arcmin from the MAXI position. The light curve is consistent with a constant source of mean count rate 8.3e-01 ct/sec. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.78 (+0.28, -0.26). The best-fitting absorption column is 9.8 (+8.3, -6.0) x 10^20 cm^-2, consistent with the Galactic value of 3.8 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 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 9.8 (+8.3, -6.0) x 10^20 cm^-2 Galactic foreground: 3.8 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.78 (+0.28, -0.26) The results of the XRT-team automatic analysis of the likely afterglow are at http://www.swift.ac.uk/xrt_products/TILED_GRB00045/index_2.php. The results of the full analysis of the tiled XRT observations are available at http://www.swift.ac.uk/xrt_products/TILED_GRB00045. This circular is an official product of the Swift-XRT team.