TITLE: GCN CIRCULAR NUMBER: 21103 SUBJECT: GRB 170510A: Swift-XRT afterglow detection DATE: 17/05/18 15:59:27 GMT FROM: Antonino D'Ai at IASF-PA A. Cholden-Brown (PSU), S. J. LaPorte (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), 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 Fermi/LAT-detected burst GRB 170510A (Bissaldi et al. GCN Circ. 21080). The observations now extend from T0+24.7 ks to T0+713.5 ks, and are entirely in Photon Counting (PC) mode. The source previously reported by Melandri et al. (GCN Circ. 21086), "Source 1", is fading with 3-sigma significance, and is therefore likely the GRB afterglow. Using 1264 s of PC mode data and 4 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 160.15394, -39.21244 which is equivalent to: RA (J2000): 10h 40m 36.95s Dec(J2000): -39d 12' 44.8" with an uncertainty of 2.3 arcsec (radius, 90% confidence). This position is 13.2 arcmin from the Fermi/LAT position. The source is fading with alpha >0.9. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.8, -0.4). The best-fitting absorption column is consistent with the Galactic value of 6.1 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 3.2 x 10^-11 (3.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6 (+/-24) x 10^20 cm^-2 Galactic foreground: 6.1 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.8 (+0.8, -0.4) The results of the XRT-team automatic analysis of the likely afterglow are at http://www.swift.ac.uk/xrt_products/TILED_GRB00067/Source1.php. The results of the full analysis of the tiled XRT observations are available at http://www.swift.ac.uk/xrt_products/TILED_GRB00067. Swift/UVOT observed "Source 1" at T0+696 ks. We did not find an optical counterpart. We measured a 3 sigma upper limit >21 mag in the UVW1 filter, using an exposure of 1671 s. This circular is an official product of the Swift-XRT and UVOT teams.