TITLE: GCN CIRCULAR NUMBER: 22413 SUBJECT: GRB 180210A: Swift-XRT afterglow detection DATE: 18/02/12 14:45:50 GMT FROM: Valerio D'Elia at ASDC A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), S.L. Gibson (U. Leicester) 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 180210A (Dirirsa et al. GCN Circ. 22408), collecting 7.5 ks of Photon Counting (PC) mode data between T0+31.0 ks and T0+117.4 ks. Four uncatalogued X-ray sources are detected, of which one ("Source 1", the brightest) is fading with 3-sigma significance, and is therefore likely the GRB afterglow. Using 4922 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 = 1.84177, +18.55274 which is equivalent to: RA (J2000): 00h 07m 22.02s Dec(J2000): +18d 33' 09.9" with an uncertainty of 1.7 arcsec (radius, 90% confidence). This position is 12.4 arcmin from the Fermi/LAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.57 (+0.36, -0.28). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.3 (+/-0.4). The best-fitting absorption column is 3.6 (+1.7, -1.4) x 10^21 cm^-2, in excess of the Galactic value of 3.7 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 2.7 x 10^-11 (5.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.6 (+1.7, -1.4) x 10^21 cm^-2 Galactic foreground: 3.7 x 10^20 cm^-2 Excess significance: 3.7 sigma Photon index: 2.3 (+/-0.4) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020787. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020787. This circular is an official product of the Swift-XRT team.