TITLE: GCN CIRCULAR NUMBER: 22462 SUBJECT: GRB 180305A: Swift-XRT afterglow detection DATE: 18/03/06 15:38:42 GMT FROM: Boris Sbarufatti at PSU P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA) 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 180305A (Axelsson et al. GCN Circ. 22457), collecting 4.7 ks of Photon Counting (PC) mode data between T0+30.6 ks and T0+53.0 ks. Four uncatalogued X-ray sources are detected, of which one ("Source 2") is above the RASS limit and fading with 3-sigma significance, and is therefore likely the GRB afterglow. Using 3574 s of PC mode data and 6 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 = 49.61821, +32.10994 which is equivalent to: RA (J2000): 03h 18m 28.37s Dec(J2000): +32d 06' 35.8" with an uncertainty of 1.7 arcsec (radius, 90% confidence). This position is 2.7 arcmin from the Fermi/LAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=2.7 (+0.5, -0.8). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.5 (+/-0.4). The best-fitting absorption column is 4.7 (+3.2, -2.3) x 10^21 cm^-2, consistent with the Galactic value of 2.4 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 5.4 x 10^-11 (6.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.7 (+3.2, -2.3) x 10^21 cm^-2 Galactic foreground: 2.4 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.5 (+/-0.4) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020789. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020789. This circular is an official product of the Swift-XRT team.