TITLE: GCN CIRCULAR NUMBER: 25251 SUBJECT: GRB 190731A: Swift-XRT afterglow detection DATE: 19/08/01 21:24:27 GMT FROM: Andy Beardmore at U Leicester J.P. Osborne (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (ASDC), J. D. Gropp (PSU), A. Tohuvavohu (PSU), J.A. Kennea (PSU), K.L. Page (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 190731A (Ohno et al. GCN Circ. 25244), collecting 5.0 ks of Photon Counting (PC) mode data between T0+33.4 ks and T0+62.9 ks. Eleven uncatalogued X-ray sources are detected, of which one ("Source 1") is fading with 3-sigma significance, and is therefore likely the GRB afterglow. Using 1953 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 = 339.72015, -76.57744 which is equivalent to: RA (J2000): 22h 38m 52.84s Dec(J2000): -76d 34' 38.8" with an uncertainty of 2.2 arcsec (radius, 90% confidence). This position is 4.0 arcmin from the Fermi/LAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.3 (+0.6, -0.7). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.5 (+/-0.5). The best-fitting absorption column is 5.9 (+2.8, -2.2) x 10^21 cm^-2, in excess of the Galactic value of 9.8 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 3.3 x 10^-11 (7.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 5.9 (+2.8, -2.2) x 10^21 cm^-2 Galactic foreground: 9.8 x 10^20 cm^-2 Excess significance: 3.6 sigma Photon index: 2.5 (+/-0.5) If the light curve continues to decay with a power-law decay index of 1.3, the count rate at T+24 hours will be 0.015 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.9 x 10^-13 (1.2 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/ToO_GRBs/00020914/Source1.php. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00020914. This circular is an official product of the Swift-XRT team.