TITLE: GCN CIRCULAR NUMBER: 19558 SUBJECT: GRB 160623A: Swift-XRT afterglow detection DATE: 16/06/23 21:03:42 GMT FROM: Alessandro Maselli at INAF/IASF Palermo B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), 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 Fermi/LAT-detected burst GRB 160623A (Vianello et al. GCN Circ. 19553), collecting 1.9 ks of Photon Counting (PC) mode data between T0+40.3 ks and T0+45.4 ks. Three uncatalogued X-ray sources are detected, of which one ("Source 1") is above the RASS limit, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=315.2967, +42.2205 which is equivalent to: RA (J2000): 21:01:11.22 Dec(J2000): +42:13:13.7 with an uncertainty of 3.5 arcsec (radius, 90% confidence). This position is 2.5 arcmin from the Fermi/LAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=2.3 (+0.5, -1.4). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.79 (+0.21, -0.20). The best-fitting absorption column is 1.69 (+0.32, -0.29) x 10^22 cm^-2, in excess of the Galactic value of 7.2 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 6.3 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.69 (+0.32, -0.29) x 10^22 cm^-2 Galactic foreground: 7.2 x 10^21 cm^-2 Excess significance: 5.5 sigma Photon index: 1.79 (+0.21, -0.20) If the light curve continues to decay with a power-law decay index of 2.3, the count rate at T+24 hours will be 0.12 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.6 x 10^-12 (1.4 x 10^-11) erg cm^-2 s^-1. The results of the XRT-team automatic analysis of the likely afterglow are at http://www.swift.ac.uk/ToO_GRBs/00020666/index_1.php. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020666. This circular is an official product of the Swift-XRT team.