TITLE: GCN CIRCULAR NUMBER: 18629 SUBJECT: GRB 151120A: Swift-XRT afterglow detection DATE: 15/11/20 20:16:45 GMT FROM: Sarah Gibson at U.of Leicester A.P. Beardmore (U. Leicester), A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R. Roegiers (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 INTEGRAL-detected burst GRB 151120A (Mereghetti et al. GCN Circ. 18623), collecting 3.9 ks of Photon Counting (PC) mode data between T0+16.4 ks and T0+23.6 ks. An uncatalogued X-ray source is detected inside or close to the INTEGRAL error region and is above the RASS limit, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=157.2410, -32.5262 which is equivalent to: RA (J2000): 10:28:57.83 Dec(J2000): -32:31:34.2 with an uncertainty of 3.6 arcsec (radius, 90% confidence). This position is 29 arcsec from the INTEGRAL position and 1.6 arcsec from the afterglow detected by GROND (GCN 18624). The light curve can be modelled with a power-law decay with a decay index of alpha=2.3 (+1.0, -0.9). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.00 (+0.31, -0.28). The best-fitting absorption column is 2.7 (+1.3, -1.1) x 10^21 cm^-2, in excess of the Galactic value of 9.0 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 3.7 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.7 (+1.3, -1.1) x 10^21 cm^-2 Galactic foreground: 9.0 x 10^20 cm^-2 Excess significance: 2.8 sigma Photon index: 2.00 (+0.31, -0.28) 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 2.1 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.5 x 10^-14 (1.1 x 10^-13) 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/00020559/index_1.php. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020559. This circular is an official product of the Swift-XRT team.