TITLE: GCN CIRCULAR NUMBER: 22984 SUBJECT: GRB 180720B: Swift-XRT refined Analysis DATE: 18/07/21 02:17:13 GMT FROM: Phil Evans at U of Leicester K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.A. Kennea (PSU), P.A. Evans (U. Leicester) and M.H. Siegel report on behalf of the Swift-XRT team: We have analysed 7.2 ks of XRT data for GRB 180720B (Siegel et al. GCN Circ. 22973), from 90 s to 19.4 ks after the BAT trigger. The data comprise 3.3 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 0.5286, -2.9189 which is equivalent to: RA (J2000): 00 02 06.86 Dec(J2000): -02 55 08.1 with an uncertainty of 3.5 arcsec (radius, 90% confidence). The late-time light curve (from T0+5.6 ks) can be modelled with an initial power-law decay with an index of alpha=1.18 (+0.16, -0.28), followed by a break at T+7030 s to an alpha of -0.17 (+0.26, -1.33). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.770 (+/-0.011). The best-fitting absorption column is 1.42 (+/-0.04) x 10^21 cm^-2, in excess of the Galactic value of 3.9 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.85 (+/-0.07) and a best-fitting absorption column of 1.75 (+0.23, -0.22) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (4.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.75 (+0.23, -0.22) x 10^21 cm^-2 Galactic foreground: 3.9 x 10^20 cm^-2 Excess significance: 10.1 sigma Photon index: 1.85 (+/-0.07) If the light curve continues to decay with a power-law decay index of -0.17, the count rate at T+24 hours will be 21 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.4 x 10^-10 (1.1 x 10^-9) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00848890. This circular is an official product of the Swift-XRT team. [GCN OPS NOTE(21jul18): In response to GCN Circular 22986, this archived copy of 22984 has been modified with the correct GRB maining "A" --> "B".]