TITLE: GCN CIRCULAR NUMBER: 22685 SUBJECT: GRB 180504A: Swift-XRT refined Analysis DATE: 18/05/05 08:09:42 GMT FROM: Phil Evans at U of Leicester V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), S. J. LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), A. Melandri (INAF-OAB) and A.Y. Lien report on behalf of the Swift-XRT team: We have analysed 12 ks of XRT data for GRB 180504A (Lien et al. GCN Circ. 22682), from 98 s to 30.5 ks after the BAT trigger. The data comprise 163 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 331.1442, -14.6583 which is equivalent to: RA (J2000): 22 04 34.61 Dec(J2000): -14 39 30.0 with an uncertainty of 2.0 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=0.7 (+nan, -nan). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.80 (+/-0.11). The best-fitting absorption column is 6.9 (+3.2, -2.8) x 10^20 cm^-2, in excess of the Galactic value of 3.5 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.86 (+0.25, -0.23) and a best-fitting absorption column of 1.5 (+0.8, -0.7) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (4.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.5 (+0.8, -0.7) x 10^21 cm^-2 Galactic foreground: 3.5 x 10^20 cm^-2 Excess significance: 2.7 sigma Photon index: 1.86 (+0.25, -0.23) If the light curve continues to decay with a power-law decay index of 0.7, the count rate at T+24 hours will be 7.4 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x 10^-13 (3.4 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00830822. This circular is an official product of the Swift-XRT team.