TITLE: GCN CIRCULAR NUMBER: 29942 SUBJECT: GRB 210504A: Swift-XRT refined Analysis DATE: 21/05/05 04:52:24 GMT FROM: Phil Evans at U of Leicester D.N. Burrows (PSU), J. D. Gropp (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. Melandri (INAF-OAB), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB) and A.P. Beardmore report on behalf of the Swift-XRT team: We have analysed 6.7 ks of XRT data for GRB 210504A (Beardmore et al. GCN Circ. 29929), from 218 s to 24.9 ks after the BAT trigger. The data comprise 146 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. Using 2804 s of PC mode data and 3 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 = 222.39118, -30.53345 which is equivalent to: RA (J2000): 14h 49m 33.88s Dec(J2000): -30d 32' 00.4" with an uncertainty of 2.0 arcsec (radius, 90% confidence). The late-time light curve (from T0+11.6 ks) can be modelled with a power-law decay with a decay index of alpha=1.9 (+/-0.8). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.00 (+0.13, -0.11). The best-fitting absorption column is 1.50 (+0.41, -0.20) x 10^21 cm^-2, consistent with the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.11 (+0.25, -0.16) and a best-fitting absorption column of 1.35 (+0.75, -0.06) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.1 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.35 (+0.75, -0.06) x 10^21 cm^-2 Galactic foreground: 1.3 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 2.11 (+0.25, -0.16) If the light curve continues to decay with a power-law decay index of 1.9, the count rate at T+24 hours will be 1.1 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.3 x 10^-14 (4.4 x 10^-14) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01046782. This circular is an official product of the Swift-XRT team.