TITLE: GCN CIRCULAR NUMBER: 29750 SUBJECT: GRB 210402A: Swift-XRT refined Analysis DATE: 21/04/03 12:53:41 GMT FROM: Phil Evans at U of Leicester P.A. Evans (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), J.P. Osborne (U. Leicester) and K.L. Page report on behalf of the Swift-XRT team: We have analysed 6.0 ks of XRT data for GRB 210402A (Page et al. GCN Circ. 29743), from 160 s to 46.0 ks after the BAT trigger. The data comprise 1.1 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. Using 1629 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 = 198.25927, +44.51454 which is equivalent to: RA (J2000): 13h 13m 02.22s Dec(J2000): +44d 30' 52.3" with an uncertainty of 2.0 arcsec (radius, 90% confidence). The late-time light curve (from T0+5.4 ks) can be modelled with a power-law decay with a decay index of alpha=1.03 (+0.21, -0.27). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.811 (+0.027, -0.026). The best-fitting absorption column is 3.25 (+/-0.13) x 10^21 cm^-2, in excess of the Galactic value of 1.5 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.09 (+0.20, -0.19) and a best-fitting absorption column of 3.2 (+/-0.8) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (5.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.2 (+/-0.8) x 10^21 cm^-2 Galactic foreground: 1.5 x 10^20 cm^-2 Excess significance: 6.5 sigma Photon index: 2.09 (+0.20, -0.19) If the light curve continues to decay with a power-law decay index of 1.03, the count rate at T+24 hours will be 0.017 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.0 x 10^-13 (9.4 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/01040123. This circular is an official product of the Swift-XRT team.