TITLE: GCN CIRCULAR NUMBER: 30023 SUBJECT: GRB 210515C: Swift-XRT refined Analysis DATE: 21/05/16 07:05:16 GMT FROM: Phil Evans at U of Leicester J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), A.P. Beardmore (U. Leicester) and A. D'Ai report on behalf of the Swift-XRT team: We have analysed 7.0 ks of XRT data for GRB 210515C (D'Ai et al. GCN Circ. 30020), from 76 s to 29.8 ks after the BAT trigger. The data comprise 98 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. Using 1614 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 = 1.09417, -14.95334 which is equivalent to: RA (J2000): 00h 04m 22.60s Dec(J2000): -14d 57' 12.0" with an uncertainty of 2.3 arcsec (radius, 90% confidence). The light curve can be modelled with an initial power-law decay with an index of alpha=2.63 (+0.13, -0.12), followed by a break at T+853 s to an alpha of 0.57 (+/-0.14). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.06 (+0.24, -0.21). The best-fitting absorption column is 1.7 (+/-0.4) x 10^21 cm^-2, in excess of the Galactic value of 2.6 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.11 (+0.26, -0.23) and a best-fitting absorption column of 6.4 (+5.7, -3.8) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.1 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6.4 (+5.7, -3.8) x 10^20 cm^-2 Galactic foreground: 2.6 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 2.11 (+0.26, -0.23) If the light curve continues to decay with a power-law decay index of 0.57, the count rate at T+24 hours will be 3.8 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x 10^-13 (1.4 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/01048618. This circular is an official product of the Swift-XRT team.