TITLE: GCN CIRCULAR NUMBER: 41359 SUBJECT: GRB 250813B: Swift-XRT afterglow detection DATE: 25/08/14 08:09:11 GMT FROM: K.L. Page at U Leicester <klp5@leicester.ac.uk> K.L. Page, A.P. Beardmore and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: We have analysed 1.6 ks of XRT data for GRB 250813B, from 9.6 to 16.3 ks after the SVOM/ECLAIRs trigger (GCN Circ. 41354). The data are entirely in Photon Counting (PC) mode. We find an uncatalogued X-ray source at RA, Dec = 336.7505, +12.4613 which is equivalent to: RA (J2000): 22 27 00.13 Dec(J2000): +12 27 40.5 with an uncertainty of 3.7 arcsec (radius, 90% confidence). There is no evidence for fading during this interval, with the X-ray source showing a mean count rate of 0.17 +/- 0.02 count s^-1. However, the localisation of this source is consistent with the fading optical afterglow found by Ondrejov D50 (GCN Circ. 41353, 41355) and SVOM/COLIBRI (GCN Circ. 41356), and so we consider it the X-ray afterglow. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.1 (+0.4, -0.3). The best-fitting absorption column is 2.5 (+1.3, -1.1) × 10^21 cm^-2, in excess of the Galactic value of 6.2 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.2 x 10^-12 (9.3 x 10^-12) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.5 (+1.3, -1.1) x 10^21 cm^-2 Galactic foreground: 6.2 x 10^20 cm^-2 Photon index: 2.1 (+0.4, -0.3) This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41359. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...