TITLE: GCN CIRCULAR NUMBER: 41746 SUBJECT: GRB 250903A: Swift-XRT afterglow detection DATE: 25/09/08 09:47:11 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-OAR), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Perri (SSDC & INAF-OAR) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 250903A. We searched for X-ray sources in 2.4 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 6.3 ks, obtained between T0+12.7 ks and T0+344.1 ks. Four uncatalogued X-ray sources are detected within the estimated 3-sigma SVOM/ECLAIRs error region (509 arcsec), of which one ("Source 1") is fading with >3-sigma significance and its position is consistent with the optical afterglow (An et al., GCN 41679). Therefore, this is the GRB afterglow. Using 2370 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 = 356.79798, -75.96644 which is equivalent to: RA (J2000): 23h 47m 11.52s Dec(J2000): -75d 57' 59.2" with an uncertainty of 3.0 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=0.9 (+/-0.9). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.95 (+0.73, -0.21). The best-fitting absorption column is consistent with the Galactic value of 4.7 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 3.4 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4 (+/-10) x 10^20 cm^-2 Galactic foreground: 4.7 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.95 (+0.73, -0.21) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000070. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00034. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41746. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...