TITLE: GCN CIRCULAR NUMBER: 42445 SUBJECT: GRB 251025B: Swift-XRT afterglow detection DATE: 25/10/25 19:52:19 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 251025B. We searched for X-ray sources in 1.7 ks of Photon Counting (PC) mode data obtained between T0+2.8 ks and T0+4.5 ks. An uncatalogued X-ray source is detected within the estimated 3-sigma SVOM/ECLAIRs error region (473 arcsec) and is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. Using 1712 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 29.53107, +30.30373 which is equivalent to: RA (J2000): 01h 58m 07.46s Dec(J2000): +30d 18' 13.4" with an uncertainty of 2.7 arcsec (radius, 90% confidence). This source is 1.25 arcsec from the SVOM/C-GFT position (Wu et al., GCN Circ. 42438) and 37 arcsec from the SVOM/MXT position (Hussein et al., GCN Circ. 42437). At present, the X-ray source shows no evidence of fading. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.00 (+0.27, -0.25). The best-fitting absorption column is 1.9 (+0.9, -0.8) x 10^21 cm^-2, in excess of the Galactic value of 5.6 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 3.5 x 10^-11 (4.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.9 (+0.9, -0.8) x 10^21 cm^-2 Galactic foreground: 5.6 x 10^20 cm^-2 Excess significance: 2.8 sigma Photon index: 2.00 (+0.27, -0.25) The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/03000153. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00044. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/42445. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...