TITLE: GCN CIRCULAR NUMBER: 40394 SUBJECT: GRB 250506A: Swift-XRT afterglow detection DATE: 25/05/08 08:42:38 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), M. A. Williams (PSU), S. Dichiara (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has conducted further observations of the field of the SVOM/MXT-detected burst GRB 250506A. The observations now extend from T0+63.4 ks to T0+138.3 ks and have a total exposure time of 6.3 ks. The source previously reported, "Source 1", is fading with 2.8 sigma significance and thus is believed to be the GRB afterglow. Using 2772 s of PC mode data and 2 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 = 219.28269, +28.87835 which is equivalent to: RA (J2000): 14h 37m 07.85s Dec(J2000): +28d 52' 42.0" with an uncertainty of 2.6 arcsec (radius, 90% confidence). This position is 84 arcsec from the SVOM/MXT position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.7 (+0.5, -0.4). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.74 (+0.34, -0.21). The best-fitting absorption column is 2.1 (+7.8, -0.4) x 10^20 cm^-2, consistent with the Galactic value of 1.6 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 3.8 x 10^-11 (3.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.1 (+7.8, -0.4) x 10^20 cm^-2 Galactic foreground: 1.6 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.74 (+0.34, -0.21) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021828. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021828. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/40394. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...