TITLE: GCN CIRCULAR NUMBER: 42611 SUBJECT: GRB 251106A: Swift-XRT afterglow detection DATE: 25/11/07 10:12:12 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (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 251106A. We searched for X-ray sources in 2.0 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 2.0 ks, obtained between T0+4.0 ks and T0+9.9 ks. Three uncatalogued X-ray sources are detected, of which one ("Source 1") is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. Using 2020 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 = 234.46260, +63.36779 which is equivalent to: RA (J2000): 15h 37m 51.02s Dec(J2000): +63d 22' 04.0" with an uncertainty of 2.6 arcsec (radius, 90% confidence). This position is consistent with the optical afterglow candidate identified by Angulo et al. (GCN 42607). The light curve can be modelled with a power-law decay with a decay index of alpha=0.87 (+0.33, -0.29). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.96 (+0.23, -0.22). The best-fitting absorption column is 1.1 (+0.6, -0.5) x 10^21 cm^-2, in excess of the Galactic value of 1.7 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 3.2 x 10^-11 (3.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.1 (+0.6, -0.5) x 10^21 cm^-2 Galactic foreground: 1.7 x 10^20 cm^-2 Excess significance: 2.8 sigma Photon index: 1.96 (+0.23, -0.22) If the light curve continues to decay with a power-law decay index of 0.87, the count rate at T+24 hours will be 0.038 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x 10^-12 (1.5 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/03000182. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00048. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/42611. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...