TITLE: GCN CIRCULAR NUMBER: 43050 SUBJECT: GRB 251208B: Swift-XRT afterglow detection DATE: 25/12/09 13:53:09 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> J.A. Kennea (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), 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 the GUANO (DeLaunay et al. GCN 43033) and Fermi/LAT-detected (Bissaldi et al. GCN 43034) burst GRB 251208B. We searched for X-ray sources in 4.8 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 4.8 ks, obtained between T0+33.1 ks and T0+45.4 ks. Three uncatalogued X-ray sources are detected within the estimated 3-sigma Fermi/LAT error region (414 arcsec), of which one ("Source 1") is 2.9 sigma above the RASS limit and it is fading with 2.2 sigma significance and thus is believed to be the GRB afterglow. Using 4576 s of PC mode data and 5 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 = 228.48108, +29.04575 which is equivalent to: RA (J2000): 15h 13m 55.46s Dec(J2000): +29d 02' 44.7" with an uncertainty of 2.3 arcsec (radius, 90% confidence). This position is 87 arcsec from the Fermi/LAT position. The light curve is consistent with a constant source of mean count rate 5.0e-02 ct/sec. A power-law fit gives an index of 1.2 (+/-1.2). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+0.4, -0.3). The best-fitting absorption column is 1.4 (+1.1, -0.8) x 10^21 cm^-2, in excess of the Galactic value of 2.6 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 3.1 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.4 (+1.1, -0.8) x 10^21 cm^-2 Galactic foreground: 2.6 x 10^20 cm^-2 Excess significance: 2.3 sigma Photon index: 2.2 (+0.4, -0.3) The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/00021893. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021893. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/43050. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...