TITLE: GCN CIRCULAR NUMBER: 38713 SUBJECT: GRB 241228B: Swift-XRT afterglow detection DATE: 24/12/29 21:04:55 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> D.N. Burrows (PSU), M. A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Fermi/GBM-detected burst GRB 241228B. We searched for X-ray sources in 1.8 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 1.8 ks, obtained between T0+40.0 ks and T0+41.8 ks. An uncatalogued X-ray source is detected within the estimated 3-sigma Fermi/GBM error region. Using 1809 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 = 127.77288, +6.84823 which is equivalent to: RA (J2000): 08h 31m 05.49s Dec(J2000): +06d 50' 53.6" with an uncertainty of 3.2 arcsec (radius, 90% confidence). This position is 0.6 arcsec from the optical source GOTO24jmz and is believed to be the afterglow. The light curve is consistent with a constant source of mean count rate 5.7e-02 ct/sec. A power-law fit gives an index of -1.281 (+4.492, -0.030). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.4 (+0.5, -0.3). The best-fitting absorption column is 3 (+15, -0) x 10^20 cm^-2, consistent with the Galactic value of 2.9 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 4.8 x 10^-11 (5.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3 (+15, -0) x 10^20 cm^-2 Galactic foreground: 2.9 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.4 (+0.5, -0.3) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021750. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021750. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/38713. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...