{
  "bibcode": "2024GCN.38713....1B",
  "createdOn": 1735506295619,
  "submittedHow": "email",
  "eventId": "GRB 241228B",
  "circularId": 38713,
  "subject": "GRB 241228B: Swift-XRT afterglow detection",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "body": "D.N. Burrows (PSU), M. A. Williams (PSU), S. Dichiara (PSU), J.P.\nOsborne (U. Leicester), K.L. Page (U. Leicester), M. Ferro (INAF-OAB),\nR. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA) and P.A. Evans (U.\nLeicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nFermi/GBM-detected burst GRB 241228B. We searched for X-ray sources in \n1.8 ks of Photon Counting (PC) mode data. The total exposure at the\nposition of the afterglow (see below) is 1.8 ks, obtained between\nT0+40.0 ks and T0+41.8 ks.\n\nAn uncatalogued X-ray source is detected within the estimated 3-sigma\nFermi/GBM error region. Using 1809 s of PC mode data and 1 UVOT image,\nwe find an enhanced XRT position (using the XRT-UVOT alignment and\nmatching UVOT field sources to the USNO-B1 catalogue): RA, Dec =\n127.77288, +6.84823 which is equivalent to:\n\nRA (J2000): 08h 31m 05.49s\nDec(J2000): +06d 50' 53.6\"\n\nwith an uncertainty of 3.2 arcsec (radius, 90% confidence). This\nposition is 0.6 arcsec from the optical source GOTO24jmz and is\nbelieved to be the afterglow. The light curve is consistent with a\nconstant source of mean count rate 5.7e-02 ct/sec. A power-law fit\ngives an index of -1.281 (+4.492, -0.030).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.4 (+0.5, -0.3). The\nbest-fitting absorption column is  3 (+15, -0) x 10^20 cm^-2,\nconsistent with the Galactic value of 2.9 x 10^20 cm^-2 (Willingale et\nal. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux\nconversion factor deduced from this spectrum  is 4.8 x 10^-11 (5.0 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     3 (+15, -0) x 10^20 cm^-2\nGalactic foreground: 2.9 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.4 (+0.5, -0.3)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00021750.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021750.\n\nThis circular is an official product of the Swift-XRT team.\n\n"
}