{
  "eventId": "GRB 250813B",
  "submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "circularId": 41359,
  "createdOn": 1755158951585,
  "bibcode": "2025GCN.41359....1P",
  "subject": "GRB 250813B: Swift-XRT afterglow detection",
  "body": "K.L. Page, A.P. Beardmore and P.A. Evans (U. Leicester) report on behalf\nof the Swift-XRT team:\n\nWe have analysed 1.6 ks of XRT data for GRB 250813B, from 9.6 to 16.3\nks after the SVOM/ECLAIRs trigger (GCN Circ. 41354). The data are\nentirely in Photon Counting (PC) mode. We find an uncatalogued X-ray\nsource at RA, Dec = 336.7505, +12.4613 which is equivalent to:\n\nRA (J2000): 22 27 00.13\nDec(J2000): +12 27 40.5\n\nwith an uncertainty of 3.7 arcsec (radius, 90% confidence).\n\nThere is no evidence for fading during this interval, with the X-ray\nsource showing a mean count rate of 0.17 +/- 0.02 count s^-1. However,\nthe localisation of this source is consistent with the fading optical\nafterglow found by Ondrejov D50 (GCN Circ. 41353, 41355) and\nSVOM/COLIBRI (GCN Circ. 41356), and so we consider it the X-ray\nafterglow.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.1 (+0.4, -0.3). The\nbest-fitting absorption column is 2.5 (+1.3, -1.1) × 10^21 cm^-2, in\nexcess of the Galactic value of 6.2 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 6.2 x 10^-12 (9.3 x\n10^-12) erg cm^-2 count^-1.\n\nA summary of the PC-mode spectrum is thus:\nTotal column:        2.5 (+1.3, -1.1) x 10^21 cm^-2\nGalactic foreground: 6.2 x 10^20 cm^-2\nPhoton index:        2.1 (+0.4, -0.3)\n\nThis circular is an official product of the Swift-XRT team.\n",
  "submittedHow": "email"
}