{
  "subject": "GRB 250407A: Swift-XRT afterglow detection",
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Simone Dichiara at Pennsylvania State University <sbd5667@psu.edu>",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "body": "S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.\nLeicester), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B.\nSbarufatti (INAF-OAB), D.N. Burrows (PSU), M. A. Williams (PSU) and\nP.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nFermi/GBM-detected burst GRB 250407A (GCN circ. 40104, GCN circ.\n40105). We searched for X-ray sources in  293 s of Photon Counting (PC)\nmode data. The total exposure at the position of the afterglow (see\nbelow) is 293 s, obtained between T0+18.8 ks and T0+19.1 ks.\n\nAn uncatalogued X-ray source is detected within the estimated 1-sigma\n(statistical) SRG/ART-XC error region (20 arcsec; GCN circ. 40110) and \nis above the RASS 3-sigma upper limit at this position, and is therefore \nlikely the GRB afterglow. Using 293 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 = \n110.75876, +36.79827 which is equivalent to:\n\nRA (J2000): 07h 23m 02.10s\nDec(J2000): +36d 47' 53.8\"\n\nwith an uncertainty of 2.8 arcsec (radius, 90% confidence). This\nposition is 6.9 arcsec from the SRG/ART-XC position reported by Molkov\net al (GCN circ. 40110). We note that the source has faded since the\nSRG/ART-XC observation.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.8 (+0.4, -0.3). The\nbest-fitting absorption column is  3.0 (+1.7, -1.4) x 10^21 cm^-2, in\nexcess of the Galactic value of 7.0 x 10^20 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 4.3 x 10^-11 (5.8 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     3.0 (+1.7, -1.4) x 10^21 cm^-2\nGalactic foreground: 7.0 x 10^20 cm^-2\nExcess significance: 2.8 sigma\nPhoton index:\t     1.8 (+0.4, -0.3)\n\nThe results of the XRT-team automatic analysis of the likely afterglow\nare at https://www.swift.ac.uk/ToO_GRBs/00021826/Source1.php.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021826.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "circularId": 40113,
  "createdOn": 1744071286085,
  "submittedHow": "email",
  "bibcode": "2025GCN.40113....1D",
  "version": 2,
  "format": "text/plain",
  "eventId": "GRB 250407A",
  "editedOn": 1744124239849
}