{
  "subject": "GRB 251103A: Swift-XRT afterglow detection",
  "circularId": 42551,
  "bibcode": "2025GCN.42551....1O",
  "createdOn": 1762183081373,
  "submittedHow": "email",
  "eventId": "GRB 251103A",
  "body": "J.P. Osborne (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB),\nA. D'Ai (INAF-IASFPA), S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea\n(PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A.\nEvans (U. Leicester) reports on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of GRB 251103A (Hu et\nal., GCN Circ. 42534). We searched for X-ray sources in  1.6 ks of\nPhoton Counting (PC) mode data. The total exposure at the position of\nthe afterglow (see below) is 1.6 ks, obtained between T0+4.8 ks and\nT0+10.2 ks.\n\nAn uncatalogued X-ray source is detected within the estimated 3-sigma\nSVOM/ECLAIRs error region (210 arcsec) and is above the LSXPS 3-sigma\nupper limit at this position, and is therefore likely the GRB\nafterglow. Using 1768 s of PC mode data and 2 UVOT images, we find an\nenhanced XRT position (using the XRT-UVOT alignment and matching UVOT\nfield sources to the USNO-B1 catalogue): RA, Dec = 146.83866, +16.12272\nwhich is equivalent to:\n\nRA (J2000): 09h 47m 21.28s\nDec(J2000): +16d 07' 21.8\"\n\nwith an uncertainty of 2.6 arcsec (radius, 90% confidence).\nThe source position is consistent with the one of the optical afterglow\nreported by Schneider et al. (GCN Circ. 42535, 42549), Turpin et al.\n(GCN Circ. 42538), Gritsevich et al. (GCN Circ. 42539), Izzo et al.\n(GCN Circ. 42540), O'Neill et al. (GCN Circ. 42543) and Ghosh et al.\n(GCN Circ. 42545).\n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.2 (+0.4, -0.3).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.82 (+0.16, -0.15). The\nbest-fitting absorption column is  9.9 (+4.7, -4.2) x 10^20 cm^-2, in\nexcess of the Galactic value of 4.0 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 3.8 x 10^-11 (4.4 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     9.9 (+4.7, -4.2) x 10^20 cm^-2\nGalactic foreground: 4.0 x 10^20 cm^-2\nExcess significance: 2.3 sigma\nPhoton index:\t     1.82 (+0.16, -0.15)\n\nIf the light curve continues to decay with a power-law decay index of\n1.2, the count rate at T+24 hours will be 0.035 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x\n10^-12 (1.5 x 10^-12) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/xrt_products/03000169.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/SVOM/SVOM_FIELD00047.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>"
}