{
  "bibcode": "2021GCN.30252....1O",
  "body": "J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore\n(U. Leicester), E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA),\nJ.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU) and\nM. G. Bernardini (INAF-OAB) report on behalf of the Swift-XRT team:\n\nWe have analysed 5.2 ks of Photon Counting (PC) mode XRT data for the\nSwift-BAT-detected burst GRB 210618A (Bernardini et al. GCN Circ.\n30250), collected between T0+65 s and T0+22.6 ks. \n\nThree uncatalogued X-ray sources have been detected consistent with\nbeing within 296 arcsec of the Swift-BAT position, however none of them\nis above the RASS limit or shows definitive signs of fading. Two\nsources (Source 1 and Source 4) are consistent within the XRT error\ncircle with two spectroscopically identified quasars, and the third one\n(Source 2) is consistent with a SDSS galaxy with photo_z=0.59 and\nmarginally consistent with a candidate quasar. Therefore, for the\nmoment the most plausible afterglow candidate is Source 2. Swift will\nperform follow-up observations to confirm this candidate.\n\nDetails of these sources are given below:\n\nSource 1:\n  RA (J2000.0):  235.8870  =  15:43:32.87\n  Dec (J2000.0): +46.0253  =  +46:01:31.0\n  Error: 8.9 arcsec (radius, 90% conf.)\n  Count-rate: (3.82 [+1.19, -1.00])e-3 ct s^-1\t \n  Distance: 179 arcsec from Swift-BAT position.\n  Flux: (1.17 [+0.36, -0.30])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)\nThis source is consistent with SDSS J154332.15+460133.2,\nspectroscopically classified as a quasar at z = 0.657. \n\nSource 2:\n  RA (J2000.0):  235.7698  =  15:43:4.75\n  Dec (J2000.0): +46.0691  =  +46:04:08.6\n  Error: 9.3 arcsec (radius, 90% conf.)\n  Count-rate: (4.0 +/- 1.0)e-3 ct s^-1\t \n  Distance: 156 arcsec from Swift-BAT position.\n  Flux: (1.35 +/- 0.35)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV) \nThis source is positionally consistent with SDSS J154305.56+460405.0, a\ngalaxy with photo_z = 0.590 +/- 0.153 and 12.2 arcsec away from SDSS\nJ154305.40+460418.7, a candidate quasar (Richards et al., 2015, ApJS,\n219, 39).\n\nSource 4:\n  RA (J2000.0):  235.7183  =  15:42:52.39\n  Dec (J2000.0): +46.0336  =  +46:02:01.0\n  Error: 8.7 arcsec (radius, 90% conf.)\n  Count-rate: (1.48 [+0.78, -0.60])e-3 ct s^-1\t \n  Distance: 256 arcsec from Swift-BAT position.\n  Flux: (7.2 [+3.8, -2.9])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)\nThis source is positionally consistent with SDSS J154251.96+460206.1,\nspectroscopically classified as a quasar at z = 2.130.\n\nSix uncatalogued sources were also detected too far from the GRB\nposition to be likely afterglow candidates.\n\nThe results of the XRT-team automatic analysis of the XRT observations,\nincluding a position-specific upper limit calculator, are available at\nhttps://www.swift.ac.uk/ToO_GRBs/01056426.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 30252,
  "createdOn": 1624024042000,
  "email": "grazia.bernardini@brera.inaf.it",
  "subject": "GRB 210618A: Swift-XRT observations",
  "submitter": "Maria Grazia Bernardini at INAF/Brera  <grazia.bernardini@brera.inaf.it>",
  "eventId": "GRB 210618A"
}