{
  "body": "P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi  (INAF-IASFPA) , A.P.\nBeardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.\nBurrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-IASFPA), A. D'Ai\n(INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara\n(PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester),\nK.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T.\nSbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf\nof the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected source\nGRB 250530A (sb25053002), collecting 1.7 ks of Photon Counting (PC) mode data\nbetween T0+994 s and T0+2.8 ks after the trigger. \n\nWe have detected a total of 2 sources. These have been automatically classified as\nfollows:\n  * 0 likely counterparts\n  * 1 candidate counterpart (Source 1)\n  * 1 uncatalogued X-ray source\n  * 0 known X-ray sources\n\nA candidate counterpart has been found. The details of this source are:\n\n  Source 1 (SWIFT J122650.6-023432):\n  ==================================\n    RA (J2000.0):  186.7111  =\t12h 26m 50.66s\n    Dec (J2000.0): -2.5758  =  -02d 34' 32.9\"\n    Error:\t   3.5 (arcsec, radius, 90% confidence).\n    Detect flag:   GOOD\n    Distance:\t   4.1 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t   0.761 +/- 0.043 ct s^-1\n    Mean flux:\t   (4.86 +/- 0.27)e-11 erg cm^-2 s^-1\n    Peak rate:\t   3.73 +/- 0.84 ct s^-1\n    Peak flux:\t   (2.38 +/- 0.53)e-10 erg cm^-2 s^-1\n    ECF:\t   6.39e-11 erg cm^-2 ct^-1, assuming NH=2.47e+22 cm^-2,\n\t\t   gamma=2.04; determined from a spectral fit.\n    XMM UL:\t   1.8e-12 erg cm^-2 s^-1, (0.3-10 keV)\n\t so the source is 4.4-sigma above this 3-sigma upper limit.\n    The source IS FADING at the 4.3-sigma level.\n    There is 1 2MASS object within the source's 3-sigma error radius.\n\n\nDetails of the other uncatalogued X-ray source:\n\n  Source 2 (SWIFT J122602.0-023341):\n  ==================================\n    RA (J2000.0):  186.5087  =\t12h 26m 02.09s\n    Dec (J2000.0): -2.5614  =  -02d 33' 41.0\"\n    Error:\t   6.3 (arcsec, radius, 90% confidence).\n    Detect flag:   GOOD\n    Distance:\t   8.1 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t   (8.4 [+3.1, -2.5])e-3 ct s^-1\n    Mean flux:\t   (1.34 [+0.50, -0.40])e-12 erg cm^-2 s^-1\n    Peak rate:\t   (8.4 [+3.1, -2.5])e-3 ct s^-1\n    Peak flux:\t   (1.34 [+0.50, -0.40])e-12 erg cm^-2 s^-1\n    ECF:\t   1.58e-10 erg cm^-2 ct^-1, assuming NH=1.47e+22 cm^-2,\n\t\t   gamma=0.98; determined from a spectral fit.\n    XMM UL:\t   2.9e-12 erg cm^-2 s^-1, (0.3-10 keV)\n\t so the source is 3.4-sigma above this 3-sigma upper limit.\n    There is no evidence for fading.\n    There is 1 2MASS object within the source's 3-sigma error radius.\n\nAll fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with\ncatalogues and upper limits  from other missions, we assumed a power-law spectrum\nwith NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.\n\nThe results of the XRT-team automatic analysis of the XRT observations, including a\nposition-specific upper limit calculator, are available at\nhttps://www.swift.ac.uk/SVOM.\n\nThis circular is an official product of the Swift-XRT team.\n\n\n",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "bibcode": "2025GCN.40579....1E",
  "circularId": 40579,
  "submittedHow": "email",
  "createdOn": 1748601076232,
  "subject": "GRB 250530A: Swift-XRT counterpart detection",
  "eventId": "GRB 250530A"
}