{
  "submittedHow": "email",
  "bibcode": "2025GCN.40662....1E",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "eventId": "EP250610a",
  "createdOn": 1749545722912,
  "subject": "EP250610a: Swift-XRT counterpart detection",
  "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-OAR), A. D'Ai (INAF-IASFPA),\nP. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro\n(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.\nLeicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato\n(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the\nSwift-XRT team:\n\nSwift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected\nsource EP250610a, collecting 2.1 ks of Photon Counting (PC) mode data between T0+2.8\nks and T0+9.4 ks after the trigger. A candidate counterpart has been found. The\ndetails of this source are:\n\n  Source 1 (SWIFT J163325.5+383159):\n  ==================================\n    RA (J2000.0):  248.3564  =\t16h 33m 25.54s\n    Dec (J2000.0): +38.5332  =\t+38d 31' 59.5\"\n    Error:\t   4.6 (arcsec, radius, 90% confidence).\n    Detect flag:   GOOD\n    Distance:\t   2.2 arcmin from the Einstein Probe/WXT position.\n    Mean rate:\t   0.0111 +/- 0.0027 ct s^-1\n    Mean flux:\t   (4.4 +/- 1.1)e-13 erg cm^-2 s^-1\n    Peak rate:\t   0.0111 +/- 0.0027 ct s^-1\n    Peak flux:\t   (4.4 +/- 1.1)e-13 erg cm^-2 s^-1\n    ECF:\t   3.96e-11 erg cm^-2 ct^-1, assuming NH=1.07e+20 cm^-2,\n\t\t   gamma=1.73; determined from a spectral fit.\n    LSXPS UL:\t   2.4e-03 ct/sec, (0.3-10 keV)\n\t so the source is 3.1-sigma above this 3-sigma upper limit.\n    There is no evidence for fading.\n\n\nWe have detected a total of 2 sources. These have been automatically classified as\nfollows:\n  * 0 likely counterparts\n  * 1 candidate counterpart\n  * 1 uncatalogued X-ray source\n  * 0 known X-ray sources\n\nUncatalogued X-ray sources\n--------------------------\n\n  Source 2 (SWIFT J163309.1+383226):\n  ==================================\n    RA (J2000.0):  248.2883  =\t16h 33m 09.19s\n    Dec (J2000.0): +38.5406  =\t+38d 32' 26.2\"\n    Error:\t   7.1 (arcsec, radius, 90% confidence).\n    Detect flag:   REASONABLE\n    Distance:\t   78 arcsec from the Einstein Probe/WXT position.\n    Mean rate:\t   (2.4 [+1.6, -1.1])e-3 ct s^-1\n    Mean flux:\t   (1.02 [+0.70, -0.49])e-13 erg cm^-2 s^-1\n    Peak rate:\t   (2.4 [+1.6, -1.1])e-3 ct s^-1\n    Peak flux:\t   (1.02 [+0.70, -0.49])e-13 erg cm^-2 s^-1\n    LSXPS UL:\t   3.1e-03 ct/sec, (0.3-10 keV)\n\t so the source is not 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/EP.\n\nThis circular is an officicial product of the Swift-XRT team.\n\n\n",
  "circularId": 40662
}