{
  "createdOn": 1755019364673,
  "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 EP250812a, collecting 1.7 ks of Photon Counting (PC) mode data between T0+1.5\nks and T0+3.4 ks after the trigger. A likely counterpart has been found. The\nposition is consistent with the uncatalogued X-ray source detected by EP-FXT (Zhang\net al., GCN 41327). The details of this source are:\n\n  Source 1 (SWIFT J020358.1-091907):\n  ==================================\n    RA (J2000.0):   30.9921  =\t02h 03m 58.10s\n    Dec (J2000.0):  -9.3187  =\t-09d 19' 07.3\"\n    Error:\t    3.7 (arcsec, radius, 90% confidence).\n    Detect flag:    GOOD\n    Distance:\t    67 arcsec from the Einstein Probe/WXT position.\n    Mean rate:\t    0.1069 +/- 0.0091 ct s^-1\n    Mean flux:\t    (4.00 +/- 0.34)e-12 erg cm^-2 s^-1\n    Peak rate:\t    0.203 +/- 0.046 ct s^-1\n    Peak flux:\t    (7.6 +/- 1.7)e-12 erg cm^-2 s^-1\n    ECF:\t    3.75e-11 erg cm^-2 ct^-1\n\t\t      assuming NH=4.71e+20 cm^-2, gamma=1.83\n\t\t      determined from a spectral fit.\n    LSXPS UL:\t    1.5e-03 ct/sec, (0.3-10 keV)\n\t\t      so the source is 4.4-sigma above this 3-sigma upper limit.\n    The source may be fading, at the 2.6-sigma level.\n\n\nWe have detected a total of 3 sources.\tThese have been automatically classified as\nfollows:\n  * 1 likely counterpart\n  * 0 candidate counterparts\n  * 0 uncatalogued X-ray sources\n  * 2 known X-ray sources\n\nKnown X-ray sources\n-------------------\n\n  Source 2 (SWIFT J020340.7-092134):\n  ==================================\n    RA (J2000.0):   30.9196  =\t02h 03m 40.70s\n    Dec (J2000.0):  -9.3596  =\t-09d 21' 34.6\"\n    Error:\t    5.8 (arcsec, radius, 90% confidence).\n    Detect flag:    GOOD\n    Distance:\t    4.8 arcmin from the Einstein Probe/WXT position.\n    Mean rate:\t    (8.5 [+3.0, -2.5])e-3 ct s^-1\n    Mean flux:\t    (2.42 [+0.87, -0.71])e-13 erg cm^-2 s^-1\n    Peak rate:\t    (8.5 [+3.0, -2.5])e-3 ct s^-1\n    Peak flux:\t    (2.42 [+0.87, -0.71])e-13 erg cm^-2 s^-1\n    ECF:\t    2.86e-11 erg cm^-2 ct^-1\n\t\t      assuming NH=4.16e+20 cm^-2, gamma=2.23\n\t\t      determined from a spectral fit.\n    This matches a catalogued X-ray source LSXPS J020340.7-092134\n    in the LSXPS catalogue. Details:\n    Separation:     1.0\" from the XRT source\n    Cat Rate:\t    4.6e-03 +/- 8.6e-04 ct s^-1 \n    Cat Flux:\t    1.3e-13 +/- 2.5e-14 erg cm^-2 s^-1 (0.3-10 keV)\n\t\t      so the source  is 1.3-sigma above the catalogued flux.\n    There is no evidence for fading.\n    A SIMBAD object `[VV2006] J020341.0-092134' is 3.4\" away.\n\n  Source 4 (SWIFT J020404.7-091504):\n  ==================================\n    RA (J2000.0):   31.0199  =\t02h 04m 04.78s\n    Dec (J2000.0):  -9.2513  =\t-09d 15' 04.7\"\n    Error:\t    5.8 (arcsec, radius, 90% confidence).\n    Detect flag:    GOOD\n    Distance:\t    4.0 arcmin from the Einstein Probe/WXT position.\n    Mean rate:\t    (6.6 [+2.8, -2.2])e-3 ct s^-1\n    Mean flux:\t    (2.14 [+0.92, -0.72])e-13 erg cm^-2 s^-1\n    Peak rate:\t    (6.6 [+2.8, -2.2])e-3 ct s^-1\n    Peak flux:\t    (2.14 [+0.92, -0.72])e-13 erg cm^-2 s^-1\n    ECF:\t    3.25e-11 erg cm^-2 ct^-1\n\t\t      assuming NH=2.48e+20 cm^-2, gamma=1.94\n\t\t      determined from a spectral fit.\n    This matches a catalogued X-ray source LSXPS J020405.1-091509\n    in the LSXPS catalogue. Details:\n    Separation:     7.3\" from the XRT source\n    Cat Rate:\t    5.6e-03 +/- 1.5e-03 ct s^-1 \n    Cat Flux:\t    1.8e-13 +/- 4.9e-14 erg cm^-2 s^-1 (0.3-10 keV)\n\t\t      so the source  is 0.3-sigma above the catalogued flux.\n    There is no evidence for fading.\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/EP_FIELD00058.\n\nThis circular is an official product of the Swift-XRT team.\n\n\n",
  "subject": "EP250812a: Swift-XRT counterpart detection",
  "eventId": "EP250812a",
  "bibcode": "2025GCN.41330....1E",
  "circularId": 41330,
  "submittedHow": "email",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>"
}