{
  "eventId": "GRB 250831A",
  "createdOn": 1756647652596,
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "bibcode": "2025GCN.41617....1E",
  "circularId": 41617,
  "subject": "GRB 250831A: 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 SVOM/ECLAIRs-detected source\nGRB 250831A (trigger sb25083102), collecting 1.5 ks of Photon Counting (PC) mode\ndata between T0+3.0 ks and T0+4.7 ks after the trigger. A likely counterpart has\nbeen found. The details of this source are:\n\n  Source 1 (SWIFT J013907.0-175644):\n  ==================================\n    RA (J2000.0):   24.7795  =\t01 39 07.08\n    Dec (J2000.0):  -17.9457  =  -17 56 44.5\n    Error:\t    3.5 (arcsec, radius, 90% confidence).\n    Detect flag:    GOOD\n    Distance:\t    3.4 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    5.60 +/- 0.16 ct s^-1\n    Mean flux:\t    (1.571 +/- 0.046)e-10 erg cm^-2 s^-1\n    Peak rate:\t    9.9 +/- 2.2 ct s^-1\n    Peak flux:\t    (2.78 +/- 0.61)e-10 erg cm^-2 s^-1\n    ECF:\t    2.81e-11 erg cm^-2 ct^-1\n\t\t      assuming NH=1.22e+21 cm^-2, gamma=2.40\n\t\t      determined from a spectral fit.\n    LSXPS UL:\t    6.6e-03 ct/sec, (0.3-10 keV)\n\t\t      so the source is 4.6-sigma above this 3-sigma upper limit.\n    There is no evidence for fading.\n\nGCN Circs. 41615 and 41616 noted that the high-proper-motion M-dwarf binary star\nsystem G 272-61 was within the SVOM field of view; with a proper motion of ~3.3\narcsec yr^-1, its position appears consistent with this XRT source.\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  * 1 uncatalogued X-ray source\n  * 1 known X-ray source\n\nUncatalogued X-ray sources\n--------------------------\n\n  Source 2 (SWIFT J013909.3-175732):\n  ==================================\n    RA (J2000.0):   24.7888  =\t01 39 09.31\n    Dec (J2000.0):  -17.9590  =  -17 57 32.4\n    Error:\t    5.4 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    3.2 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    0.242 +/- 0.015 ct s^-1\n    Mean flux:\t    (6.73 +/- 0.42)e-12 erg cm^-2 s^-1\n    Peak rate:\t    0.392 +/- 0.082 ct s^-1\n    Peak flux:\t    (1.09 +/- 0.23)e-11 erg cm^-2 s^-1\n    ECF:\t    2.78e-11 erg cm^-2 ct^-1\n\t\t      assuming NH=8.33e+20 cm^-2, gamma=2.42\n\t\t      determined from a spectral fit.\n    LSXPS UL:\t    5.7e-04 ct/sec, (0.3-10 keV)\n\t\t      so the source is 4.8-sigma above this 3-sigma upper limit.\n    There is no evidence for fading.\n\n\nKnown X-ray sources\n-------------------\n\n  Source 3 (SWIFT J013932.8-175532):\n  ==================================\n    RA (J2000.0):   24.8869  =\t01 39 32.86\n    Dec (J2000.0):  -17.9257  =  -17 55 32.5\n    Error:\t    8.1 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    2.9 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    (-1.0 +/- -1.0)e0 ct s^-1\n    Mean flux:\t    (-2.2 +/- -2.2)e-24 erg cm^-2 s^-1\n    Peak rate:\t    (3.0 [+2.2, -1.6])e-3 ct s^-1\n    Peak flux:\t    (6.6 [+4.9, -3.7])e-27 erg cm^-2 s^-1\n    ECF:\t    2.24e-24 erg cm^-2 ct^-1\n\t\t      assuming NH=3.77e+20 cm^-2, gamma=98.61\n\t\t      determined from a spectral fit.\n    This matches a catalogued X-ray source LSXPS J013933.3-175540\n    in the LSXPS catalogue. Details:\n    Separation:     10.3\" from the XRT source\n    Cat Rate:\t    9.9e-04 +/- 2.4e-04 ct s^-1 \n    Cat Flux:\t    2.2e-27 +/- 5.5e-28 erg cm^-2 s^-1 (0.3-10 keV)\n\t\t      so the source  is 1.0-sigma above the catalogued flux.\n    There is no evidence for fading.\n    A SIMBAD object `SDSS J013934.38-175541.9' is 23.8\" away.\n    There are 2 2MASS objects 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/SVOM_FIELD00032.\n\nThis circular is an official product of the Swift-XRT team.\n\n\n",
  "submittedHow": "email"
}