{
  "circularId": 42311,
  "eventId": "sb25101602",
  "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\nGRB251016A (sb25101602), collecting 749 s of Photon Counting (PC) mode data between\nT0+6.6 ks and T0+7.4 ks after the trigger. A likely counterpart has been found. The\ndetails of this source are:\n\n  Source 1 (SWIFT J174512.5-393337):\n  ==================================\n    RA (J2000.0):   266.3024  =  17 45 12.58\n    Dec (J2000.0):  -39.5606  =  -39 33 38.2\n    Error:\t    4.6 (arcsec, radius, 90% confidence).\n    Detect flag:    GOOD\n    Distance:\t    4.5 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    0.057 +/- 0.011 ct s^-1\n    Mean flux:\t    (3.41 +/- 0.64)e-12 erg cm^-2 s^-1\n    Peak rate:\t    0.072 +/- 0.019 ct s^-1\n    Peak flux:\t    (4.4 +/- 1.1)e-12 erg cm^-2 s^-1\n    ECF:\t    6.02e-11 erg cm^-2 ct^-1\n\t\t      assuming NH=7.85e+21 cm^-2, gamma=1.55\n\t\t      determined from a spectral fit.\n    XMM UL:\t    6.5e-13 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is 3.2-sigma above this 3-sigma upper limit.\n    The source may be fading, at the 1.1-sigma level.\n    There are 2 2MASS objects within the source's 3-sigma error radius.\n\n\nWe have detected a total of 9 sources.\tThese have been automatically classified as\nfollows:\n  * 1 likely counterpart\n  * 0 candidate counterparts\n  * 8 uncatalogued X-ray sources\n  * 0 known X-ray sources\n\nUncatalogued X-ray sources\n--------------------------\n\n  Source 2 (SWIFT J174439.5-394550):\n  ==================================\n    RA (J2000.0):   266.1646  =  17 44 39.50\n    Dec (J2000.0):  -39.7642  =  -39 45 51.1\n    Error:\t    6.1 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    11.4 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    0.0300 [+0.0096, -0.0079] ct s^-1\n    Mean flux:\t    (1.29 [+0.41, -0.34])e-12 erg cm^-2 s^-1\n    Peak rate:\t    0.0300 [+0.0096, -0.0079] ct s^-1\n    Peak flux:\t    (1.29 [+0.41, -0.34])e-12 erg cm^-2 s^-1\n    XMM UL:\t    7.9e-13 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is not above this 3-sigma upper limit.\n    There is no evidence for fading.\n    There are 5 2MASS objects within the source's 3-sigma error radius.\n\n  Source 3 (SWIFT J174431.8-393913):\n  ==================================\n    RA (J2000.0):   266.1326  =  17 44 31.82\n    Dec (J2000.0):  -39.6537  =  -39 39 13.3\n    Error:\t    5.9 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    5.7 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    (-1.0 +/- -1.0)e0 ct s^-1\n    Mean flux:\t    (-4.3 +/- -4.3)e-11 erg cm^-2 s^-1\n    Peak rate:\t    0.0123 [+0.0069, -0.0053] ct s^-1\n    Peak flux:\t    (5.3 [+2.9, -2.3])e-13 erg cm^-2 s^-1\n    XMM UL:\t    1.3e-12 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is not above this 3-sigma upper limit.\n    There is no evidence for fading.\n    There are 5 2MASS objects within the source's 3-sigma error radius.\n\n  Source 4 (SWIFT J174456.1-392739):\n  ==================================\n    RA (J2000.0):   266.2340  =  17 44 56.16\n    Dec (J2000.0):  -39.4609  =  -39 27 39.2\n    Error:\t    14.0 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    7.1 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    (-1.0 +/- -1.0)e0 ct s^-1\n    Mean flux:\t    (-3.5 +/- -3.5)e-9 erg cm^-2 s^-1\n    Peak rate:\t    (6.3 [+5.6, -4.0])e-3 ct s^-1\n    Peak flux:\t    (2.2 [+1.9, -1.4])e-11 erg cm^-2 s^-1\n    ECF:\t    3.48e-09 erg cm^-2 ct^-1\n\t\t      assuming NH=3.52e+22 cm^-2, gamma=5.05\n\t\t      determined from a spectral fit.\n    XMM UL:\t    1.0e-12 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is 1.3-sigma above this 3-sigma upper limit.\n    There is no evidence for fading.\n    There are 34 2MASS objects within the source's 3-sigma error radius.\n\n  Source 5 (SWIFT J174354.6-392827):\n  ==================================\n    RA (J2000.0):   265.9777  =  17 43 54.65\n    Dec (J2000.0):  -39.4743  =  -39 28 27.5\n    Error:\t    9.8 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    12.2 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    0.0128 [+0.0066, -0.0050] ct s^-1\n    Mean flux:\t    (5.5 [+2.8, -2.1])e-13 erg cm^-2 s^-1\n    Peak rate:\t    0.0128 [+0.0066, -0.0050] ct s^-1\n    Peak flux:\t    (5.5 [+2.8, -2.1])e-13 erg cm^-2 s^-1\n    XMM UL:\t    7.2e-13 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is not above this 3-sigma upper limit.\n    There is no evidence for fading.\n    There are 22 2MASS objects within the source's 3-sigma error radius.\n\n  Source 6 (SWIFT J174500.5-394402):\n  ==================================\n    RA (J2000.0):   266.2523  =  17 45 00.55\n    Dec (J2000.0):  -39.7341  =  -39 44 02.8\n    Error:\t    11.1 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    9.7 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    0.0156 [+0.0070, -0.0055] ct s^-1\n    Mean flux:\t    (6.7 [+3.0, -2.4])e-13 erg cm^-2 s^-1\n    Peak rate:\t    0.0156 [+0.0070, -0.0055] ct s^-1\n    Peak flux:\t    (6.7 [+3.0, -2.4])e-13 erg cm^-2 s^-1\n    XMM UL:\t    1.3e-12 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is not above this 3-sigma upper limit.\n    There is no evidence for fading.\n    There are 18 2MASS objects within the source's 3-sigma error radius.\n\n  Source 7 (SWIFT J174411.2-392428):\n  ==================================\n    RA (J2000.0):   266.0470  =  17 44 11.28\n    Dec (J2000.0):  -39.4079  =  -39 24 28.4\n    Error:\t    13.9 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    12.5 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    0.0167 [+0.0073, -0.0057] ct s^-1\n    Mean flux:\t    (7.2 [+3.2, -2.4])e-13 erg cm^-2 s^-1\n    Peak rate:\t    0.0167 [+0.0073, -0.0057] ct s^-1\n    Peak flux:\t    (7.2 [+3.2, -2.4])e-13 erg cm^-2 s^-1\n    XMM UL:\t    1.1e-12 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is not above this 3-sigma upper limit.\n    There is no evidence for fading.\n    A SIMBAD object `OGLE BLG-DSCT-13120' is 43.9\" away.\n    There are 33 2MASS objects within the source's 3-sigma error radius.\n\n  Source 8 (SWIFT J174427.1-393500):\n  ==================================\n    RA (J2000.0):   266.1131  =  17 44 27.14\n    Dec (J2000.0):  -39.5835  =  -39 35 00.6\n    Error:\t    11.9 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    4.3 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    (-1.0 +/- -1.0)e0 ct s^-1\n    Mean flux:\t    (-4.3 +/- -4.3)e-11 erg cm^-2 s^-1\n    Peak rate:\t    (8.7 [+6.1, -4.5])e-3 ct s^-1\n    Peak flux:\t    (3.7 [+2.6, -1.9])e-13 erg cm^-2 s^-1\n    XMM UL:\t    6.6e-13 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is not above this 3-sigma upper limit.\n    There is no evidence for fading.\n    There are 28 2MASS objects within the source's 3-sigma error radius.\n\n  Source 9 (SWIFT J174502.9-392734):\n  ==================================\n    RA (J2000.0):   266.2622  =  17 45 02.93\n    Dec (J2000.0):  -39.4596  =  -39 27 34.6\n    Error:\t    8.8 (arcsec, radius, 90% confidence).\n    Detect flag:    POOR\n    Distance:\t    7.5 arcmin from the SVOM/ECLAIRs position.\n    Mean rate:\t    (-1.0 +/- -1.0)e0 ct s^-1\n    Mean flux:\t    (-4.3 +/- -4.3)e-11 erg cm^-2 s^-1\n    Peak rate:\t    (8.9 [+6.4, -4.7])e-3 ct s^-1\n    Peak flux:\t    (3.8 [+2.7, -2.0])e-13 erg cm^-2 s^-1\n    XMM UL:\t    7.1e-13 erg cm^-2 s^-1, (0.3-10 keV)\n\t\t      so the source is not above this 3-sigma upper limit.\n    There is no evidence for fading.\n    There are 11 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_FIELD00042.\n\nThis circular is an official product of the Swift-XRT team.\n\n\n",
  "submittedHow": "email",
  "createdOn": 1760644835016,
  "subject": "sb25101602: Swift-XRT counterpart detection",
  "bibcode": "2025GCN.42311....1E",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>"
}