TITLE: GCN CIRCULAR NUMBER: 41617 SUBJECT: GRB 250831A: Swift-XRT counterpart detection DATE: 25/08/31 13:40:52 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected source GRB 250831A (trigger sb25083102), collecting 1.5 ks of Photon Counting (PC) mode data between T0+3.0 ks and T0+4.7 ks after the trigger. A likely counterpart has been found. The details of this source are: Source 1 (SWIFT J013907.0-175644): ================================== RA (J2000.0): 24.7795 = 01 39 07.08 Dec (J2000.0): -17.9457 = -17 56 44.5 Error: 3.5 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 3.4 arcmin from the SVOM/ECLAIRs position. Mean rate: 5.60 +/- 0.16 ct s^-1 Mean flux: (1.571 +/- 0.046)e-10 erg cm^-2 s^-1 Peak rate: 9.9 +/- 2.2 ct s^-1 Peak flux: (2.78 +/- 0.61)e-10 erg cm^-2 s^-1 ECF: 2.81e-11 erg cm^-2 ct^-1 assuming NH=1.22e+21 cm^-2, gamma=2.40 determined from a spectral fit. LSXPS UL: 6.6e-03 ct/sec, (0.3-10 keV) so the source is 4.6-sigma above this 3-sigma upper limit. There is no evidence for fading. GCN Circs. 41615 and 41616 noted that the high-proper-motion M-dwarf binary star system G 272-61 was within the SVOM field of view; with a proper motion of ~3.3 arcsec yr^-1, its position appears consistent with this XRT source. We have detected a total of 3 sources. These have been automatically classified as follows: * 1 likely counterpart * 0 candidate counterparts * 1 uncatalogued X-ray source * 1 known X-ray source Uncatalogued X-ray sources -------------------------- Source 2 (SWIFT J013909.3-175732): ================================== RA (J2000.0): 24.7888 = 01 39 09.31 Dec (J2000.0): -17.9590 = -17 57 32.4 Error: 5.4 (arcsec, radius, 90% confidence). Detect flag: POOR Distance: 3.2 arcmin from the SVOM/ECLAIRs position. Mean rate: 0.242 +/- 0.015 ct s^-1 Mean flux: (6.73 +/- 0.42)e-12 erg cm^-2 s^-1 Peak rate: 0.392 +/- 0.082 ct s^-1 Peak flux: (1.09 +/- 0.23)e-11 erg cm^-2 s^-1 ECF: 2.78e-11 erg cm^-2 ct^-1 assuming NH=8.33e+20 cm^-2, gamma=2.42 determined from a spectral fit. LSXPS UL: 5.7e-04 ct/sec, (0.3-10 keV) so the source is 4.8-sigma above this 3-sigma upper limit. There is no evidence for fading. Known X-ray sources ------------------- Source 3 (SWIFT J013932.8-175532): ================================== RA (J2000.0): 24.8869 = 01 39 32.86 Dec (J2000.0): -17.9257 = -17 55 32.5 Error: 8.1 (arcsec, radius, 90% confidence). Detect flag: POOR Distance: 2.9 arcmin from the SVOM/ECLAIRs position. Mean rate: (-1.0 +/- -1.0)e0 ct s^-1 Mean flux: (-2.2 +/- -2.2)e-24 erg cm^-2 s^-1 Peak rate: (3.0 [+2.2, -1.6])e-3 ct s^-1 Peak flux: (6.6 [+4.9, -3.7])e-27 erg cm^-2 s^-1 ECF: 2.24e-24 erg cm^-2 ct^-1 assuming NH=3.77e+20 cm^-2, gamma=98.61 determined from a spectral fit. This matches a catalogued X-ray source LSXPS J013933.3-175540 in the LSXPS catalogue. Details: Separation: 10.3" from the XRT source Cat Rate: 9.9e-04 +/- 2.4e-04 ct s^-1 Cat Flux: 2.2e-27 +/- 5.5e-28 erg cm^-2 s^-1 (0.3-10 keV) so the source is 1.0-sigma above the catalogued flux. There is no evidence for fading. A SIMBAD object `SDSS J013934.38-175541.9' is 23.8" away. There are 2 2MASS objects within the source's 3-sigma error radius. All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00032. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41617. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...