TITLE: GCN CIRCULAR NUMBER: 41180 SUBJECT: GRB 250727A: Swift-XRT observations DATE: 25/07/27 21:03:14 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 250727A, collecting 1.6 ks of Photon Counting (PC) mode data between T0+2.2 ks and T0+8.3 ks after the trigger. We have detected 2 sources. These have been automatically classified as follows: * 0 likely counterparts * 0 candidate counterparts * 2 uncatalogued X-ray sources * 0 known X-ray sources

Uncatalogued X-ray sources --------------------------

Source 1 (SWIFT J234539.1-811248): ================================== RA (J2000.0): 356.4132 = 23h 45m 39.17s Dec (J2000.0): -81.2136 = -81d 12' 49.0" Error: 4.9 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 7.7 arcmin from the SVOM/ECLAIRs position. Mean rate: 0.0155 +/- 0.0041 ct s^-1 Mean flux: (5.2 +/- 1.4)e-13 erg cm^-2 s^-1 Peak rate: 0.0155 +/- 0.0041 ct s^-1 Peak flux: (5.2 +/- 1.4)e-13 erg cm^-2 s^-1 ECF: 3.37e-11 erg cm^-2 ct^-1, assuming NH=9.71e+20 cm^-2, gamma=1.97; determined from a spectral fit. XMM UL: 4.6e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 29.9-sigma below this 3-sigma upper limit. There is no evidence for fading.

Source 2 (SWIFT J234406.8-810315): ================================== RA (J2000.0): 356.0284 = 23h 44m 06.82s Dec (J2000.0): -81.0543 = -81d 03' 15.5" Error: 8.5 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 4.9 arcmin from the SVOM/ECLAIRs position. Mean rate: (5.2 [+2.6, -2.0])e-3 ct s^-1 Mean flux: (1.52 [+0.77, -0.59])e-13 erg cm^-2 s^-1 Peak rate: (5.2 [+2.6, -2.0])e-3 ct s^-1 Peak flux: (1.52 [+0.77, -0.59])e-13 erg cm^-2 s^-1 ECF: 2.93e-11 erg cm^-2 ct^-1, assuming NH=3.26e+22 cm^-2, gamma=4.66; determined from a spectral fit. XMM UL: 1.9e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 25.1-sigma below this 3-sigma upper limit. There is no evidence for fading.

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_FIELD00025.

This circular is an official product of the Swift-XRT team.

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