TITLE: GCN CIRCULAR NUMBER: 40579 SUBJECT: GRB 250530A: Swift-XRT counterpart detection DATE: 25/05/30 10:31:16 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-IASFPA), 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 250530A (sb25053002), collecting 1.7 ks of Photon Counting (PC) mode data between T0+994 s and T0+2.8 ks after the trigger.

We have detected a total of 2 sources. These have been automatically classified as follows: * 0 likely counterparts * 1 candidate counterpart (Source 1) * 1 uncatalogued X-ray source * 0 known X-ray sources

A candidate counterpart has been found. The details of this source are:

Source 1 (SWIFT J122650.6-023432): ================================== RA (J2000.0): 186.7111 = 12h 26m 50.66s Dec (J2000.0): -2.5758 = -02d 34' 32.9" Error: 3.5 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 4.1 arcmin from the SVOM/ECLAIRs position. Mean rate: 0.761 +/- 0.043 ct s^-1 Mean flux: (4.86 +/- 0.27)e-11 erg cm^-2 s^-1 Peak rate: 3.73 +/- 0.84 ct s^-1 Peak flux: (2.38 +/- 0.53)e-10 erg cm^-2 s^-1 ECF: 6.39e-11 erg cm^-2 ct^-1, assuming NH=2.47e+22 cm^-2, gamma=2.04; determined from a spectral fit. XMM UL: 1.8e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 4.4-sigma above this 3-sigma upper limit. The source IS FADING at the 4.3-sigma level. There is 1 2MASS object within the source's 3-sigma error radius.

Details of the other uncatalogued X-ray source:

Source 2 (SWIFT J122602.0-023341): ================================== RA (J2000.0): 186.5087 = 12h 26m 02.09s Dec (J2000.0): -2.5614 = -02d 33' 41.0" Error: 6.3 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 8.1 arcmin from the SVOM/ECLAIRs position. Mean rate: (8.4 [+3.1, -2.5])e-3 ct s^-1 Mean flux: (1.34 [+0.50, -0.40])e-12 erg cm^-2 s^-1 Peak rate: (8.4 [+3.1, -2.5])e-3 ct s^-1 Peak flux: (1.34 [+0.50, -0.40])e-12 erg cm^-2 s^-1 ECF: 1.58e-10 erg cm^-2 ct^-1, assuming NH=1.47e+22 cm^-2, gamma=0.98; determined from a spectral fit. XMM UL: 2.9e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 3.4-sigma above this 3-sigma upper limit. There is no evidence for fading. There is 1 2MASS object 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.

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

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