GCN Circular 40579

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.