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GCN Circular 40951

Subject
EP250704a / GRB250704B: Swift-XRT counterpart detection
Event
EP250704a
Date
2025-07-04T14:06:18Z (14 hours ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
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 Einstein Probe/WXT-detected
source EP250704a (temporally coincident with the SVOM GRB 250704B, Wang et al., GCN
40940Loading...
 
 ), collecting 1.7 ks of Photon Counting (PC) mode data between T0+1.8 ks and
T0+3.7 ks after the trigger. A candidate counterpart has been found. The details of
this source are:

  Source 2 (SWIFT J200329.1+120123):
  ==================================
    RA (J2000.0):  300.8716  =	20h 03m 29.18s
    Dec (J2000.0): +12.0233  =	+12d 01' 23.9"
    Error:	   3.6 (arcsec, radius, 90% confidence).
    Detect flag:   REASONABLE
    Distance:	   24 arcsec from the Einstein Probe/WXT position.
    Mean rate:	   0.247 +/- 0.017 ct s^-1
    Mean flux:	   (8.79 +/- 0.59)e-12 erg cm^-2 s^-1
    Peak rate:	   0.389 +/- 0.088 ct s^-1
    Peak flux:	   (1.39 +/- 0.31)e-11 erg cm^-2 s^-1
    ECF:	   3.56e-11 erg cm^-2 ct^-1, assuming NH=1.47e+21 cm^-2,
		   gamma=1.97; determined from a spectral fit.
    RASS UL:	   2.9e-02 ct s^-1 (converted to XRT; 0.3-10 keV)
	 so the source is not above this 3-sigma upper limit.
    The source may be fading, at the 1.7-sigma level.
    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/EP.

The detected afterglow position is consistent with the optical counterpart of the EP
trigger, reported by Schneider et al. (GCN 40942Loading...
 
 ).

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