TITLE:   GCN CIRCULAR
NUMBER:  40951
SUBJECT: EP250704a / GRB250704B: Swift-XRT counterpart detection
DATE:    25/07/04 14:06:18 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 Einstein Probe/WXT-detected
source EP250704a (temporally coincident with the SVOM GRB 250704B, Wang et al., GCN
40940), 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 40942).

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