TITLE:   GCN CIRCULAR
NUMBER:  41563
SUBJECT: EP250827a: Swift-XRT counterpart detection
DATE:    25/08/27 15:35:30 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), R.A.J.
Eyles-Ferris (U. Leicester), 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 EP250827a, collecting 1.6 ks of Photon Counting (PC) mode data between T0+3.1
ks and T0+9.2 ks after the trigger. A likely counterpart has been found. The details
of this source are:

  Source 1 (SWIFT J001351.7-562952):
  ==================================
    RA (J2000.0):   3.4656  =  00 13 51.74
    Dec (J2000.0):  -56.4979  =  -56 29 52.4
    Error:	    3.7 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    2.2 arcmin from the Einstein Probe/WXT position.
    Mean rate:	    0.161 +/- 0.011 ct s^-1
    Mean flux:	    (5.48 +/- 0.39)e-12 erg cm^-2 s^-1
    Peak rate:	    0.253 +/- 0.057 ct s^-1
    Peak flux:	    (8.6 +/- 1.9)e-12 erg cm^-2 s^-1
    ECF:	    3.41e-11 erg cm^-2 ct^-1
		      assuming NH=1.45e+20 cm^-2, gamma=1.93
		      determined from a spectral fit.
    XMM UL:	    2.2e-12 erg cm^-2 s^-1, (0.3-10 keV)
		      so the source is 3.3-sigma above this 3-sigma upper limit.
    The source may be fading, at the 1.5-sigma level.

This position is consistent with the optical counterpart previously reported (Levan
et al. GCN 41554, Li et al. GCN 41555, Lipunov et al. GCN 41558, Gritsevich et al.
GCN 41559, An et al. GCN 41560) at a redshift of 1.613 (Levan et al. GCN 41557).

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/EP_FIELD00062.

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