GCN Circular 42908

Subject
EP251130a: Swift-XRT counterpart detection
Event
EP251130a
Date
2025-11-30T19:00:56Z (a day 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 EP251130a, collecting 2.0 ks of Photon Counting (PC) mode data between T0+5.8
ks and T0+11 ks after the trigger. A likely counterpart has been found. The details
of this source are:

  Source 1 (SWIFT J064404.8+272302):
  ==================================
    RA (J2000.0):   101.0203  =  06 44 04.87
    Dec (J2000.0):  +27.3839  =  +27 23 02.0
    Error:	    3.9 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    75 arcsec from the Einstein Probe/WXT position.
    Mean rate:	    0.0618 [+0.0070, -0.0068] ct s^-1
    Mean flux:	    (2.21 [+0.25, -0.24])e-12 erg cm^-2 s^-1
    Peak rate:	    0.139 +/- 0.031 ct s^-1
    Peak flux:	    (5.0 +/- 1.1)e-12 erg cm^-2 s^-1
    ECF:	    3.57e-11 erg cm^-2 ct^-1
		      assuming NH=1.80e+21 cm^-2, gamma=1.97
		      determined from a spectral fit.
    XMM UL:	    3.5e-12 erg cm^-2 s^-1, (0.3-10 keV)
		      so the source is 1.4-sigma above this 3-sigma upper limit.
    The source may be fading, at the 2.6-sigma level. This position is consistent
with the candidate optical counterpart reported by COLIBRI (GCN 42904Loading...
 
 ).


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_FIELD00085.

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