GCN Circular 41624

Subject
EP250831a: Swift-XRT observations
Event
EP250831a
Date
2025-09-01T04:39:35Z (a month 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 EP250831a, collecting 1.8 ks of Photon Counting (PC) mode data between T0+24
ks and T0+30 ks after the trigger. We have detected 1 source 4.6 arcmin from the WXT
position, somewhat outside the 90% error region of 2.9 arcmin (GCN 41619Loading...
 
 ).  This has
been automatically classified as an uncatalogued X-ray source.

Uncatalogued X-ray sources
--------------------------

  Source 1 (SWIFT J025130.9-360728):
  ==================================
    RA (J2000.0):   42.8789  =	02 51 30.94
    Dec (J2000.0):  -36.1245  =  -36 07 28.2
    Error:	    5.0 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    4.6 arcmin from the Einstein Probe/WXT position.
    Mean rate:	    (7.6 [+2.8, -2.3])e-3 ct s^-1
    Mean flux:	    (1.97 [+0.73, -0.59])e-13 erg cm^-2 s^-1
    Peak rate:	    (7.6 [+2.8, -2.3])e-3 ct s^-1
    Peak flux:	    (1.97 [+0.73, -0.59])e-13 erg cm^-2 s^-1
    ECF:	    2.58e-11 erg cm^-2 ct^-1
		      assuming NH=3.05e+20 cm^-2, gamma=2.54
		      determined from a spectral fit.
    LSXPS UL:	    1.7e-02 ct/sec, (0.3-10 keV)
		      so the source is not above this 3-sigma upper limit.
    There is no evidence for fading.

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_FIELD00066.

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