GCN Circular 41698
Subject
EP250903a: Swift-XRT observations
Event
Date
2025-09-04T13:34:42Z (a month ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
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 EP250903a, collecting 3.7 ks of Photon Counting (PC) mode data between T0+1.3
ks and T0+35 ks after the trigger. We have detected 1 source inside the EP/WXT error
region. This has been automatically classified as follows:
* 0 likely counterparts
* 0 candidate counterparts
* 1 uncatalogued X-ray source
* 0 known X-ray sources
Uncatalogued X-ray sources
--------------------------
Source 2 (SWIFT J223627.0-495113):
==================================
RA (J2000.0): 339.1126 = 22 36 27.02
Dec (J2000.0): -49.8538 = -49 51 13.7
Error: 7.1 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 37 arcsec from the Einstein Probe/WXT position.
Mean rate: (3.8 [+1.4, -1.2])e-3 ct s^-1
Mean flux: (2.20 [+0.83, -0.67])e-13 erg cm^-2 s^-1
Peak rate: (3.8 [+1.4, -1.2])e-3 ct s^-1
Peak flux: (2.20 [+0.83, -0.67])e-13 erg cm^-2 s^-1
ECF: 5.78e-11 erg cm^-2 ct^-1
assuming NH=1.17e+20 cm^-2, gamma=1.15
determined from a spectral fit.
XMM UL: 1.9e-12 erg cm^-2 s^-1, (0.3-10 keV)
so the source is not above this 3-sigma upper limit.
There is no evidence for fading.
There are 2 2MASS objects 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 position of Source 2 is consistent with the optical counterpart detected by LCO
(GCN 41680) and SVOM/VT (GCN 41683).
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_FIELD00068.
This circular is an official product of the Swift-XRT team.