GCN Circular 41802
Subject
EP250911a: Swift-XRT observations
Event
Date
2025-09-11T17:36:52Z (a day 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 EP250911a (Liang et al., GCN Circ 41788), collecting 1.7 ks of Photon
Counting (PC) mode data between T0+5.5 ks and T0+11 ks after the trigger. We have
detected 1 source consistent with the EP/FXT position (Liang et al., GCN Circ
41788). This has been automatically classified as an uncatalogued X-ray source.
Uncatalogued X-ray sources
--------------------------
Source 1 (SWIFT J235004.5+291757):
==================================
RA (J2000.0): 357.5191 = 23 50 04.58
Dec (J2000.0): +29.2992 = +29 17 57.1
Error: 4.6 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 62 arcsec from the Einstein Probe/WXT position.
Mean rate: 0.0239 +/- 0.0047 ct s^-1
Mean flux: (8.3 +/- 1.6)e-13 erg cm^-2 s^-1
Peak rate: 0.0239 +/- 0.0047 ct s^-1
Peak flux: (8.3 +/- 1.6)e-13 erg cm^-2 s^-1
ECF: 3.47e-11 erg cm^-2 ct^-1
assuming NH=5.84e+20 cm^-2, gamma=1.92
determined from a spectral fit.
XMM UL: 7.6e-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.
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.
We note that source 1 also matches the position of the optical counterpart to
EP250911a reported by Quirola-Vasquez et al. (GCN Circ. 41790) and Angulo et al.
(GCN Circ. 41793).
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_FIELD00073.
This circular is an official product of the Swift-XRT team.