GCN Circular 42401
Subject
EP#01709247298: Swift-XRT counterpart detection
Date
2025-10-23T15:35:42Z (2 days 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 EP#01709247298, collecting 1.6 ks of Photon Counting (PC) mode data between
T0+6.6 ks and T0+8.3 ks after the trigger. A likely counterpart has been found. This
source is 43 arcmin from the position of the MAXI-detected transient GRB 251023B /
MAXI J0451+122 (Kang et al., GCN Circ. 42392), well outside their error region, thus
its association, while likely on temporal grounds, remains unconfirmed. The details
of this source are:
Source 1 (SWIFT J045400.4+123929):
==================================
RA (J2000.0): 73.5018 = 04 54 00.43
Dec (J2000.0): +12.6583 = +12 39 29.9
Error: 3.7 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 22 arcsec from the Einstein Probe/WXT position.
Mean rate: 0.152 +/- 0.011 ct s^-1
Mean flux: (5.94 +/- 0.44)e-12 erg cm^-2 s^-1
Peak rate: 0.221 +/- 0.049 ct s^-1
Peak flux: (8.6 +/- 1.9)e-12 erg cm^-2 s^-1
ECF: 3.91e-11 erg cm^-2 ct^-1
assuming NH=3.15e+21 cm^-2, gamma=1.97
determined from a spectral fit.
XMM UL: 2.3e-12 erg cm^-2 s^-1, (0.3-10 keV)
so the source is 3.2-sigma above this 3-sigma upper limit.
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_FIELD00077.
This circular is an official product of the Swift-XRT team.