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GCN Circular 36332

Subject
LIGO/Virgo S240422ed: Swift XRT observation of EP240426a
Date
2024-04-27T08:41:47Z (13 days ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
K.L. Page (U. Leicester), P.A. Evans (U. Leicester), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto), S.B. Cenko (NASA/GSFC), R.A.J. Eyles-Ferris (U.
Leicester), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB),
A.A. Breeveld (UCL-MSSL), S. Campana (INAF-OAB), J.J. Delaunay (PSU), M.
De Pasquale (University of Messina), S. Dichiara (PSU), P. D’Avanzo
(INAF-OAB), A. D’Aì (INAF-IASFPA) , V. D’Elia (ASI-SSDC & INAF-OAR), C.
Gronwall (PSU), D. Hartmann (Clemson University), N. Klingler (NASA-GSFC /
UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha (NASA/GSFC), S.R. Oates
(U. Birmingham), J.P. Osborne (U. Leicester), P. O’Brien (U. Leicester),
M.J. Page (UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri
(INAF-OAB), E. Troja (U Tor Vergata, INAF) report on behalf of the Swift
team:

Swift has conducted a follow-up observation of the candidate counterpart
EP240426a (GCN Circ. 36313Not found
 
 
), starting 28.8 ks after the EP-FXT discovery.
We detect an X-ray source 6.2" from, and thus consistent with, the
localisation reported by EP-FXT:

Source S240422ed_X255
=====================
    RA:          121.85742 ( = 08h 07m 25.78s) J2000
    Dec:         -29.4593  ( = -29° 27′ 33.5″) J2000
    Error:       5.3 (arcsec, radius, 90% confidence)
    Peak Rate:   0.012 +/- 0.003 ct s^-1 (0.3-10 keV)
    Peak Flux:   5.1e-13 +/- 1.3e-13 erg cm^-2 s^-1
    RASS UL:     0.034 ct s^-1 (converted to XRT; 0.3-10 keV)
                 so the source is not above the RASS 3-sigma
                 upper limit

For the flux conversions and comparisons with catalogues and upper limits
from other missions given above, we assumed a power-law spectrum with
NH=3x10^20 cm^-2, and photon index (Gamma)=1.7.

However, for direct comparison with the flux of 9.2 x 10 ^-13 erg cm^-2
s^-1 given in GCN Circ. 36313Not found
 
 
, we estimate a value of 7.0 x 10^-13 erg
cm^-2 s^-1, calculated over 0.5-10 keV, using Gamma = 2.0, corrected for
the higher Galactic absorption of 4.3x10^21 cm^-2 in this direction.

The results of the XRT automated analysis, including details of the
sources detected during the follow-up of S240422ed, are online at
https://www.swift.ac.uk/LVC/S240422ed/

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