TITLE:   GCN CIRCULAR
NUMBER:  40204
SUBJECT: EP250421a: Swift/XRT counterpart detection
DATE:    25/04/22 09:04:31 GMT
FROM:    Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>

C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), R. Brivio (INAF-OAB), M.
Ferro (INAF-OAB), K. Page (U. Leicester), D. Malesani (DAWN/NBI and
Radboud) and P.A. Evans (U. Leicester) report on behalf the Swift-XRT team:

Swift/XRT has performed follow-up observations of EP-WXT detected transient
EP250421a (trigger 01709135179, Zhao et al. GCN 40198).
We searched for X-ray sources in 1.8 ks of Photon Counting (PC) mode data,
from T0+5.8 ks to T0+11.7 ks after the EP-WXT trigger.
We found an uncatalogued X-ray source within the estimated 3-sigma EP-WXT
error region. Using 1772 s of XRT PC mode data and 2 UVOT images, we find
an enhanced XRT position: RA, Dec = 172.420,-24.67568 which is equivalent
to:

RA (J2000): 11h 29m 40.80s
Dec (J2000): -24° 40' 32.4"

with an uncertainty of 2.7 arcsec (radius, 90% confidence).
This position is 35 arcsec from the EP-WXT position, and consistent with
the X-ray counterpart found in EP/FXT data (Zhao et al., GCN 40198) and
with the optical counterpart reported by Lee et al. (GCN 40192), Fu et al.
(GCN 40193), Liu et al. (GCN 40197).

The source has a mean count rate of 0.272 ct/s and shows signs of fading at
2.8 sigma significance.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/EP/EP_FIELD00035/

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