TITLE: GCN CIRCULAR NUMBER: 40163 SUBJECT: EP20250416a: Swift/XRT counterpart detection DATE: 25/04/17 15:06:58 GMT FROM: Tullia Sbarrato at INAF-OAB tullia.sbarrato@inaf.it

T. Sbarrato (INAF-OAB), R. Brivio (INAF-OAB), M. Ferro (INAF-OAB), C. Salvaggio (INAF-OAB) 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 EP 250416a (Zhao et al., GCN 40154). We searched for X-ray sources in 2.0 ks of Photon Counting (PC) mode data, from T0+17.1 ks to T0+23.2 ks after the EP/WXT trigger. An uncatalogued X-ray source is detected within the estimated 3-sigma EP/WXT error region. Using 2000 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 256.41935, +25.7751 which is equivalent to:

RA (J2000): 17h 05m 40.64s Dec(J2000): +25d 46’ 30.4"

with an uncertainty of 3.1 arcsec (radius, 90% confidence). This position is 1 arcmin from the EP/WXT position, and consistent with the optical candidate reported by Levan et al., (GCN 40160).

The source has a mean count rate of 9.9e-02 ct/sec and currently shows no signs of fading.

The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/EP/EP_FIELD00034/

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

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