TITLE: GCN CIRCULAR NUMBER: 40160 SUBJECT: EP250416a: Gemini South-GMOS optical counterpart DATE: 25/04/17 10:42:52 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group antonio.martin-carrillo@ucd.ie

Andrew J. Levan (Radboud), Jonathan Quirola-Vásquez (Radboud), Peter G. Jonker (Radboud), Franz E. Bauer (UTA), Antonio Martin-Carrillo (UCD), Daniele B. Malesani (DAWN/ NBI and Radboud), Javi Sánchez-Sierras (Radboud), Agnes van Hoof (Radboud), Jennifer Chacon (PUC), Joyce van Dalen (Radboud), Gregory Corcoran (UCD) report on behalf of a larger collaboration:

We observed the field of the Einstein Probe (EP) transient EP250416a (Zhao et al., GCN 40154) using the Gemini South-GMOS in imaging mode located on Cerro Pachón, Chile.

Photometric observations started at 2025-04-17 06:51:03 UTC (i.e., ~0.54 days after the EP detection), consisting of 4 exposures of 60 s each, carried out using filter r'.

Consistent with the position of the X-ray source detected by XRT (target ID 19721, PI J. Kennea; https://www.swift.ac.uk/EP/) and the EP/FXT localization, we notice a source at coordinates measured from the WCS-calibrated Gemini image:

RA = 17:05:40.83 Dec = +25:46:31.5

This source had magnitude r = 22.8 +/- 0.1 (AB), calibrated against the Pan-STARRS objects in the field. An underlying, fainter source is catalogued in the Legacy Survey (DR10; Dey et al. 2019), with a quoted magnitude of r = 23.16 and photometric redshift of 0.72+-0.24 (Zhou et al. 2021).

Based on the position and brighter magnitude relative to the previously catalogued object, we conclude that the source is very likely the optical counterpart of EP250416a.

We acknowledge excellent support from the Gemini South staff.

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