TITLE: GCN CIRCULAR NUMBER: 39027 SUBJECT: EP250125a: Gemini-South optical afterglow discovery and redshift z = 2.89 DATE: 25/01/25 09:26:10 GMT FROM: Andrew Levan at Radboud University a.levan@astro.ru.nl

A. J. Levan (Radboud Univ. and Warwick Univ.), J. A. Quirola-Vasquez (Radboud Univ.), F. E. Bauer (PUC), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud Univ.) report on behalf of a larger collaboration.

We observed the field of the EP transient 01709130802 (EP250125a, trigger time: 2025-01-25 02:37:31.749 UT) using the Gemini South telescope equipped with the GMOS-S camera and spectrograph. A set of images in the r and z bands were secured, starting on 2025 25.294 UT (4.43 hr after the trigger).

A point-like object not present in the archival Pan-STARRS images of the field is detected in both filters at coordinates (J2000), well within the 3 arcmin-radius WXT error circle:

RA = 11:41:27.39 Dec = -21:42:51.5

For this object, we measure an AB magnitude r = 21.60 +- 0.03, calibrated against nearby objects from the Pan-STARRS catalog.

We suggest this object is the optical counterpart of EP250125a.

A spectrum of the source was taken with the B480 grism, starting at 08:39 UT. In the first 600 s exposure, the source is well detected across the spectral range from 4300 to 8200 AA (preliminary wavelength calibration). A strong absorption trough is visible at ~4730 AA, which we interpret as a DLA. From the detection of several narrow absorption features, including those from Si II, C II, Si IV, Al II, and Fe II we determine z = 2.89, which we suggest to be the redshift of EP250125a.

We thank the Gemini staff, in particular Karleyne Silva, for masterfully executing the requested observations

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