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 View this GCN Circular online at https://gcn.nasa.gov/circulars/39027. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...