TITLE: GCN CIRCULAR NUMBER: 38877 SUBJECT: GRB 250108B: Gemini-North likely redshift z = 2.197 DATE: 25/01/09 17:37:07 GMT FROM: Jillian Rastinejad at Northwestern Univ. jillianrastinejad2024@u.northwestern.edu

Daniele B. Malesani (DAWN/ NBI and Radboud), Jillian C. Rastinejad (Northwestern), Andrew J. Levan (Radboud), Wen-fai Fong, Charlie Kilpatrick (Northwestern), Gavin P. Lamb (LJMU), and Nial R. Tanvir (Leicester) report on behalf of a larger collaboration:

Following the detection of the optical afterglow (Rastinejad et al., GCN 38855) of GRB 250108B (Klinger et al., GCN 38847), we obtained optical spectroscopy using the Gemini Multi-Object Spectrograph (GMOS-N) mounted on Gemini-North, under Program GN-2024B-Q-107 (P.I. Rastinejad). The mean time of the observation was 15:12 UT on 2025 Jan 8 (4.81 hr post-burst). The total exposure time was 4x400 s using the R400 grating, which covers the wavelength range 4650-9450 AA.

Given the faintness of the target (r ~ 23.4; Rastinejad et al., GCN 38855), only a weak trace is detected, with most of the signal apparent at the red end of the spectrum. A few absorption features are weakly detected. A possible redshift solution, based on the detection of Mg I 2852, Mg II 2796,2803, Fe II 2600, and Fe II 2586, yields z = 2.197, which we suggest to be the redshift of GRB 250108B.

The slit was aligned in order to cover also the nearby galaxy (Rastinejad et al., GCN 38855) at coordinates RA = 13:25:18.72, Dec = +25:37:12.7, with photometric redshift z = 0.2-0.3, resulting in a bright trace. We detect Halpha, [N II], and [S II] at z = 0.302, consistent with the Legacy Survey photometric redshift of z = 0.29 +/- 0.04 (Zhou et al., 2021). Our observations indicate that this object is unlikely to be physically connected to the GRB.

We thank Jen Miller, Leila Alamos, Brian Lemaux and additional Gemini staff for excellent support in the rapid planning and execution of these observations.

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