TITLE: GCN CIRCULAR NUMBER: 38855 SUBJECT: GRB 250108B: Gemini-North Candidate Optical Afterglow Detection DATE: 25/01/08 16:51:54 GMT FROM: Jillian Rastinejad at Northwestern Univ. jillianrastinejad2024@u.northwestern.edu

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

We observed the location of the X-ray afterglow of GRB 250108B (Klinger et al., GCN 38847; Osborne et al., GCN 38848) with the Gemini Multi-Object Spectrograph (GMOS-N) mounted on Gemini-North under Program GN-2024B-Q-107. We obtained 15x120-s imaging in r-band at a mid-time of 2025-01-08 12:42:45.8 UT (2.33 hours post-burst), at a median airmass of 1.7 and seeing < 1.2''.

We clearly detect a faint source not present in archival Legacy Survey DR10 r-band imaging but within the XRT localization at a position of

RA (J2000) = 13:25:18.50 Dec (J2000) = 25:36:55.82

with an uncertainty of 0.7''. We suggest that this is the afterglow of GRB 250108B.

We note that the candidate afterglow is 16.8'' offset from a bright galaxy (r = 19.5 AB mag). This galaxy has a Legacy Survey DR9 photometric redshift of z = 0.29 +/- 0.04 (Zhou et al., 2021) and an SDSS photometric redshift of z = 0.22 +/- 0.04 (Alam et al., 2015), corresponding to projected separations of 74 kpc and 60 kpc from the transient, respectively. We determine a probability of chance coincidence between this galaxy and the candidate afterglow of Pcc = 0.13 (Bloom et al., 2002). With present data we cannot rule out the presence of a faint, underlying host at the position of the transient.

Calibrated to SDSS, we measure a preliminary brightness for the optical source of r = 23.4 +/- 0.1 AB mag, not corrected for Galactic extinction. At a redshift of z = 0.29, this corresponds to an optical luminosity of 2.1 x 10^42 erg/s.

Further observations are planned to monitor the variability of the source. 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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