TITLE: GCN CIRCULAR NUMBER: 38874 SUBJECT: GRB 250108B: REM optical/NIR upper limits DATE: 25/01/09 16:12:35 GMT FROM: Riccardo Brivio at INAF-OAB riccardo.brivio@inaf.it

R. Brivio, M. Ferro, P. D’Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:

We observed the field of GRB 250108B detected by Swift (Klingler et al., GCN 38847) with the REM 60cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, K bands, starting on 2025 January 09 at 07:02:48 UT (i.e. 21.07 hours after the Swift trigger), and lasting for about 1 hour. Within the XRT-enhanced position (Osborne et al., GCN 38848), from preliminary photometry we do not detect any optical or NIR counterpart consistent with the candidate optical afterglow (Rastinejad et al., GCN 38855), down to the following 3sigma magnitude upper limits:

r > 20.1 (AB; calibrated against the PanSTARRS catalogue) at a mid-time of t - t0 = 21.2 hr after the trigger,

H > 17.0 (Vega; calibrated against the 2MASS catalogue) at a mid-time of t - t0 = 21.1 hr after the trigger.

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