TITLE: GCN CIRCULAR NUMBER: 39202 SUBJECT: GRB 250207A: REM optical afterglow detection DATE: 25/02/07 13:23:26 GMT FROM: Riccardo Brivio at INAF-OAB riccardo.brivio@inaf.it

R. Brivio, M. Ferro, P. D’Avanzo, S. Covino (INAF-OAB), A. Melandri (INAF-OAR), D. B. Malesani (DAWN/NBI and Radboud Univ.), G. Tagliaferri, D. Fugazza (INAF-OAB) report on behalf of the REM team:

We observed the field of GRB 250207A, detected by Fermi/GBM (Fermi GBM Team, GCN 39181) and Swift/BAT (Ferro et al., GCN 39182) 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 February 07 at 01:17:08 UT (i.e. 61 s after the Swift trigger), and lasting for about 2 hours. From preliminary photometry we detect the counterpart in the optical images at the position of the optical afterglow (Angulo et al., GCN 39186; Kuin & Ferro, GCN 39199) with the following magnitude:

r = 12.7 +/- 0.1 (AB; calibrated against the Pan-STARRS catalogue) at a mid-time of t - t0 = 66 s after the trigger.

The analysis of the NIR data is ongoing.

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