TITLE: GCN CIRCULAR NUMBER: 40676 SUBJECT: GRB 250610B: optical counterpart candidate DATE: 25/06/10 23:26:37 GMT FROM: Benjamin Schneider at MIT bschn@mit.edu

B. Schneider (LAM), A. de Ugarte Postigo (LAM), C. Adami (LAM/Pytheas/AMU), J.P Troncin (Pytheas/OHP), A. Martin-Carrillo (UCD), S. Basa (LAM/OHP/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), report on behalf of a larger collaboration:

We observed the field of the GRB 250610B (Saccardi et al., GCN 40671) using the T120cm telescope at Observatoire de Haute-Provence (France). We obtained 30 minutes of exposure in the r-band starting at 20:31:03.29 UT on 2025-06-10 (3.97 hr after the trigger).

In the stacked image, we detected a new source at RA, DEC (J2000) 200.1796, +31.1047, not visible in the Legacy Survey and consistent with the XRT error:

RA(J2000) = 13:20:43.11 DEC(J2000) = +31:06:16.8 with an uncertainty of 0.5 arcsec.

The preliminary magnitude derived for that source is:

r = 20.96 +/- 0.15 mag (AB)

The photometric calibration was performed using nearby stars from the SDSS catalog and the magnitude is not corrected for Galactic extinction.

We acknowledge the excellent support from Observatoire de Haute-Provence and in particular the students and professors from the summer camp OHP 2025.

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