TITLE:   GCN CIRCULAR
NUMBER:  42478
SUBJECT: GRB 251023B / EP251023b: Mondy optical upper limit
DATE:    25/10/27 13:44:29 GMT
FROM:    Alina Volnova at IKI RAS <alinusss@gmail.com>

A. Volnova (IKI), A. Pozanenko (IKI) , N. Pankov (HSE, IKI), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN:

We observed the field of the GRB 251023B / EP251023b (Kang et al., GCN Circ. 42392; Nakahira et al., GCN Circ. 42402; Wu et al., GCN Circ. 42415; Cheung et al., GCN Circ. 42407) in the R-filter with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) starting Oct. 25, 2025, 17:50:36 UT, and taking several frames with 120 s exposures. The optical candidate reported by Mandarakas et al. (GCN Circ. 42404) is not detected in the stacked frame. Photometry and observational details are the following:

Date        UT start  t-T0       Exp.    Filter  OT     Err.  UL
                     (mid,days)  (n*s)                       (3sigma)
2025-10-25  17:50:36  2.40386    40*120  R       n/d    n/d   22.7

The photometry is based on several nearby SDSS stars (R-magnitudes Lupton's transformation) and is not corrected for the Galactic extinction.