TITLE:   GCN CIRCULAR
NUMBER:  39865
SUBJECT: GRB 250321B: ISON-Kitab RC-36 Optical Upper Limit
DATE:    25/03/24 17:11:05 GMT
FROM:    Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>

N. Pankov (HSE, IKI), A. Schmalz (KIAM), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:

We performed optical observations of the field of GRB 250321B (Schanne et. al, GCN 39818; Capalbi et. al, GCN 39856) in the Clear light with the 0.36-meter RC-36 telescope of the ISON-Kitab observatory. The observations started on (UT) 2025-03-21 17:31:49, i.e. about 0.59 days since trigger. We do not detect an optical afterglow candidate in the stacked image. The preliminary photometry is as follows:

Date       UT start  t-T0         Exp.    Filter OT     Err   UL(3sigma)
                     (mid, days)  (s)            
2025-03-21 17:31:49  0.59330      38*60   Clear  n/d    n/d   17.0

Ref. stars
RA Dec r
169.5809 +39.4176 13.063
169.7213 +39.4809 13.341

The photometry is based on nearby stars of SDSS-DR12 (r-magnitudes) and has not been corrected for the Galactic extinction. Our result is consistent with non-detections of an optical (Ducoin et. al, GCN 39840) and an X-Ray (Capalbi et. al, GCN 39856) counterpart.