TITLE:   GCN CIRCULAR
NUMBER:  41931
SUBJECT: GRB 250920B: Kitab-ISON optical upper limit
DATE:    25/09/21 17:38:23 GMT
FROM:    Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>

N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Novichonok (Petrozavodsk State University,  KIAM) report on behalf of GRB-IKI-FuN:

We observed the field of a long GRB 250920B detected by Fermi/GBM (The Fermi GBM team GCN 41897), Swift (Gupta et. al, GCN 41898), with the RC-36 telescope of Kitab-ISON. The series of unfiltered 325x15 sec exposures was obtained starting on 2025-09-20 at 14:31 UT, i.e. ~0.26 days since Fermi/GBM trigger. The optical afterglow initially detected by Swift/UVOT (Gupta et. al, GCN 41898) and further observed by (WU et. al, GCN 41908; Hall et. al, GCN 41914) is not visible in our stacked image of 298x15 sec. Preliminary results of our observations provided below:

Date       UTstart Exptime       t-T0   Filter OT  Err. UL
                    (nxs)   (mid, days)                 (3sigma)
2025-09-20 14:31:30 298x15  0.26550     Clear  n/d n/d  15.4

The photometry was calibrated against nearby USNO-B1.0 stars (R2 mags) and not corrected for the Galactic extinction.

Ref. stars
USNO-B1.0
RA Dec R2
187.73536 47.71878 13.75
187.52490 47.74643 13.74