TITLE:   GCN CIRCULAR
NUMBER:  40133
SUBJECT: GRB 250407A: Mondy and AbAO Optical Observations
DATE:    25/04/10 09:17:26 GMT
FROM:    Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>

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

We observed the field of GRB 250407A detected by Fermi (the Fermi GBM team, GCN 40104; Preis & Greiner, GCN 40105; Mukherjee & Meegan, GCN 40114), SRG (Molkov et. al, GCN 40110), Swift (Evans, GCN 40111; Dichiara et. al, GCN 40113), EP (Zheng et. al, GCN 40119), Konus-Wind (Frederiks et. al, GCN 40121) with the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy), and the 0.7-meter AS-32 telescope of the Abastumani Astrophysical Observatory (AbAO). The R-band observations began on 2025-04-08 at 18:01:15 UT, i.e. ~1.12 days since trigger at AbAO. The optical counterpart (Zheng & Fillipenko, GCN 40116; Ducoin et. al, GCN 40117; Pérez-Fournon et. al, GCN 40125) is marginally detected in the stacked image taken with AZT-33IK. The preliminary photometry is as follows:

Date       UTstart  Exptime     t-T0     Filter OT   Err  UL      Telescope
                      (s)   (mid, days)                  (3sigma)
2025-04-08 18:01:15 89*60    1.12319       R    n/d  n/d 17.5     AS-32
2025-04-09 14:42:55 34*120   1.97818       R    21.0 0.3 21.2     AZT-33IK

Ref. stars
USNO-B1.0
RA Dec R2mag
110.81407 36.79950 15.86
110.78017 36.81867 14.55

The photometry is based on nearby USNO-B1.0 stars (R2 magnitudes) and has not been corrected for the Galactic extinction.