TITLE:   GCN CIRCULAR
NUMBER:  36118
SUBJECT: GRB 240415A: Mondy optical transient detection
DATE:    24/04/15 16:15:54 GMT
FROM:    Alexei Pozanenko at IKI, Moscow <apozanen@iki.rssi.ru>

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

We observed the field of GRB 240415A (Fermi GBM team, GCN 36107; Moss et al., GCN 36108, Evans et al., GCN 36109) with 1.5-meter AZT-33IK telescope of Sayan observatory at Mondy. The observations were started on 2024-04-15 (UT) 14:33:22, i.e. ~8 hr since the Fermi GBM trigger. We took 30 frames 120 s each in the R-filter. We detect a possible optical counterpart in the stacked image at coordinates of (J2000)  08:36:45.26    73:08:47.6 with uncertainties of 0.4 arcsec in both coordinates. 
Preliminary photometry of the optical source is following:

Date       UT_start  MJD           Filter Exp.  OT    err  UL(3sigma)
                    (mid, days)           (s)
2024-04-15 14:33:22  60415.627345  R     44*120 18.84 0.08 21.9

The magnitudes were calibrated using nearby USNO-B1.0 stars.