TITLE:   GCN CIRCULAR
NUMBER:  37244
SUBJECT: GRB 240809A: Mondy and AbAO Optical Observations
DATE:    24/08/23 11:19:52 GMT
FROM:    Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>

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

We observed the field of GRB 240809A, detected by Swift (Evans et al., GCN
37110) and SVOM (Wang et al., GCN 37113), using the AZT-33IK 1.5-meter
telescope at the Sayan Observatory (Mondy) and the AS-32 0.7-meter
telescope at the Abastumani Observatory (AbAO). The observations began on
2024-08-09 at 13:54:52 (UT) and 17:27:38 (UT), respectively, i.e., about
0.2 and 7 hours after the Swift trigger. The preliminary photometry of the
optical counterpart (Evans et al., GCN 37110; Dubay et al., GCN 37114;
Jiang et al., GCN 37116; Gottumukkala et al., GCN 37122; Shilling et al.,
GCN 37123; Mohan et al., GCN 37125; Zheng & Filippenko, GCN 37127;
Schneider et al., GCN 37129; Quadri et al., GCN 37131; Brivio et al., GCN
37132; Midavaine et al., GCN 37159; Ruocco et al., GCN 37194) is provided
below:

Date UT Start t-T0 Exp. Filter OT Err. UL Telescope
(mid, days) (s) (3sigma)
2024-08-09 13:54:52 0.24228 49*60 R 19.45 0.05 22.3 AZT-33IK
2024-08-09 17:27:38 0.37302 44*60 R n/d n/d 20.0 AS-32

The magnitudes were calibrated using nearby comparison stars from the
USNO-B1.0 catalog (R2 magnitudes). No correction was made for the Galactic
extinction toward the GRB, corresponding to E(B-V) = 0.213 (S&F 2011). The
optical light curve is well fitted by a power-law model with an index of
-1.3, which is typical for GRBs.