TITLE:   GCN CIRCULAR
NUMBER:  37049
SUBJECT: EP240801a: BTA BVRI photometry
DATE:    24/08/05 17:22:19 GMT
FROM:    Moskvitin Alexander at SAO RAS <mosk@sao.ru>

A. Moskvitin, D. Oparin (SAO RAS), N. Pankov (HSE, IKI), A. Pozanenko (IKI)
report:

We observed the field of the fast X-ray transient EP240801a
(Zhou et al., GCN 36997) with the BTA 6-meter telescope
of SAO RAS equipped with the focal reducer Scoprio-I on August, 3
(t_mid - T0 = 2.629 days) and August, 4 (t_mid - T0 = 3.485 days).

The OT (Fu et al., GCN 36998; Li et al., GCN 36999; Zheng et al.,
GCN 37000; Aryan et al., GCN 37002; An et al., GCN 37004;
Perez-Fournon et al., GCN 37007; Malesani et al., GCN 37008;
Zhu et al., GCN 37010; Moskvitin et al., GCNs 37012, 37017;
Quirola-Vasquez et al., GCN 37013; Moretti et al., GCN 37014;
Turpin et al., GCN 37015; Pankov et al., GCNs 37016, 37040) is clearly
detected in stacked frames. Preliminary photometry of OT in the both epochs
as is follows.

Filter, exp, t_start                t_end (UTC)         OT, mag
        s,  (UT)                    (UT)
B    620    2024-08-03T23:44:36 -- 2024-08-04T00:37:41 23.88 +/- 0.13
V    600    2024-08-03T23:50:17 -- 2024-08-04T00:33:45 23.13 +/- 0.09
Rc   510    2024-08-03T23:47:46 -- 2024-08-04T00:35:28 22.82 +/- 0.07
Ic   510    2024-08-03T23:52:56 -- 2024-08-04T00:30:14 22.05 +/- 0.10

V    900    2024-08-04T20:28:02 -- 2024-08-04T21:01:52 23.55 +/- 0.08
Rc   930    2024-08-04T20:23:36 -- 2024-08-04T21:03:41 22.91 +/- 0.06

The magnitudes were calibrated against nearby Pan-STARRS stars
and not corrected for Galactic extinction.

We note significant reddening (V-Rc) between the two epochs.