TITLE:   GCN CIRCULAR
NUMBER:  37146
SUBJECT: EP240801a: Assy-Turgen optical observations
DATE:    24/08/12 06:35:28 GMT
FROM:    Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>

N. Pankov (HSE, IKI), V. Kim (FAI), M. Krugov (FAI), S. Belkin (IKI, HSE),
A. Pozanenko (IKI) report on
behalf of GRB-IKI-FuN:

We observed the field of EP240801a (Zhou et al, GCN 36997) with AZT-20
telescope of Assy-Turgen observatory. The observation were conducted on
epochs
2024-08-04 17:16, 2024-08-07 19:03, and 2024-08-08 19:36 (UT, starting
time).
The optical source (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; Pérez-Fournon et
al,
GCN 37007; Malesani et al, GCN 37008; Zhu et al, GCN 37010;
Moskvitin & Spiridonova, GCN 37012; Quirola-Vásquez et al, GCN 37013;
Moretti et al, GCN 37014; Turpin et al, GCN 37015; Pankov et al, GCN 37016;
Moskvitin & Spiridonova, GCN 37017; Ruocco et al, GCN 37024; Pankov et al,
GCN
37040; Moskvitin et al, GCN 37049) is detected only on the first epoch.
Preliminary photometry is the following:

Date       UT start  t-T0     Exp.    Filter   OT     Err.     UL
                  (mid, days)  (s)                          (3sigma)
2024-08-04 17:16:21 3.373472  95*60   r'       23.29  0.14    23.6
2024-08-07 19:03:15 6.437292  65*60   r'       n/d    n/d     23.9
2024-08-08 19:36:10 7.458070  59*60   r'       n/d    n/d     24.0

The magnitudes were calibrated using nearby stars from SDSS-DR12 catalog
and are not corrected for the expected Galactic extinction A(r) = 0.252 mag
(S&F 2011) towards the EP240801a source.

The light curve in the r and R filters (AB mags) can be fitted with a
smoothed broken power law (Beuermann et al, 1999) with parameters:
alpha_1 = 1.1, alpha_2 = 0.33, t_break = 1.5 days with fixed smoothing
parameter omega = 1.0. We also note, that there
is a point-like source in the LS DR10 at the position of EP240801a. The
source magnitudes are g = 25.14, r = 24.40, and the photometric redshift of
0.995 +- 0.352, which is marginally consistent with the redshift of 1.673
obtained with GTC spectroscopy (Quirola-Vásquez et al, GCN 37013).