TITLE:   GCN CIRCULAR
NUMBER:  39836
SUBJECT: EP250321a: Mondy AZT-33IK Optical Observations
DATE:    25/03/22 17:46:10 GMT
FROM:    Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>

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

We performed optical observations of the field of EP250321a (Hu et. al, GCN 39800; Page et. al, GCN 39811, Hu et. al, GCN 39833) at the redshift of z = 4.368 (Zhu et. al, GCN 39769) in the R filter with the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy). The observations started on (UT) 2025-03-21 16:42:58, i.e. about 0.46 days since trigger. The optical afterglow (Fu et. al, GCN 39804; Perez-Garcia et. al, GCN 39805; Brivio et. al, GCN 39807; Zhu et. al, GCN 39809; Becerra et. al, GCN 39810; Pérez-Fournon et. al, GCN 39812; Lee et. al, GCN 39815; Han et. al, GCN 39817; Jin et. al, GCN 39822; Zhu et. al, GCN 39827; Gill et. al, GCN 39831; Zheng et. al, GCN 39832) is detected in the stacked image. The preliminary photometry is as follows:

Date       UT start  t-T0         Exp.    Filter OT     Err   UL(3sigma)
                     (mid, days)  (s)            
2025-03-21 16:42:58  0.46042      30*120  R      22.41  0.28  22.8

The photometry is based on nearby stars of SDSS-DR12 and has not been corrected for the Galactic extinction.

SDSS-DR12
RA Dec R(Lupton transformations, 2005)
179.2552 +17.3825 18.123 +/- 0.049
179.2436 +17.3713 18.846 +/- 0.012