TITLE: GCN CIRCULAR NUMBER: 39870 SUBJECT: EP250321a: Mondy AZT-33IK Continued Optical Observations DATE: 25/03/25 17:27:37 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 continued 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) 2025-03-22 15:35:42, i.e. about 1.4 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; Bochenek & Perley, GCN 39837; Zhu et. al, GCN 39854) is not detected in the stacked images. The preliminary photometry is as follows: Date UT start t-T0 Exp. Filter OT Err UL(3sigma) (mid, days) (s) 2025-03-22 15:35:42 1.40467 17*120 R n/d n/d 22.3 2025-03-23 16:02:48 2.45336 60*120 R n/d n/d 23.6 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 View this GCN Circular online at https://gcn.nasa.gov/circulars/39870. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...