TITLE: GCN CIRCULAR NUMBER: 40339 SUBJECT: GRB 250502A: Mondy and Tien-Shan Optical Observations DATE: 25/05/04 11:59:20 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), E. Klunko (ISTP), I. Reva (FAI), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN: We performed follow-up observations of the optical counterpart (Rakotondrainibe et. al, GCN 40315) to GRB 250502A (Wang et al., GCN 40313) using the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy), and the 1-meter Zeiss-1000 telescope of the Tien-Shan Astronomical Observatory (TSHAO). The optical source was observed also by (An et. al, GCN 40319; Li et. al, GCN 40320; Ghosh et. al, GCN 40322; Ugarte Postigo et. al, GCN 40328; Pérez-Fournon et. al, GCN 40330; Odeh et. al, GCN 40331; Bochenek & Perley, GCN 40333; Corcoran et. al, GCN 40334; Leonini et. al, GCN 40337; Zheng et. al, GCN 40338). The redshift z = 2.163 was determined by Ugarte Postigo et. al, GCN 40328. The X-Ray counterpart was detected by XRT (Dichiara et. al, GCN 40336). Our observations began on 02.05.2025 (UT) 15:48 at Mondy Observatory and 2025-05-03 18:25:43 at Tien-Shan Observatory. We detect the optical counterpart in the stacked image in the first observation. The preliminary photometry is as follows: Date UTstart Exptime t-T0 Filter OT Err UL (s) (mid, days) (3sigma) 2025-05-02 15:48:58 30*120 0.31425 R 19.74 0.10 22.5 2025-05-03 18:25:43 20*120 1.41617 R n/d n/d 21.0 The photometry is based on nearby USNO-B1.0 stars and has not been corrected for the Galactic extinction. Reference stars RA Dec R2 206.5862 -10.7663 14.69 206.4706 -10.7622 16.20 206.5453 -10.7644 17.95 View this GCN Circular online at https://gcn.nasa.gov/circulars/40339. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...