TITLE: GCN CIRCULAR NUMBER: 37313 SUBJECT: GRB 240825A: SAO RAS optical observations DATE: 24/08/28 00:51:15 GMT FROM: Moskvitin Alexander at SAO RAS mosk@sao.ru

A. S. Moskvitin and O. I. Spiridonova (SAO RAS) report on behalf of the GRB follow-up team.

We observed the field of the GRB 240825A (The Fermi GBM team, GCN 37273; Gupta et al., GCN 37274; Lalla et al., GCN 37288; Evans et al., 37290; Gropp et al., GCN 37294; Joshi et al., GCN 37298; Sharma et al., GCN 37301; Frederiks et al., GCN 37302) with the SAO RAS 1-meter telescope Zeiss-1000 equipped with the CCD-photometer on August 25/26, 26/27 and 27/28. The weather conditions and seeing during the first and the second nights were poor. The third night was good. We obtained series of exposures in Rc band on 2024-08-25T20:37:53 -- 2024-08-25T22:32:59; 2024-08-26T22:33:48 -- 2024-08-27T01:29:03; 2024-08-27T19:51:04 -- 2024-08-27T21:15:46.

The OT (Gupta et al., GCN 37274; Jiang et al., GCN 37275; Dutton et al., GCN 37276; Odeh and Guessoum, GCNs 37277, 37299; Zhang et al., GCN 37278; Li et al., GCN 37280; Izzo and Malesani, GCN 37287; Lipunov et al., GCN 37289; Leonini et al., GCN 37291; Wu et al., GCN 37292; Martin-Carrillo et al., GCN 37293; Brivio et al., GCN 37295; Kuin et al., GCN 37296; Le Floc’h et al., GCN 37300; Zheng and Filippenko, GCN 37304; Wang et al., GCN 37306; Maksut et al., GCN 37307; Melandri et al., GCN 37310) is clearly detected in the stacked frame only during the third night. The non-detection of OT during our first and second nights is consistent with the magnitudes reported by the other teams. Our results are following.

Date T_mid - T0, d exp, s magnitude Aug 25 0.2378 1080 R_lim = 19.5 Aug 26/27 1.3392 1483 R_lim = 21.0 Aug 27 2.1948 4200 R = 21.88 +/- 0.07

Our stacked frames are calibrated against nearby Pan-STARRS objects and not corrected for the MW extinction.

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