TITLE:   GCN CIRCULAR
NUMBER:  44600
SUBJECT: GRB 260511B: further SAO RAS optical observations
DATE:    26/05/13 22:29:56 GMT
FROM:    Alexander Moskvitin at SAO RAS <mosk@sao.ru>

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

We observed the field of the GRB 260511B discovered by Fermi
(The Fermi GBM team, GCN 44542; Roberts, GCN 44554), SVOM
(Godet et al., GCN 44543), NuSTAR (Waratkar and Grefenstette,
GCN 44556), AstroSat CZTI (Arya et al., GCN 44560),
EP-FXT (Fu et al., GCN 44578), Konus-Wind (Ridnaia et al. GCN 44597)
with the Zeiss-1000, 1m telescope of the SAO RAS
equipped with the CCD photometer. The observations were performed
in Rc band on May 12 and 13 under moderate weather conditions.

The OT (Wu et al., GCN 44547; Sasada et al., GCN 44549;
Jiang et al., GCN 44551; Wu et al., GCN 44552; Vijaykumar et al.,
GCN 44561; de Ugarte Postigo et al., GCN 44565; Lee et al., GCN 44567;
Li et al., GCN 44568; Wang et al., GCN 44571; Belkin et al.,
GCN 44572; Adami et al., GCN 44573; Saccardi et al., GCN 44574;
Rajabov et al., GCN 44576; Pérez-Fournon et al., GCN 44577;
Novotný et al., GCN 44580; Moskvitin and Pozanenko, GCN 44582;
Calapai, GCN 44592; also detected in NIR by Li et al.,
GCN 44557; GCN 44593) is clearly detected in the stacked frames
with the following brightness.

Date       T_start   T_end     t_mid-T0, exp, filter mag   +/- err   UL
           UT        UT        days      s    
2026-05-12T18:57:30--20:17:50  1.35912   2760 Rc     20.93 +/- 0.05  23.1
2026-05-13T19:12:53--21:21:03  2.38641   4800 Rc     21.49 +/- 0.07  23.3

This preliminary photometry is based on nearby SDSS stars
(using Lupton 2005 transformation equations)
and is not corrected for Galactic extinction.