TITLE: GCN CIRCULAR NUMBER: 44618 SUBJECT: GRB 260511B: NOT optical observations DATE: 26/05/15 16:42:48 GMT FROM: sqjiang at NAOC <sqjiang@bao.ac.cn> S. Q. Jiang, D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), J. P. U. Fynbo (DAWN/NBI), A. M. Kadela (NOT & NBI) report on behalf of a large collaboration: We observed the optical counterpart (Wu et al., GCNs 44547, 44552; Sasada et al., GCN 44549; Jiang et al., GCN 44551; Li et al., GCNs 44557, 44593; 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 et al., GCNs 44582, 44600; Calapai et al., GCN 44592; Liu et al., GCN 44610; Li et al., GCN 44615) of GRB 260511B, detected by Fermi/GBM (Fermi GBM Team, GCN 44542), SVOM (Godet et al., GCN 44543), NuSTAR (Waratkar & Grefenstette, GCN 44556), AstroSat CZTI (Arya et al., GCN 44560), and Konus-Wind (Ridnaia et al., GCN 44597) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera, and obtained 3 x 90 s frames in each of the Sloan g, r, and i bands, and 3x200 s frames in the Sloan z band, starting at 21:12:00.450 UT on 2026-05-12, i.e., 34.192 hr after the Fermi/GBM trigger. The afterglow is clearly detected in our stacked images, with a magnitude r = 21.33 +/- 0.06 (AB) at a median time of 34.337 hr after the trigger, calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44618. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...