TITLE: GCN CIRCULAR NUMBER: 41426 SUBJECT: GRB 250818B: NOT optical observations DATE: 25/08/19 04:07:05 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> Jonas Broe Bendtsen (SDU), Johanne Hein Pedersen (SDU), Isabella Alexandra Koch (SDU), Sophie Lund Wagner (SDU), Julie Magaard Knudsen (SDU), Roar Holmberg Rasmussen (NOT and Aarhus), Kostas Valeckas (NOT and NBI), Johan P. U. Fynbo (DAWN/NBI), Daniele B. Malesani (DAWN/NBI and Radboud Univ.), Antonio de Ugarte Postigo (LAM), report on behalf of a larger collaboration: We observed the optical counterpart (Kumar et al., GCN 41406; Yao et al., GCN 41409; Zheng et al., GCN 41417; Fong et al., GCN 41419) of the short SVOM GRB 250818B (Wang et al., GCN 41405) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried in the SDSS r band out using the ALFOSC camera, for a total exposure time of 3x300 s. The optical afterglow is clearly detected in our images. Its coordinates, calibrated against the Gaia catalog, are: RA(J2000) = 03:04:13.483 Dec(J2000) = -03:07:30.74 with an error of 0.1". At a mean epoch 2025 Aug 19.128 UT (23.60 hr after the trigger), we measure r = 21.15 +- 0.04 (AB), calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41426. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...