TITLE: GCN CIRCULAR NUMBER: 40229 SUBJECT: GRB 250424A: BlackGEM optical afterglow detection DATE: 25/04/24 09:43:29 GMT FROM: Simon de Wet at University of Cape Town <simdewet@gmail.com> S. de Wet (DTU), P.J. Groot (Radboud/UCT/SAAO) and P.M. Vreeswijk (Radboud) report on behalf of the BlackGEM consortium: The BlackGEM Unit Telescope 4 (BG4) located at ESO La Silla, Chile, responded automatically to the Swift trigger on GRB 250424A (Cenko et al., GCN 40224) and obtained a repeating series of 60 s exposures in the q,u,g,r,i,z bands. The first exposure started 264 s after the trigger time at 06:56:53 UT on 2025 April 24. A total of 26 exposures were obtained. We detect the optical afterglow with the following AB magnitudes: q = 18.30 +/- 0.03 at 06:57:24 UT u = 19.33 +/- 0.18 at 06:58:47 UT g = 18.98 +/- 0.05 at 07:01:32 UT r = 18.58 +/- 0.04 at 07:04:17 UT i = 18.26 +/- 0.04 at 07:07:02 UT z = 18.22 +/- 0.10 at 07:09:46 UT BlackGEM is an array of wide-field telescopes designed, built and operated by a consortium consisting of Radboud University, the Netherlands Research School for Astronomy NOVA, KU Leuven, the University of Manchester, Tel Aviv University, the Weizmann Institute, the Hebrew University of Jerusalem, the University of Potsdam, Texas Tech University, the University of California at Davis, the Danish Technical University and the Armagh Observatory and Planetarium. View this GCN Circular online at https://gcn.nasa.gov/circulars/40229. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...