TITLE: GCN CIRCULAR NUMBER: 38692 SUBJECT: GRB 241228B: LCOGT photometry of the likely optical counterpart GOTO24jmz DATE: 24/12/28 18:26:47 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es> I. Ortega-Casas, B. Armas-Chinea, F. Dobrindt, P. Escudero-Coca, G. Fernández-Rodríguez, Á. García Lozano, A. Huertas Ferrer, C. Méndez-Lapido, M. Torreiro Martínez, G. Villa (ULL), S.R. Berlanas, F. Poidevin, and I. Pérez-Fournon (IAC and ULL) We report on multi-band optical imaging of the GOTO optical counterpart candidate GOTO24jmz (Kumar et al., GCN 38684 and GCN 38691) of GRB 241228B detected by Fermi (Fermi GBM team, GCN 38682) with Las Cumbres Observatory global telescope network (LCOGT) telescopes. We used first the two 40-cm telescopes of the LCOGT node at Haleakala Observatory (Maui, Hawaii). The observations (600 sec in the SDSS r', i', and g' filters and in the PanSTARRS zs filter) started at 09:33:19 UT on 2024-12-28, i.e. 5.33 hr after the Fermi GBM trigger. We used also one of the LCOGT 1-m telescopes located at the LCOGT node at Siding Spring Observatory (Australia) to obtain a 180-sec SDSS r' image starting at 14:09:01 on 2024-12-28, i.e. 9.93 hr after the Fermi GBM trigger. The likely optical counterpart of GRB 241228B is clearly detected in all the LCOGT observations at the position reported by Kumar et al. (GCN 38684 and GCN 38691). This source has also been detected by An et al. (GCN 38687). We measure the following magnitudes, calibrated against PanSTARRS and not corrected for Galactic extinction: r' = 18.62 +/- 0.06, on 2024-12-28 09:38:19 UT i' = 18.31 +/- 0.09, on 2024-12-28 10:07:03 UT g' = 19.40 +/- 0.06, on 2024-12-28 10:18:46 UT zs = 18.25 +/- 0.28, on 2024-12-28 10:49:38 UT r' = 19.62 +/- 0.05, on 2024-12-28 14:10:31 UT These results are based on observations made with the Las Cumbres Observatory’s education network telescopes that were upgraded through generous support from the Gordon and Betty Moore Foundation and are part of a course on Astrophysical Techniques of the Master in Astrophysics of the Universidad de La Laguna, Tenerife, Spain (LCOGT observing programme IAC2024B-010, ULL-ASTRO-MASTER). This work makes use of observations from the Las Cumbres Observatory global telescope network (1-m telescopes, LCOGT observing programme IAC2024B-004, SGLF). View this GCN Circular online at https://gcn.nasa.gov/circulars/38692. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...