TITLE: GCN CIRCULAR NUMBER: 40584 SUBJECT: GRB 250521C/AT2025mgj: NOT optical observations of the afterglow candidate DATE: 25/05/30 16:12:30 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie> A. Martin-Carrillo (UCD), A. de Ugarte Postigo (LAM), G. Corcoran (UCD), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Acebron Munoz (UNICAN), report on behalf of a larger collaboration. We observed the position of AT2025mgj (Andreoni et al., GCN 40566), a candidate optical counterpart to GRB 250521C (Fermi GBM team, GCN 40515), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained exposures in the SDSS r band (3x300 s) and SDSS z band (5x200 s) starting at 21:06 UT on 2025-05-29 (8.32 days after the Fermi trigger). The proposed candidate is well detected in both bands with an AB magnitudes of r = 19.22 +/- 0.06 z = 19.67 +/- 0.08. The photometric calibration was performed using nearby stars from the Pan-STARRS catalog and the magnitudes are not corrected for Galactic extinction. Assuming the explosion time of GRB 250521C, when compared to the last magnitude reported by Andreoni et al., (GCN 40566), our observation shows a power-law decay index of ~1.3, consistent with those typically seen in GRB afterglows. However, it should be noted that the object is much brighter than a typical GRB afterglow at T0+8 days. Further observations are planned (and encouraged) to establish the nature of this object and its association to GRB 250521C. View this GCN Circular online at https://gcn.nasa.gov/circulars/40584. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...