TITLE: GCN CIRCULAR NUMBER: 40608 SUBJECT: GRB 250521C: NOT spectroscopic observations exclude the afterglow candidate AT2025mgj DATE: 25/06/02 16:07:14 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie> A. Martin-Carrillo (UCD), B. Schneider (LAM), L. Izzo (INAF/OACN and DARK/NBI), A. de Ugarte Postigo (LAM), G. Corcoran (UCD), B. Gompertz (U. Birmingham), D. B. Malesani (DAWN/NBI and Radboud), R. H. Rasmussen (NOT and Aarhus Univ.), report on behalf of a larger collaboration. We carried out spectroscopic observations 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 4x900 s exposures starting at 21:16 UT on 2025-05-31 (10.3 days after the Fermi trigger) with a wavelength coverage of 3800-9500 AA. The spectrum shows a blue, featureless continuum, peaking at around 4500 AA. Redward of the peak, the continuum is consistent with a power law F_lambda propto lambda^-2.7. The overall shape does not resemble a power-law spectrum, which would be typical of GRB afterglows, nor a SN spectrum, given the lack of any features. Although the spectrum does not fully resemble those of dwarf novae in outbursts, the spectral energy distribution is overall consistent with previous examples (e.g. Aviles et al. 2018, doi:10.14482/INDES.30.1.303.661). In summary, our spectroscopic observations confirm that AT2025mgj is not associated with GRB250521C, as already suggested by Busmann et al. (GCN 40597) and Gillanders et al. (GCN 40605) based on photometric observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/40608. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...