Long term evolution of the nightly high-energy gamma-ray flux emitted by OP 313. Left: Fermi-LAT light curve (1-day binning) of OP 313 above 100 MeV during the major 2023–2024 flare episode, for integral flux and spectral index. Blue and red bars mark the two nights with NuSTAR observations that are the focus of this study. Right: spectral index as a function of the integral flux for each night over six months of LAT data. The density of points is color-coded, with yellow points showing larger number density of nights and purple-blue tones indicating more isolated bins.