Probing the details of relativistic electrons with multifrequency observations of M87 black hole

RESUMO

Beginning in March 2026, the Event Horizon Telescope (EHT) conducted a two-month observing campaign aimed at obtaining the first time-resolved sequence of images of the black hole M87* at the center of the Virgo A galaxy. The primary scientific objectives of this campaign are to constrain the black hole spin and to investigate the temporal variability of the magnetic field and plasma properties within the horizon-scale jet-launching region. In this work, we present theoretical predictions for the spectral index maps expected from these observations. Our predictions are based on Magnetically Arrested Disk simulations performed with the EBHLIGHT code and post-processed using the IPOLE general relativistic ray-tracing code. We investigate the temporal evolution of the spectral index for a range of electron distribution models and five different black hole spin values. The analysis includes both frequency bands currently present within the EHT and neighboring frequencies proposed for future extensions of the EHT array. In addition to extending previous studies to a broader range of observing frequencies, our work systematically investigates the time variability of the predicted spectral index maps. In purely thermal electron models, the temporal variability of the spectral index maps closely traces variations in the magnetic field strength and electron temperature in the immediate vicinity of the event horizon. By contrast, models incorporating a non-thermal electron population exhibit substantially weaker spectral index variability, owing to the contribution of a power-law component with a fixed spectral slope. Our numerical results are consistent with theoretical expectations. Finally, a comparison with the recently measured integrated spectral index of the M87* enables us to place preliminary constraints on the model parameters.