Large-scale magnetic fields enabling fitting of the high-frequency QPOs observed around supermassive black holes

Abstract Recently, it has been argued that the high-frequency quasi-periodic oscillations (QPOs) observed in black hole systems of various scales in mass in cases of supermassive black holes (SMBH) are not consistent with any of the simple physical models, based on frequencies of the geodesic epicyclic motion (Smith et al. 2021, ApJ, 906, 92). We test if such a disease can be simply cured by geodesic models based on epicyclic frequencies modified by the effect of electromagnetic interaction of slightly charged orbiting matter, with large-scale magnetic fields with values observed around SMBHs in active nuclei. Inspired by GRAVITY/ESO observations, we assume a slightly charged hot spot, as the relativistic motion of a plasma in magnetic field leads to charge separation and non-negligible charge density in the orbiting plasma. Its electromagnetic interaction with the large-scale magnetic field around the black hole can be weak enough, allowing for nearly harmonic epicyclical oscillatory motion of the hot spot with frequencies given by modification of those applied in the geodesic model. Even the simplest epicyclic resonance variant of the geodesic model, modified by slight electromagnetic interaction admitted by observations, can fit the QPOs in the case of both stellar-mass and supermassive black holes. We have shown that even a tiny excess of charged particles in the quasi-neutral plasma of the radiating hot spot, allowed by observations, enable an explanation of QPOs observed in active galactic nuclei. We also estimate the effect of the electromagnetic interaction on the shift of the innermost stable circular orbits, implying the degeneracy in the measurements of spins of the black hole candidates.

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