SIMBAD references

Periodic outbursts are observed in many active galactic nuclei (AGNs) and are usually explained with a supermassive black hole binary (SMBHB) scenario. However, multiple periods are observed in some AGNs and cannot be explained in this way. Here we analyze the periodicity of the radio light curves of AO 0235+164 at multiple frequencies and report the discovery of six quasi-periodic oscillations (QPOs) in the integer frequency ratio 1:2:3:4:5:6, of which the second, with period P₂=5.46±0.47 yr, is the strongest. We fit the radio light curves and show that the initial phases of the six QPOs have differences of 0 or π relative to each other. We suggest a harmonic relationship among the QPOs. The centroid frequency, relative strength, harmonic relationship, and relative initial phases of the QPOs are independent of radio frequency. The harmonic QPOs are likely due to quasi-periodic injection of plasma from an oscillating accretion disk into the jet. We estimate a supermassive black hole mass of M_BH≃(4.72±2.04)x10⁸ M_☉and an accretion rate m{dot}≃0.007. With knowledge of the accretion disk, this implies that the inner region of the AO 0235+164 accretion disk is a radiatively inefficient accretion flow. The oscillatory accretion is due to p-mode oscillations of the thick disk, probably excited by a SMBHB. Theoretical predictions for the fundamental oscillation frequency and the harmonics show good consistency with the observations. Harmonic QPOs would disappear if the thick disk became geometrically thin as the result of an increase in accretion rate. We discuss the observations of AO 0235+164 in the context of the SMBHB-thick disk oscillation scenario.