Harmonic QPOs and Thick Accretion Disk Oscillations in the BL Lacertae Object AO 0235+164

Periodic outbursts are observed in many AGNs and usually explained with asupermassive black hole binary (SMBHB) scenario. However, multiple periods areobserved in some AGNs and cannot be explained with it. Here we analyze theperiodicity of the radio light curves of AO 0235+164 at multi-frequencies andreport the discovery of six QPOs in integer ratio 1:2:3:4:5:6 of QPOfrequencies, of which the second with period $P_2 = (5.46 \pm 0.47) {\rm yr}$is the strongest. We fit the radio light curves and show that the initialphases of six QPOs have zero or $\pi$ differences relative to each other. Wesuggest a harmonic relationship of QPOs. The centroid frequency, relativestrength, harmonic relationship and relative initial phases of QPOs areindependent of radio frequency. The harmonic QPOs are likely due to thequasi-periodic injection of plasma from an oscillating accretion disk into thejet. We estimate the supermassive black hole mass $M_{\rm BH} \simeq (4.72\pm2.04) \times 10^8 M_\odot$ and the accretion rate $\dot{m}\simeq 0.007$. Withthe knowledge of accretion disk, it implies that the inner region of accretiondisk of AO 0235+164 is a radiatively inefficient accretion flow. Theoscillation accretion is due to the p-mode oscillation of the thick diskprobably excited by a SMBHB. The theoretical predications of fundamentaloscillation frequency and the harmonics are well consistent with theobservations. Harmonic QPOs would be absent when the thick disk becomesgeometrically thin due to the increase of accretion rate. We discuss theobservations of AO 0235+164 basing on the SMBHB-thick disk oscillationscenario.Comment: 40 pages, 10 figures, 1 table; accepted for publication in Ap