In this paper, we used the database of the university of Michigan Radio Astronomy Observatory (UMRAO) at three (4.8 GHz, 8.0 GHZ, and 14.5 GHz) radio frequency to analyze the radio light curves by the power spectral analysis method in search of possible periodicity. The analysis results showed that the radio sources display astrophysically meaningful periodicity ranging from 2.2 to 20.8 years in their light curves at the three frequencies. We also calculated the variability parameters and investigated the correlations between the variability parameter and the flux density. For the variability parameters, we found that the parameters at higher frequency are higher than those in the lower frequency. In addition, the variability parameters of BL Lacertae objects are larger than those of flat-spectrum radio quasars. suggesting that they are more variable than flat spectrum radio quasars.
Optically thin two-temperature accretion flows may be thermally and viscously stable, but acoustically unstable. Here we propose that the O-mode instability of a cooling-dominated optically thin two-temperature inner disk may explain the 23day quasi-periodic oscillation (QPO) period observed in the TeV and X-ray light curves of Mkn 501 during its 1997 high state. In our model the relativistic jet electrons Compton upscatter the disk soft X-ray photons to TeV energies, so that the instability-driven X-ray periodicity will lead to a corresponding quasi-periodicity in the TeV light curve and produce correlated variability. We analyse the dependence of the instability-driven quasi-periodicity on the mass (M) of the central black hole, the accretion rate (Ṁ) and the viscous parameter (α) of the inner disk. We show that in the case of Mkn 501 the first two parameters are constrained by various observational results, so that for the instability occurring within a two-temperature disk where α = 0.05 − 1.0, the quasi-period is expected to lie within the range of 8 to 100 days, as indeed the case. In particular, for the observed 23-day QPO period our model implies a viscosity coefficient α ≤ 0.28, a sub-Eddington accretion ratė M ≃ 0.02Ṁ Edd and a transition radius to the outer standard disk of r 0 ∼ 60 r g , and predicts a period variation δP/P ∼ 0.23 due to the motion of the instability region.
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