Periodic radio variabilities of the blazar 1156+295: harmonic oscillations

Abstract

(Quasi)-periodic oscillations of the radio luminosity of a typical blazar 1156+295 were analysed to investigate the dynamic processes associated with the central engine of the active galactic nucleus. Three numerical techniques, the Lomb-Scargle periodogram, the Weighted Wavelet Z-transform and the improved Phase Dispersion Minimization, were used to search for candidate periodicities in the radio light curves of 1156+295 at 14.5, 8.0 and 4.8 GHz. The results of these methods are consistent with the detection of four periodic components with characteristic periods of similar to 1.7 (P-4), similar to 2.4 (P-3), similar to 3.4 (P-2) and similar to 7.5 yr (P-1), which suggest a harmonic relationship in frequency of 4: 3: 2: 1 with f(1) as the fundamental frequency. Except for the fact that P-1 persistently shows in the whole time span, the three others appear intermittent and are only prominent during certain time ranges. The second harmonic mode (P-2) is most powerful at all three observing frequency bands. The multiplicity and harmonic relation of the periodicities in 1156+295 reconfirms that global p-mode oscillations of the accretion disc are coupled to the jet. The oscillation of the disc is likely driven by Kelvin-Helmholtz instabilities in the inner edge of the accretion disc.