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 ~1.7 (P4), ~2.4 (P3), ~3.4 (P2) and ~7.5 yr (P1), which suggest a harmonic relationship in frequency of 4: 3: 2: 1 with f1 as the fundamental frequency. Except for the fact that P1 persistently shows in the whole time span, the three others appear intermittent and are only prominent during certain time ranges. The second harmonic mode (P2) 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. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.