Periodic radio variabilities in NRAO 530: a jet–disc connection?

Abstract

Time series analysis techniques, Lomb-Scargle periodogram and weighted wavelet Z-transform, have been employed to search for year-time-scale periodicities from the radio light curves of a typical blazar NRAO 530. The periodicity analysis using the two methods show consistent results: two strong and persistent periods of similar to 10 and similar to 6 yr are prominent at all three wavelengths; three other weaker periodicities with shorter time-scales (similar to 5, similar to 3.5 and similar to 3 yr) were also seen; in addition, the 14.5-GHz WWZ power spectrum revealed a similar to 2-yr periodic component during the time interval 1983-1990. Statistical significance analysis suggests that these periodicities might result from physically periodic process. The characteristic frequencies of these periodicities seem to have a harmonic relationship, and the fundamental frequency, i.e. the first-order harmonic, is about 0.05 yr(-1) (P-0 = 20 yr). But the fundamental frequency was not pronounced. The multiplicity and apparent harmonic relationship of the periodicities could be interpreted by the global p-mode oscillation of the accretion disc, if the jet and disc are coupled. In this scenario, the periodic fluctuations in the accretion rate consequently lead to the observed periodic variabilities of the radio luminosity.