Evidence for quasiperiodicity in orbital period modulation of WW Cygni

Abstract

The Algol type binary system WW Cygni is known to display an intricate orbital period variability phenomenon which consists in the presence of an alternating component superposed on a secular trend. The alternating component clearly deviates from a strictly periodic pattern. In the present paper we studied WW Cyg using a data set consisting of 309 times of primary minimum, free of outliers, covering a time base of 113.8 yr. We investigated these timing data using periodicity detection methods relying on both amplitude spectrum and enhanced self-correlation (ESC) methods through Monte Carlo simulations. The problem of estimation of the statistical significance in the ESC method was further developed. The multiperiodicity of the $O-C$ residuals was investigated via iterative prewhitening using three types of models consisting of polynomial (P) + multiperiodic (MP) terms, P + light travel time effect (LTTE) terms, and P + MP terms + LTTE terms. The interpretation of the results led us to the following two hypotheses: (i) a quasiperiodic modulation at a time scale of about 46 yr induced by the cyclic magnetic activity occurring in the cool secondary component of WW Cyg, or (ii) the coexistence of a periodic modulation with a period of 50.41 yr related to the presence of an unseen compact stellar object of at least $2.22~\mbox{M}_{\odot }$ in the system and a quasiperiodic modulation at a time scale of about 19.7 yr, caused by the mechanism (i).