Is the apparent period-doubling in Blazhko stars actually an illusion?

TL;DR

This paper challenges the common belief that Blazhko stars exhibit true period-doubling, providing evidence that the observed alternations are due to near-resonant modes and beating effects rather than actual bifurcations.

Contribution

It demonstrates that the apparent period-doubling in Blazhko stars is an illusion caused by near-resonant modes, not genuine period-doubling bifurcations.

Findings

Alternating maxima are due to near-resonant modes, not period-doubling.

Phase-slip correlates with alternation parity, indicating a nonresonant process.

Side-peaks at twice the Blazhko frequency support the beating-modes model.

Abstract

The light curves of many Blazhko stars exhibit intervals in which successive pulsation maxima alternate between two levels in a way that is characteristic of period-doubling. In addition, hydrocode models of these stars have clearly demonstrated period-doubling bifurcations. As a result, it is now generally accepted that these stars do indeed exhibit period-doubling. Here we present strong evidence that this assumption is incorrect. The alternating maxima likely result from the presence of one or more near-resonant modes which appear in the stellar spectra and are slightly but significantly offset from 3/2 times the fundamental frequency. We show that a previously proposed explanation for the presence of these peaks is inadequate. The phase-slip of the dominant near-resonant peak in RR Lyr is shown to be fully correlated with the parity of the observed alternations, providing further strong evidence that the process is nonresonant and cannot be characterized as period-doubling. The dominant near-resonant peak in V808 Cyg has side-peaks spaced at twice the Blazhko frequency. This apparent modulation indicates that the peak corresponds to a vibrational mode and also adds strong support to the beating-modes model of the Blazhko effect which can account for the doubled frequency. The modulation also demonstrates the "environment" altering effect of large amplitude modes which is shown to be consistent with the amplitude equation formalism.