Is the Blazhko effect the beating of a near-resonant double-mode pulsation?

TL;DR

This paper proposes that the Blazhko effect in RR Lyrae stars results from near-resonant multi-mode pulsations, with a simple model reproducing key observational features and explaining complex phenomena like the bump feature and dual modulation.

Contribution

The paper extends previous hypotheses by developing a simple non-hydro model that accurately reproduces Kepler data features, supporting the beating-modes explanation for the Blazhko effect.

Findings

The model reproduces the pulsation waveform and Blazhko envelope functions.

It explains the motion, disappearance, and reappearance of the bump feature.

It accounts for the double-maxima waveform in V445 Lyr.

Abstract

In this paper it is shown that the Blazhko effect may result from a near-resonant type of multi-mode pulsation, where two (or sometimes more) periodic oscillations with slightly different frequencies gradually slip in phase, producing a beat frequency type of modulation. Typically one of these oscillations is strongly non-sinusoidal. Two oscillations are sufficient for the standard Blazhko effect; additional oscillations are needed to explain multi-frequency modulation. Previous work on this hypothesis by Arthur N. Cox and others is extended in this paper by developing a simple (non-hydro) model that can accurately reproduce several important features found in Kepler data for RR Lyr, including the pulsation waveform, the upper and lower Blazhko envelope functions and the motion, disappearance and reappearance of the bump feature. The non-sinusoidal oscillation is probably generated by the fundamental mode and the other oscillations are probably generated by nonradial modes. This model provides an explanation for the strong asymmetry observed in the side peak spectra of most RR Lyrae stars. The motion and disappearance of the bump feature are shown to be an illusion, just an artifact of combining the oscillations. V445 Lyr is presented as an example with dual modulation. The mysterious double-maxima waveform observed for this star is explained, providing additional strong evidence that this beating-modes hypothesis is correct. Problems with other recent explanations of the Blazhko effect are discussed in some detail.