Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables -- VI. Spin rates, propellering, and coherence

TL;DR

This paper analyzes dwarf nova oscillations and quasi-periodic oscillations, showing their behavior aligns with magnetic accretion models, and proposes new interpretations for observed periodicities and phenomena like propellering.

Contribution

It provides new insights into DNOs and QPOs, including evidence for propellering, revised understanding of coherence loss, and a novel explanation for the 28.95 s periodicity in WZ Sge.

Findings

DNO rates match magnetic accretion predictions.

Evidence for propellering during late outburst stages.

Reinterpretation of the 28.95 s periodicity in WZ Sge.

Abstract

We examine published observations of dwarf nova oscillations (DNOs) on the rise and decline of outbursts and show that their rates of change are in reasonable agreement with those predicted from the magnetic accretion model. We find evidence for propellering in the late stages of outburst of several dwarf novae, as shown by reductions in EUVE fluxes and from rapid increases of the DNO periods. Reanalysis of DNOs observed in TY PsA, which had particularly large amplitudes, shows that the apparent loss of coherence during late decline is better described as a regular switching between two nearby periods. It is partly this and the rapid deceleration in some systems that make the DNOs harder to detect. We suggest that the 28.95 s periodicity in WZ Sge, which has long been a puzzle, is caused by heated regions in the disc, just beyond the corotation radius, which are a consequence of magnetic coupling between the primary and gas in the accretion disc. This leads to a possible new interpretation of the `longer period DNOs' (lpDNOs) commonly observed in dwarf novae and nova-like variables.