On the Nature of Superhumps

TL;DR

This paper supports the idea that superhumps are caused by irradiation-modulated, periodically variable mass transfer rates, with observational evidence and simple models showing the role of disk structure and modulation amplitude decline during superoutbursts.

Contribution

It provides new observational evidence and modeling to confirm the role of non-axisymmetric disk structures and modulation amplitude decline in superhump behavior.

Findings

Superhumps are linked to irradiation-modulated mass transfer rates.

Modulation amplitude decreases during superoutbursts.

Superhump amplitude decline correlates with modulation amplitude decline.

Abstract

Further evidence is presented supporting the alternative interpretation of superhumps as being due to irradiation modulated periodically variable mass transfer rate. NZ Boo, HT Cas and PU UMa are added to the sample of high inclination dwarf novae showing -- during their superoutbursts -- modulation of the observed brightness of the disk with beat period. Simple model calculations confirm earlier hypothesis that this modulation is due to a non-axisymmetric structure of the outer parts of the disk, involving the azimuthal dependence of their geometrical thickness, rotating with the beat period. The modulation amplitude $A_{mod}$ is found to decrease during superoutbursts. In particular, it is found that during two superoutbursts of OY Car the rate of decline of the superhump amplitude $dA_{SH}/dt$ was correlated with the rate of decline $dA_{mod}/dt$. This leads to a simple explanation for the decreasing amplitudes of superhumps: it is a consequence of the decreasing modulation amplitude.