KIC 9406652: A laboratory of the tilted disk in cataclysmic variable stars

TL;DR

This study analyzes 1500 days of Kepler data on KIC 9406652, revealing a precessing, tilted accretion disk in a cataclysmic variable star, with detailed insights into its oscillations, superhumps, and disk tilt dynamics.

Contribution

It provides the first direct evidence of a tilted, retrogradely precessing accretion disk in a CV, with detailed analysis of its oscillatory behaviors and tilt angle.

Findings

Negative superhump frequency varies with brightness cycles.

The disk tilt angle is inferred to be less than 3 degrees.

The light source of negative superhumps is the bright spot on the tilted disk.

Abstract

KIC 9406652 is a cataclysmic variable (CV), sub-classified as `IW And-type star', showing a repetition of standstills with oscillatory variations terminated by brightening. This system showed negative superhumps, semi-periodic variations having periods slightly shorter than the orbital period, and super-orbital signals, both of which are believed to originate from a precessing, tilted accretion disk. We have re-examined its Kepler light curve extending over 1500 d. In accordance with a cycle of the IW And-type light variation, the frequency of negative superhumps showed a reproducible variation: a rapid drop during the brightening and a gradual increase during the standstill. They are interpreted as the drastic change in the radial mass distribution and the expansion of the tilted disk, which is not expected from the existing models for IW And stars. The constancy in flux amplitudes of negative superhumps confirms that their light source is the bright spot sweeping across the surface of the tilted disk. The frequencies of negative superhumps and super-orbital signals varied in unison on long timescales, suggesting their common origin: the tilted disk. Orbital signals at the brightening were dominated by the irradiation of the secondary star and varied with the orientation of the tilted disk; the amplitude was maximized at the minimum of super-orbital signals and the light maximum shifted to early orbital phases as the super-orbital phase advances. This is the first direct evidence that the disk was tilted out of the binary orbital plane and retrogradely precessing. The tilt angle of the disk inferred from semi-amplitudes of super-orbital signals was lower than 3 degrees. The diversity in light curves of negative superhumps supports this and suggests that a part of the gas stream overflows the disk edge. This study thus offers rich information about the tilted disk in CVs.