Reversibility of time series: revealing the hidden messages in X-ray binaries and cataclysmic variables

TL;DR

This study investigates the non-linear variability in accretion-powered systems using bispectrum and time-skewness, revealing consistent non-reversible flaring behavior and potential limitations of the fluctuating accretion disk model at certain frequencies.

Contribution

It applies high-order Fourier statistics to multiple accreting systems, uncovering universal non-reversibility patterns and challenging existing accretion disk models.

Findings

All sources show biphase amplitudes less than π/2, indicating positively skewed flux distributions.

Biphases are positive at high frequencies, suggesting non-time-reversible flaring behavior.

Biphase decreases at lower frequencies, hinting at potential breakdown of the accretion disk model.

Abstract

We explore the non-linear, high-frequency, aperiodic variability properties in the three cataclysmic variables MV Lyr, KIC 8751494 and V1504 Cyg observed with Kepler, as well as the X-ray binary Cyg X-1 observed with RXTE. This is done through the use of a high-order Fourier statistic called the bispectrum and its related biphase and bicoherence, as well as the time-skewness statistic. We show how all objects display qualitatively similar biphase trends. In particular all biphase amplitudes are found to be smaller than $\pi/2$, suggesting that the flux distributions for all sources are positively skewed on all observed timescales, consistent with the log-normal distributions expected from the fluctuating accretion disk model. We also find that for all objects the biphases are positive at frequencies where the corresponding power spectral densities display their high frequency break. This suggests that the noise-like flaring observed is rising more slowly than it is falling, and thus not time-reversible. This observation is also consistent with the fluctuating accretion disk model. Furthermore, we observe the same qualitative biphase trends in all four objects, where the biphases display a distinct decrease at frequencies below the high-frequency break in their respective power spectral densities. This behaviour can also be observed in the time-skewness of all four objects. As far as we are aware, there is no immediate explanation for the observed biphase decreases. The biphase decreases may thus suggest that the fluctuating accretion disk model begins to break down at frequencies below the high frequency break.