Flickering study of nova like systems KR Aur and UU Aqr

TL;DR

This study models flickering activity in nova-like systems KR Aur and UU Aqr, successfully fitting observations for KR Aur and suggesting spiral waves may explain discrepancies in UU Aqr.

Contribution

Introduces a statistical turbulence-based flickering model for accretion discs, successfully fitting KR Aur data and proposing spiral waves as an explanation for UU Aqr's behavior.

Findings

Model fits KR Aur's power density spectrum with specific disc parameters.

Inner disc radius in KR Aur is smaller than in quiescent dwarf novae.

Model fails to reproduce UU Aqr's power density spectrum, suggesting additional phenomena.

Abstract

We present a study of the flickering activity in two nova like systems KR Aur and UU Aqr. We applied a statistical model of flickering simulations in accretion discs based on turbulent angular momentum transport between two adjacent rings with an exponential distribution of the turbulence dimension scale. The model is based on a steady state disc model which is satisfied in the case of hot ionized discs of nova like cataclysmic variables. Our model successfully fits the observed power density spectrum of KR Aur with the disc parameter alpha = 0.10 - 0.40 and an inner disc truncation radius in the range R_in = 0.88 - 1.67 x 10^9 cm. The exact values depend on the mass transfer rate in the sense that alpha decreases and R_in increases with mass transfer rate. In any case, the inner disc radius found for KR Aur is considerably smaller than in quiescent dwarf novae, as predicted by the disc instability model. On the other hand, our simulations fail to reproduce the power density spectrum of UU Aqr. A tantalizing explanation involves the possible presence of spiral waves, which are expected in UU Aqr, because of its low mass ratio, but not in KR Aur. In general our model predicts the observed concentration of flickering in the central disc. We explain this by the radial dependence of the angular momentum gradient.