QPOs in compact binaries from small scale eruptions in an inner magnetized disk

TL;DR

This paper proposes a model where ionization instability in the inner magnetized disk of compact binaries causes quasi-periodic oscillations (QPOs) on timescales of around 1000 seconds, explaining some observed rapid fluctuations.

Contribution

It introduces a new mechanism linking ionization instability in inner magnetized disks to QPOs in dwarf novae and low mass X-ray binaries, with specific conditions for their development.

Findings

QPOs are generated by ionization instability in the inner magnetized disk.

QPOs occur only during quiescence or at the start/end of outbursts.

The model predicts a connection between disk state and QPO presence.

Abstract

Dwarf nov\ae\ (DNe) and low mass X-ray binaries (LMXBs) are compact binaries showing variability on time scales from years to less than seconds. Here, we focus on explaining part of the rapid fluctuations in DNe, following the framework of recent studies on the monthly eruptions of DNe that use a hybrid disk composed of an outer standard disk and an inner magnetized disk. We show that the ionization instability, that is responsible for the monthly eruptions of DNe, is also able to operate in the inner magnetized disk. Given the low density and the fast accretion time scale of the inner magnetized disk, the ionization instability generates small, rapid heating and cooling fronts propagating back and forth in the inner disk. This leads to quasi-periodic oscillations (QPOs) with a period of the order of $1000$ s. A strong prediction of our model is that these QPOs can only develop in quiescence or at the beginning/end of an outburst. We propose that these rapid fluctuations might explain a subclass of already observed QPOs in DNe as well as a, still to observe, subclass of QPOs in LMXBs. We also extrapolate to the possibility that the radiation pressure instability might be related to Type B QPOs in LMXBs.