Episodic Accretion on to Strongly Magnetic Stars

TL;DR

This paper investigates an instability at the disc-magnetic field interface in strongly magnetic accreting stars, explaining episodic outbursts and challenging the necessity of the propeller state at low accretion rates.

Contribution

It introduces a new model for episodic accretion driven by disc-field interaction instability near the corotation radius, supported by numerical simulations.

Findings

The instability can cause repeated accretion bursts.

Burst cycle time increases as accretion rate decreases.

Steady-state solutions exist without requiring the propeller regime.

Abstract

Some accreting neutron stars and young stars show unexplained episodic flares in the form of quasi-periodic oscillations or recurrent outbursts. In a series of two papers we present new work on an instability that can lead to episodic outbursts when the accretion disc is truncated by the star's strong magnetic field close to the corotation radius (where the Keplerian frequency matches the star's rotational frequency). In this paper we outline the physics of the instability and use a simple parameterization of the disc-field interaction to explore the instability numerically, which we show can lead to repeated bursts of accretion as well as steady-state solutions, as first suggested by Sunyaev and Shakura. The cycle time of these bursts increases with decreasing accretion rate. These solutions show that the usually assumed `propeller' state, in which mass is ejected from the system, does not need to occur even at very low accretion rates.