Inertial oscillation modes of an inclined dipolar magnetosphere as a source of band-limited noise in X-ray pulsars

TL;DR

This paper proposes that inertial oscillation modes in an inclined dipolar magnetosphere can generate band-limited noise and quasi-periodic oscillations in X-ray pulsars during outbursts, without needing changes in accretion rate.

Contribution

It introduces a new model linking magnetospheric oscillations to observed X-ray variability, emphasizing stable equilibrium points and their oscillation frequencies.

Findings

Oscillation frequencies range from zero to √3 times the spin frequency.

Variability mode can be excited without changes in accretion rate.

Explains broad-band noise and QPO features in X-ray pulsars.

Abstract

Magnetic fields of strongly magnetized stars can trap conducting matter due to frozen-in condition. In the force-free regime, the motion of the matter along the field lines may be considered in the "bead on a wire" approximation. Such a motion, if gravity and centrifugal forces are taken into account, has equilibrium points, some of which are stable. In most cases, stability is possible in about several per cent of the possible locations. Corresponding oscillation frequencies span the range from zero to $\sqrt{3}$ of the spin frequency. We suggest that this variability mode may be excited in some X-ray pulsars during the outbursts and create the peaked broad-band noise component near the break frequency in the power density spectrum, as well as produce some of the quasi-periodic oscillation features in this frequency range. Existence of this variability does not require any changes in mass accretion rate and involves only a small amount of matter infiltrating from the disc and magnetic flow due to interchange instabilities.