Observing Rayleigh-Taylor stable and unstable accretion through a Kalman filter analysis of X-ray pulsars in the Small Magellanic Cloud

TL;DR

This study uses a Kalman filter to analyze X-ray pulsar data, revealing how different accretion regimes at the disk-magnetosphere boundary affect pulsar behavior, with implications for understanding Rayleigh-Taylor instabilities.

Contribution

It introduces a novel application of an unscented Kalman filter to time series data of X-ray pulsars, linking accretion regimes with observable pulsar properties.

Findings

24 pulsars classified into stable and ordered unstable regimes

Positive correlation between magnetocentrifugal parameter and pulse amplitude

Most pulsars do not exhibit sustained chaotic unstable accretion

Abstract

Global, three-dimensional, magnetohydrodynamic simulations of Rayleigh-Taylor instabilities at the disk-magnetosphere boundary of rotating, magnetized, compact stellar objects reveal that accretion occurs in three regimes: the stable regime, the chaotic unstable regime, and the ordered unstable regime. Here we track stochastic fluctuations in the pulse period $P(t)$ and aperiodic X-ray luminosity $L(t)$ time series of 24 accretion-powered pulsars in the Small Magellanic Cloud using an unscented Kalman filter to analyze Rossi X-ray Timing Explorer data. We measure time-resolved histories of the magnetocentrifugal fastness parameter $\omega(t)$ and we connect $\omega(t)$ with the three Rayleigh-Taylor accretion regimes. The 24 objects separate into two distinct groups, with 10 accreting in the stable regime, and 14 accreting in the ordered unstable regime. None of the 24 objects except SXP 293 visit the chaotic unstable regime for sustained intervals, although several objects visit it sporadically. The Kalman filter output also reveals a positive temporal cross-correlation between $\omega(t)$ and the independently measured pulse amplitude $A(t)$, which agrees with simulation predictions regarding the pulse-forming behavior of magnetospheric funnel flows in the three accretion regimes.