A Bayesian approach for torque modelling of BeXRB pulsars with application to super-Eddington accretors

TL;DR

This paper introduces a Bayesian nested sampling method to estimate accretion torque and orbital parameters in X-ray binaries, applied to systems during super-Eddington outbursts, revealing deviations from standard models at high luminosities.

Contribution

It develops a novel Bayesian approach for parameter estimation in accretion torque models, especially for super-Eddington accretors, and applies it to multiple Be X-ray binaries and a ULX.

Findings

Identifies the more suitable torque model for each studied system.

Shows that magnetospheric and Alfvén radii are not proportional at super-Eddington luminosities.

Suggests that distance estimates influence neutron star property inferences.

Abstract

In this study we present a method to estimate posterior distributions for standard accretion torque model parameters and binary orbital parameters for X-ray binaries using a nested sampling algorithm for Bayesian Parameter Estimation. We study the spin evolution of two Be X-ray binary systems in the Magellanic Clouds, RX J0520.5-6932 and RX J0209-7427, during major outbursts, in which they surpassed the Eddington-limit. Moreover, we apply our method to the recently discovered Swift J0243.6+6124; the only known Galactic pulsating ultra-luminous X-ray source. This is an excellent candidate for studying the disc evolution at super-Eddington accretion rates, for its luminosity span several orders of magnitude during its outburst, with a maximum $L_{\rm X}$ that exceeded the Eddington limit by a factor of $\sim 10$. Our method, when applied to RX J0520.5-6932 and RX J0209-7427, is able to identify the more favourable torque model for each system, while yielding meaningful ranges for the NS and orbital parameters. Our analysis for Swift J0243.6+6124 illustrates that, contrary to the standard torque model predictions, the magnetospheric radius and the Alfv\'en radius are not proportional to each other when surpassing the Eddington limit. Reported distance estimates of this source range between 5 and 7 kpc. Smaller distances require non-typical neutron star properties (i.e. mass and radius) and possibly lower radiative efficiency of the accretion column.