A Bayesian approach for torque modelling of BeXRB pulsars with application to super-Eddington accretors. Issue 1 (11th November 2022)

• Record Type: Journal Article
• Authors: Karaferias, A S; Vasilopoulos, G; Petropoulou, M; Jenke, P A; Wilson-Hodge, C A; Malacaria, C
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 520:Issue 1(2023)
• Page Start: 281
• Page End: 299
• Publication Date: 2022-11-11
• Subjects: accretion, accretion discs -- pulsars: individual: RX J0520.5−6932, RX J0209−7427, Swift J0243.6+6124 -- X-rays: binaries; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac3208
• Languages: English
• ISSNs: 0035-8711

Abstract

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, because its luminosity spans several orders of magnitude during its outburst, with a maximum L X that exceeded the Eddington limit by a factor of ∼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 ( R m ) and the Alfvén radius ( R A ) 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.