A Bayesian approach for torque modelling of BeXRB pulsars with application to super-Eddington accretors by A. S. Karaferias et al. on Wednesday 30 November

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.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.16079v1