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

X-Ray pulsars are systems powered by accretion, the majority of which is found in Be X-
ray binaries (BeXRBs). The study of Giant outbursts (LX > 1038 erg s-1) in such systems
becomes very relevant in the advent of the recent discoveries of pulsating ultra-luminous
X-ray sources (PULXs) with an apparent isotropic luminosity above the Eddington limit
for a typical neutron star (NS) demonstrating that stable accretion onto NSs is possible at
super – Eddington rates. Given that PULXs host magnetized NSs, several attempts have
been made to estimate the magnetic field of the NS using standard torque models. At the
same time theoretical studies have demonstrated that it is required to adjust these models
due to changes in the accretion disc structure when exceeding the Eddington limit.
Motivated by these findings we studied torque models during Giant outbursts of BeXRBs
monitored by Fermi/GBM and Swift/BAT. We developed a code to estimate posterior
distributions for the parameters of standard accretion models and binary orbital
parameters using a nested sampling algorithm for Bayesian Parameter Estimation. Most
notably we applied our method to the recently discovered Swift J0243.6+6124 (i.e., the
only known Galactic PULX) and we illustrate that the standard torque models need
adjustment to explain the observed spin evolution of the NS.