Date Presented:

2022/07/1

Abstract:

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 (L$_{X}$ > 10$^{38}$ 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. Finally, we discuss the implications the newest GAIA distances have on the NS equation of state.

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