Populations of Neutron Star Ultraluminous X-ray Sources: Mind your b's and B's

Abstract: Ultraluminous X-ray sources (ULXs) with neutron star (NS) accretors challenge traditional accretion models, and have sparked a debate regarding the role of geometrical beaming and strong magnetic fields (B). The reduction of the Thomson cross-section in the presence of strong B, leads to a modification of the Eddington limit, and therefore is expected to affect significantly the observational appearance of NS-ULXs. We investigate the role of this modification using population synthesis models, and explore its effects on the X-ray luminosity functions, spin-up rates, and outflow energetics of the observed NS-ULXs. Our results show that the new prescription allows NS-ULXs to achieve super-Eddington luminosities with milder beaming compared to before, improving the agreement with observations. In addition, it broadens the range of spin-up rates allowing for more diverse conditions in NS-ULXs in terms of accretion rates and magnetic fields. More importantly, the reduced beaming increases the likelihood of observing the NS-ULXs within wind-powered nebulae such as NGC 5907 ULX-1. Our findings highlight the necessity of taking into account B effects independently of the approach: geometrical beaming or strong B, and call for magnetospheric accretion prescriptions that can be integrated in population synthesis codes.