Abstract
ABSTRACT Since the detection of X-ray pulses from ultraluminous X-ray sources (ULXs) in 2014, neutron stars have been considered as their central objects. However, it remains unclear how neutron stars can be brighter than the Eddington luminosity, and no unified view exists on the magnetic field of neutron stars and the degree of beaming. Recent observations suggest that some X-ray pulsating ULXs have Be-type donors, and some of them occupy the same region as Be-type high-mass X-ray binaries (Be-HMXBs) on the Corbet diagram, which reveals the relation between spin and orbital periods. This suggests that at least some ULXs are special cases of Be-HMXBs. In this study, we use the framework of mass accretion models for Be-HMXBs to investigate the conditions under which neutron stars achieve mass accretion rates beyond the Eddington limit and become observable as ULXs. We show that a Be-HMXB may become a ULX if the magnetic field of the neutron star and the density of the Be disc meet certain conditions. We also show that, although a stronger magnetic field increases the brightness of a neutron star ULX with a Be donor, its brightness cannot exceed the Eddington limit by a more than a factor of ≈50. Finally, we propose a scenario whereby some normal Be-HMXBs may evolve into ULXs as the donor evolves into a giant.
Highlights
Ultraluminous X-ray sources (ULXs) are X-ray sources located outside the galactic centre with luminosities above the Eddington luminosity of stellar-mass objects
A current controversy involving ULX sources debates whether they are stellar-mass black holes (BHs) with mass-accretion above the Eddington limit or intermediate-mass BHs with sub-Eddington mass-accretion rates (Kaaret et al 2017)
If the donor star is a supergiant and the system has a short orbital period, mass-transfer from the donor to the neutron stars (NSs) will occur via Roche lobe overflow (RLOF), which leads to a large masstransfer rate (Bildsten et al 1997; Frank et al 2002)
Summary
Ultraluminous X-ray sources (ULXs) are X-ray sources located outside the galactic centre with luminosities above the Eddington luminosity of stellar-mass objects. If the donor star is a supergiant and the system has a short orbital period, mass-transfer from the donor to the NS will occur via Roche lobe overflow (RLOF), which leads to a large masstransfer rate (Bildsten et al 1997; Frank et al 2002) In this case, one can imagine that the mass-transfer rate exceeds the Eddington limit, allowing it to emit X-rays above the Eddington luminosity. The NS/BH fraction of the ULX population is important for calculating the initial mass function of massive stars and for estimating the population of gravitational wave sources such as NS-NS binary stars (Mondal et al 2020) With these motivations, we examine in this study whether BeHMXBs can be ULXs. For an NS binary system with a Be donor to be a ULX, it must satisfy the following criteria:.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
