Studies On High-Mass X-ray Binaries

High-mass X-ray binaries (HMXBs) consist of an accreting compact star (i.e., neutron star or black hole) and an early-type companion with mass exceeding～10M(?) The compact star is fed by the stellar wind or beginning Roche-lobe overflow (RLOF) from the companion. HMXBs are the ideal systems to study the structure and physics of stellar winds, the processes of mass transfer, the properties of compact stars and the massive binary evolution. This thesis focuses on the subject.We first give a brief review on both observations and theories of X-ray binaries in Chapter 1.In Chapter 2 we adopt the tidal truncation model proposed by Negueruela and Okazaki for Be/X-ray binaries to investigate the influence of intermediate-mass black holes (IMBHs) on Be-star disks. We show that the viscous decretion disks around Be stars are generally truncated ineffectively under the tidal force of IMBHs. Combing this with the observations of Be/X-ray binaries, we suggest that Be/IMBH X-ray binaries may appear as recurrent luminous X-ray transients with quasi-periodic X-ray outbursts.In Chapter 3 we investigate the formation of an individual HMXB:SMC X-l. SMC X-1 is a high-mass X-ray binary with an orbital period of 3.9 days. The mass of the neutron star is as low as～1M(?), suggesting that it was likely to be formed through an electron-capture supernova rather an iron-core collapse supernova. From the present system configurations, we argue that the orbital period at the supernova was(?)10 days. Since the mass transfer process between the neutron star’s progenitor and the compan-ion star before the supernova should have increased the orbital period to tens of days, a mechanism with efficient orbit angular momentum loss and relatively small mass loss is required to account for its current orbital period. We have calculated the evolution of the progenitor binary systems from zero-age main-sequence to the pre-supernova stage with different initial parameters and various mass and angular momentum loss mechanisms. Our results show that the outflow from the outer Langrangian point or a circumbinary disk formed during the mass transfer phase may be qualified for this purpose. We point out that these mechanisms may be popular in binary evolution and significantly affect the formation of compact star binaries.Neutron stars in HMXBs generally accrete from the wind matter of their massive companion stars. Recently Shakura et al. (2012) suggested a subsonic accretion model for low-luminosity (< 4×1036 ergs-1), wind-fed X-ray pulsars. To test the feasibility of this model, in Chapter 4 we investigate the spin period distribution of wind-fed X-ray pulsars with a supergiant companion star, using a population synthesis method. We find that the modeled distribution of supergiant HMXBs in the spin period-orbital period diagram is consistent with observations provided that the winds from the donor stars have relatively low terminal velocities ((?)1000 kms-1). The measured wind velocities in several supergiant HMXBs seem to favor this viewpoint. The predicted number ratio of X-ray pulsars with persistent X-ray luminosities higher and lower than 4 x 1036 ergs-1 is about 1:11.Finally in Chapter 5 we summarize our results and give the prospects for future work.