The Study Of Supergiant Fast X-ray Transients

Supergiant Fast X-ray Transients(SFXTs)usually host a neutron star and a supergiant of spectral type O or B. In the X-ray band, they display a peculiar fast X-ray transient behavior lasting typically from a few hours to no longer than a few days. At present, the physical mechanism driving the peculiar X-ray behaviour of SFXTs is still unknown. The most popular model is the use of a inhomogeneous clumpy wind of supergiant star explain the X-ray transient behavior in SFXTs.Convection and convective overshooting significantly affect the structure and evolution of stars. The clumpy wind of supergiant star is closely related to convection. In this paper, we study the influence of convection and convective overshooting on the massive star’s interior and sub-surface convection zones with four sets of evolutionary models, and the influence on the evolution of massive stars. Combined with the observation of SFXTs, we analyze the feasibility of the X-ray transient behavior in SFXTs based on the inhomogeneous clumpy wind of supergiant stars. It has a very important significance for understanding the origin and evolution of the stellar clumpy wind, the origin and evolution of SFXTs, the formation and evolution of High-mass X-ray binaries(HMXBs).Our investigation shows as follows: First, a larger value of mixing length parameter leads to a more efficient mixing in the convective region which changes the internal and surface chemical element abundance of a star and increases the convective heat transfer efficiency. Thus, the star has a higher effective temperature and a smaller stellar radius.Second, convective overshooting not only makes a larger convective core, increases the main sequence lifetime, produces a larger luminosity and extends main sequence band in the Hertzsprung-Russell diagram, but also can reduce the chemical discontinuity outside the convective core. Third, if we assume that convective overshooting works in the convective core of a star during the main sequence phase, the star will enter the red(super)giant phase earlier than the one without core overshooting. Fourth, convective overshooting leads to adiscontinuity of hydrogen profile above the hydrogen burning shell and facilitates the occurrence of blue loop in the Hertzsprung-Russell diagram. Finally, we can not fully explain the X-ray transient behavior of SFXTs just with the properties of clumpy wind of supergiant stars.