The relation between the spin period and the orbital period in high-mass X-ray binaries is investigated.Be/X-ray binaries and SG/X-ray binaries occupy different areas in Corbet's diagram.The two SG/X-ray binaries,IGR J18483-0311 and OAO 1657-415,however,appear to occupy a transition region between Be/X-ray binaries and wind-fed SG/X-ray binaries.This can be understood by noting that the neutron stars in the two SG/X-ray binaries have not reached their equilibrium spin periods when the donors evolved off the main sequence.Here,we provide an alternative scenario to explain their location in Corbet's diagram:the SG/X-ray binaries are the descendants of Be/X-ray binaries,i.e.the supergiants in these systems were formed from O-type emission line stars instead of normal main-sequence stars.Furthermore,we suggest that there are two types of SG/X-ray binaries:one is directly evolved from a normal OB-type main sequence and neutron star system without an X-ray history,and the other evolved from a Be/X-ray binary with an X-ray history.
LIU QingZhong1,2,LI Hui1,3 & YAN JingZhi1,2 1 Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210008,China
