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Massive X-ray Binaries

The first version of the Scenario Machine was aimed especially at the simulation of massive binary star evolution.  We relate a binary star to the class of massive binaries  if the primary component mass is large enough for the star to collapse  at least to a NS without receiving any additional matter form the companion star. This limiting mass was assumed to be equal to 10 solar masses, which is the mean value of the stellar masses from tex2html_wrap_inline8945 8 to tex2html_wrap_inline10999 in different calculations of single star evolution.  In fact, the calculations we performed for different lower boundary masses showed only weak dependence on its value.

This, however, does not mean that all NS can be formed only in such systems. First, there is a large class of intermediate mass binaries (say, consisting initially of 8 + 5 solar masses) that can also produce NS and thus lead to the formation of a bright X-ray source. A possible example is Her X-1.  Thus we consider a full range of stellar masses from tex2html_wrap_inline8945 0.8 (a lower mass limit for the normal star to evolve out of the main sequence during the Hubble time) to tex2html_wrap_inline8945 tex2html_wrap_inline11005 . However, already early calculations have revealed a number of important properties of the joint evolution of normal and NS, which we discuss briefly in this section.

Until recently, only accreting NS had been observed in binaries (apart from binary pulsars),  that is, the stage of an accreting X-ray pulsar in a pair with a supergiant (II+A) and/or with a Be star (I+A). But even first inspection of the observational situation (Table 8) revealed a large diversity of NS stages in massive binaries. 


As expected, loop-like tracks of NS evolution (see, e.g. Figure 11, 10 in Section 4) were most frequently encountered. As a rule, evolutionary stages of a NS with magnetic moment tex2html_wrap_inline11007 follow the sequence



But other sequences may also appear. Since the initial distribution of mass ratios tex2html_wrap_inline11009 peaks toward tex2html_wrap_inline11011 , it is quite possible that the NS may appear when the secondary star is at stage II, III, or even IV instead of stage I. Generally, no characteristic loop tex2html_wrap_inline11013 is found in this case.

The following features of the evolution  of the NS were discovered (or confirmed):

  1. The larger magnetic field the NS has, the faster it evolves from one state to another, that is the NS can pass through a greater number of stages,
  2. Looped tracks are most typical for magnetized NS,
  3. The collapse  of the NS into a BH occurs in most cases at stage III of the evolution of the normal star (i.e. during Roche lobe overflow), the probability of such collapse  being higher for stars with larger magnetic moments,
  4. Most of the NS in a pair with a companion at stage I (MS) do not pass through the accretor stage, since the NS with large magnetic moments evolve rapidly and become georotators M (stage G), while those having weaker magnetic fields have no time to spin down  to the accretion stage due to low stellar wind  rates from the MS-companions,
  5. NS with very small magnetic moments ( tex2html_wrap_inline11015 ) show no significant evolution after birth remaining at the E stage; therefore, the phenomenon of X-ray pulsars  is unlikely in this case,
  6. Formation of the stage IV+A (an X-ray pulsar in a pair with a helium star   (WR-star in the case of massive binaries))  is unlikely since the NS becomes rapidly rotating at stage III (Roche lobe overflow by optical companion), which usually precedes the IV stage,
  7. NS with standard magnetic moments may pass twice through the ejector  stage (I+E and E+E, that is, at the beginning and after the recycling) during the course of their evolution. Most of them have small periods (less than 4 s) and may appear as recycled radiopulsars even without magnetic field decay, 
  8. Most binary systems with NS are at stages E+I and P+I, and the number of such systems is significantly larger than the number of X-ray pulsars  (see Figure 27) (Kornilov and Lipunov, 1983a;[85] Lipunov and Prokhorov, 1987[115]),
  9.   figure3188

    Figure 27: Relative fractions of NS at different evolutionary states in massive binary systems (Lipunov and Prokhorov, 1987[115]). 

  10. The number of X-ray pulsars at stage IV is small, which is in agreement with the fact that they have not so far been observed in pairs with WR stars.

As the majority of NS in massive binaries  prove to be at the ejector  stage, it worth discussing such systems in more detail.

next up previous contents index
Next: Ejectors in Massive Binary Previous: Artificial Galaxy: Full Matrix

Mike E. Prokhorov
Sat Feb 22 18:38:13 MSK 1997