Where Are Be/black-hole Binaries?

We apply the tidal truncation model proposed by Negueruela & Okazaki(2001) to arbitrary Be/compact star binaries to study the truncation efficiency dependance on the binary parameters. We find that the viscous decretion disks around the Be stars could be truncated very effectively in narrow systems. Combining this with the population synthesis results of Podsiadlowski, Rappaport and Han (2003) that binary black holes are most likely to be born in systems with orbital periods less than about 30 days, we suggest that most of the Be/black-hole binaries may be transient systems with very long quiescent states. This could explain the lack of observed Be/black-hole X-ray binaries. We also discuss the evolution of the Be/black-hole binaries and their possible observational features.Comment: 14 pages,3 figures, ApJ accepte

Luminosity function of binary X-ray sources calculated using the Scenario Machine

Using the ``Scenario Machine'' we have carried out a population synthesis of X-ray binaries for the purpose of modelling of X-ray luminosity functions (XLFs) in different types of galaxies: star burst, spiral, and elliptical. This computer code allows to calculate, by using Monte Carlo simulations, the evolution of a large ensemble of binary systems, with proper accounting for the spin evolution of magnetized neutron stars. We show that the XLF has no universal type. It depends on the star formation rate in the galaxy. Also it is of importance to take into account the evolution of binary systems and life times of X-ray stages in theoretical models of such functions. We have calculated cumulative and differential XLFs for the galaxy with the constant star formation rate. Also we have calculated cumulative luminosity functions for different intervals of time after the star formation burst in the galaxy and curves depicting the evolution of the X-ray luminosity after the star formation burst in the galaxy.Comment: 22 pages, 13 figure

Modeling the Wind of the Be Star SS 2883

Observations of eclipses of the radio pulsar B1259-63 by the disk of its Be-star companion SS 2883 provide an excellent opportunity to study the winds of stars of this type. The eclipses lead to variations in the radio flux (due to variations in the free-free absorption), dispersion measure, rotation measure, and linear polarization of the pulsar. We have carried out numerical modeling of the parameters of the Be-star wind and compared the results with observations.Comment: 6 pages, 7 figure

Population synthesis of Be/white dwarf binaries in the Galaxy

Using the "Scenario Machine"(a numerical co-de that models the evolution of large ensembles of binary systems) we study the number and physical properties of binary Be stars with white dwarfs taking account of the compact object cooling and we discuss the ways of their formation. In our calculations we take into account the influence of tidal synchronization on the evolution of stars in a close binary. The synchronization time scale may be less than the life-time of a Be star on the main sequence after the first mass transfer. It has strong effects on the resulting number distribution of binary Be stars over orbital periods. In particular, it can explain the lack of short period Be binaries. According to our calculations the number of binary systems containing a Be star paired with a white dwarf in the Galaxy is very large -70% of all Be stars formed as a result of binary evolution must have a white dwarf as a companion. Based on our calculations we conclude that the compact companion in these systems must have a high surface temperature. The number distribution over the surface temperature peaks at 2 104 K for all white dwarfs and at 4 104 K for white dwarfs paired with early-type Be stars (between B0 and B2). The registration of white dwarfs in such systems is hampered by the fact that the entire orbit of a white dwarf is embedded in the dense circumstellar envelope of the primary star (our calculations show that the majority of Be/WD systems have orbital periods less than one year) and all extreme-UV and soft X-ray photons of a compact companion are absorbed by the Be star envelope. The detection of a white dwarf is possible during the period when the Be star disc-like envelope is lacking by the detection of white dwarf extreme-UV and soft X-ray emission. This method of registration appears to be particularly promising for "single"early-type Be stars because in these systems the white dwarfs must have a very high surface temperature. However, the loss of the Be disc-like envelope does not often occur and it is a rather rare event for many Be stars. The best possibility of white dwarf detection is given by the study of helium spectral lines found in emission from several late-type Be stars. The ultraviolet continuum energy of these Be stars is found to be not enough to produce the observed helium emission. Besides, we also discuss the orbital properties of binary Be star systems with other evolved companions such as helium stars and neutron stars and give a possible explanation for the lack of Be/black hole binaries

Be/X-ray binaries and candidates

Be/X-ray binaries form the largest subclass of High Mass X-ray Binaries (HMXBs). These objects attract interests of specialists in several branches of astrophysics: stellar astrophysics, accretion theory, close binary evolution, etc. I