PhD Thesis (PDF) - Department of Astronomy - University of Virginia

2000). In most HMXBs, the accreted material is captured from the strong stellar
wind of the O or B star, although mass transfer via Roche-lobe overflow can also
occur in some HMXBs (White et al. 1995). Approximately half of the ∼300 Milky
Way X-ray binaries are HMXBs (Liu et al. 2000, 2001). Typically, HMXBs have
wide orbital separations and relatively long orbital periods (∼ 1 – 10 2 days). Given
the short lifetimes of O and B stars, HMXBs can only occur in a region with recent
star formation. This means that HMXBs occur predominately in late-type galaxies
(spirals and irregulars).
In LMXBs, the donor star has a relatively low mass (Mdonor 2 M⊙), so that these
are stars of later spectral type than A (White et al. 1995). Such stars have relatively
long lifetimes (τdonor 1 Gyr), and reach the age of the Universe for Mdonor 0.8 M⊙.
Since main sequence low mass stars have relatively weak stellar winds, most LMXBs
accrete through Roche-lobe overflow. This typically requires much tighter orbits and
shorter orbital periods. Among the ∼ 150 Galactic LMXBs, typical orbital periods
range from tens of minutes to tens of days, with a median of ∼ 9 hours (Liu et al.
2001) Since the donor stars of LMXBs live for very long periods, LMXBs can occur in
regions either with or without recent star formation. Thus, LMXBs are expected to
be the dominant class of X-ray binaries in early-type galaxies. [Although supernova
remnants (SNRs) and radio pulsars (RPs) can reach X-ray luminosities comparable
to X-ray binaries at early times, very few bright, young SNRs or RPs are expected
in E/S0 galaxies, and the bright X-ray sources (LX > 10 37 ergs s −1 ) are expected to
be predominantly LMXBs.] In fact, Roche lobe overflow generally requires that the
donor star evolve off the mass sequence (Verbunt & van den Heuvel 1995). This
suggests that there may be a delay time (comparable to the main sequence lifetime
of the donor stars) between star formation and the onset of LMXB activity.
5