Universitat de - Departament d'Astronomia i Meteorologia ...

4 Chapter 1. Introduction and background
• Be stars: B or late O stars with luminosity classes III–V with emission lines,
indicative of an equatorial decretion disk. Masses in the range ∼ 8–20 M⊙.
Orbital periods in the range ∼ 10–250 days, with eccentricities 0.3. Do not
fill their Roche lobe, and accretion onto the compact object is produced via
mass transfer through the decretion disk. Most of these systems are transient
X-ray sources during periastron passage. The compact object is typically a
strong magnetic field neutron star, and most of these systems are X-ray pulsars.
The lifetime of Be/X-ray binaries is around ∼ 10 7 years.
• OB SG stars: O or B stars with luminosity classes I–III. Masses above 15 M⊙.
Orbital periods typically below 10 days, with eccentricities 0. The mass
transfer is due to a strong stellar wind and/or Roche lobe overflow, with mass
loss rates up to 6 × 10 −6 M⊙ yr −1 . The X-ray emission is persistent, and
large variability is usual. The lifetime of SG/X-ray binaries is between ∼ 10 5 –
10 6 years.
The typical X-ray to optical luminosity ratio is in the range LX/Lopt 10 −3 –10,
although highly absorbed systems in the optical have higher values.
The most recent catalog of HMXBs was compiled by Liu et al. (2000), and
contains 130 sources. We have added to this catalog a source previously classified
as a LMXB. We show in Fig. 1.1 the distribution of catalogued HMXBs in galactic
coordinates. As can be seen, these systems are located close to the galactic plane (in
or near spiral arms) because are young stellar systems of Population I. The presence
of HMXBs in the Small and Large Magellanic Clouds (SMC and LMC) is also clearly
seen.
In Table 1.1 we list the number of different types of HMXBs within the SMC,
the LMC, the Galaxy and total values. In the Be and SG types we have considered
all the confirmed cases and also the suspected ones based on X-ray properties or
orbital solutions. NF stands for those sources that do not fit in the Be or SG types.
Under the question mark we list the number of sources without a clear identification
or with a suspected white dwarf (WD) compact object. All of them amount to 131
systems, of which some are X-ray Pulsars (XPs) and/or transient sources, as quoted
also in the table. Finally, the number of Radio Emitting X-ray Binaries (REXBs)
has also been included (see Sect. 1.1.3).