The Be X-ray Binary General Data

Since the discovery of Cen X-3 and Her X-1 as accreting neutron star pulsars found in close binary systems in 1971-72, we have had nearly 50 years during which dozens of additional accreting pulsars in binary systems have been discovered. The discoveries have been primarily categorized as “low mass x-ray binaries (LMXB‘s)”, of which Her X-1 is the original example, and “high mass x-ray binaries (HMXB‘s)”, of which Cen X-3 is the original example.

It turns out the Cen X-3 and Her X-1 are extreme examples within their own classes of high and low mass x-ray binaries. The extreme property of both Cen X-3 and Her X-1 are their close binary orbit around their companions, by which they are virtually embedded in the outer atmosphere of their companions and their orbits are almost exactly circular with eccentricity almost exactly equal to zero (e~=0).

The data on other x-ray pulsars since the original discoveries, has not been so “clean”. Eccentricities have tended to be significantly greater than zero implying orbits that are modestly or highly elliptical ( 0.0 < e < 1.0).

The data for the HMXB’s will be the primary focal point of the discussion on this page and the LMXB’s will be discussed in a page at a later time.

An HMXB generally consists of a giant star (M > 4 Msun) and neutron star (NS) companion (m ~ 1.4 Msun). A major class of HMXB consists of a NS plus a Be giant star, where the Be giants are hot giants, and neighbors of the blue giant O stars on the H-R diagram. There is a page that describes the Be X-ray binaries and their role with respect to the NS-Capture Theory here.

There are a few distinct collections of data that describe the Be X-ray binaries, that have been published during the last 40-50 years that primarily are associated with where the binaries are found:

  • Galactic Be X-ray binaries (in the main disk of the Milky Way (MW))
  • SMC Be X-ray binaries (a very high density of these binaries is found in the Small Magellanic Cloud (SMC)).
  • LMC Be X-ray binaries (a normal density of these binaries is found in the Large Magellanic Cloud (LMC)).
  • Extragalactic Be X-ray binaries (a normal density of these binaries is found in other galaxies, such as Andromeda (M31)).

We only need to concentrate on the first 2 classes: Galactic Be XRB’s and SMC Be XRB’s. The LMC category is similar to the MW galactic category, but a smaller less-defined sample. The extragalactic category, which is aside from the SMC and LMC, is also in the early stages of analysis and does not impact the major points of the first 2 classes.

The primary focus of the data analysis will be on the spin-orbit data of the Be X-ray Binary Pulsars. That data shows a correlation between the spin rate of the pulsar in one of these systems and the orbital period of the pulsar in that system. The correlation is generally that the shorter pulsar spin periods are associated with the short orbital periods of the pulsar around the Be star.

The significance of this correlation is that as the system evolves, when the pulsar is near periastron, it accretes matter causing it to spin faster. Also at the same time, near periastron, the gravitational force of the pulsar causes tidal turbulence of the Be star, and the net effect is that the neutron star’s orbital energy is lost as tidal turbulence energy in the Be star is increased, causing the neutron star to be unable to reach its previous furthest distance from the Be star. i.e. the orbit tends to circularize (become less eccentric) and that circle has a radius no wider than the distance at which the periastron takes place during each orbit.