The BeXRB's page.
Be/X-ray Binaries.
Be stars are surrounded by a circumstellar
disc. This is matter that, for some reason, has escaped from the
star. It is not very clear why circumstellar discs are formed. It is
believed that it is related to internal disturbances caused by very
rapid rotation.
If the binary system contains a Be star and a neutron star, the orbit
of the neutron star will probably take it inside the circumstellar
disc at some moment. Then the neutron star will be able to accrete
material and become an X-ray source. Another possibility is that the
Be star undergoes a sudden ejection of matter which will also turn on
the X-ray source. As these sources are only on from time to time,
they are known as X-ray transients.
A typical Be/X-ray Binary Transient. The neutron star
revolves around the Be star in a very wide orbit. At apastron
(maximum distance to the companion) it is very far away from the Be
star to capture a significant amount of matter and, therefore,there is
no X-ray emission. However, at periastron (minimum distance), it comes
inside the circumstellar envelope of the Be star and accretes matter,
becoming a transient source. The X-ray
emission is only seen in separate outburst, occurring once per orbit.
Unfortunately, these systems tend to be much more complicated than
expected, presenting many difficult physical problems. Some of them
are not transient at all, but show persistent, weak X-ray
emission. Some others do not have periodic outbursts and most of them
spend some (or most) of their time in an ``off'' state, in which the
disc is too small to reach the orbit of the neutron star. During these
``off'' phases, they are not X-ray sources at all.
Back to the X-ray Binaries Page.
Last modified 10 May 1996.