A review of basic properties of Be/X-ray binaries is presented .
These systems ( called also hard X-ray transients ) , which form the most numerous class of massive X-ray binaries in the Galaxy , are composed of Be stars and neutron stars ( X-ray pulsars ) on wide ( P _ { orb } \sim 17 - 263 d ) , usually eccentric ( e \sim 0.1 - 0.9 ) orbits .
The systems contain two quasi-Keplerian ( { { { { \mid v _ { r } \mid / v _ { orb } \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } % $ \displaystyle < $ \cr$ \displaystyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $% \textstyle < $ \cr$ \textstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $% \scriptstyle < $ \cr$ \scriptstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $% \scriptscriptstyle < $ \cr$ \scriptscriptstyle \sim$ } } } } 10 ^ { -2 } ) discs : decretion disc around Be star and accretion disc around neutron star .
Both discs are temporary : decretion disc disperses and refills on time scales \sim years ( dynamical evolution of the disc , formerly known as the ” activity of a Be star ” ) , while accretion disc disperses and refills on time scales \sim weeks to months ( which is related to the orbital motion on an eccentric orbit and , on some occasions , also to the major instabilities of the other disc ) .
Accretion disc might be absent over a longer period of time ( \sim years ) , if the other disc is very weak or absent .
The X-ray emission of Be/X-ray binaries has distinctly transient nature and is controlled by the centrifugal gate mechanism , which , in turn , is operated both by the periastron passages ( Type I bursts ) and by the dynamical evolution of the decretion disc ( both types of bursts ) .
The X-ray pulsars in these systems rotate at equilibrium periods ( with the possible exception of the slowest pulsars ) .
Be/X-ray binaries are excellent laboratories for investigation of both the evolutionary processes in the two discs and of the evolution of the neutron star rotation .