Be X-ray Binaries
Be/X-ray binary systems form the majority of accreting X-ray pulsars. In these systems the optical companion is a giant star which besides a fast low-density wind also show an equatorial disk or flattened envelope in fast rotation. This disk shows up through Balmer emission lines (see, e.g., The spectroscopic Be stars Atlas. The X-ray emitting object is usually a neutron star on an eccentric orbit (e=0.1-0.9). For a recent review of Be/X-ray binaries see Reig et al. (2011)[1].
Most Be/X-ray binaries show transient activity and only very weak emission outside outbursts. Stella et al. (1986)[2] have classified the different outbursts in two types: Type I are periodical outbursts linked to the periastron passage of the neutron star and relatively short; Type II are larger outbursts with no clear link to the orbital phase.
Historically, type I outbursts have been explained by a simple picture of the passage of the neutron star through the equatorial disk of the Be star every orbit. But this picture fails to explain the large variety of outbursts seen in many systems with periods of regular outbursts, occasional giant outbursts, but also extended periods of quiescence. A possible explanation was given by the model developed by Atsuo Okazaki in which the disk around the Be star will often be truncated due to tidal interactions and outbursts are triggered by changes in the disk. This model has been published in Negueruela & Okazaki 2001[3], Negueruela et al. (2001)[4], and Okazaki & Negueruela (2001)[5].
Sometimes, optical information can prove quite useful while studying BeXRBs. See Be stars in Gaia for a few simple checks.
The BeXRB Monitor page collects data from different monitoring missions and shows current activity for a variety of sources.
See List of Be X-ray Binaries for details about individual sources.