Be/X-ray binaries: An observational approach

Be/X-ray binaries are the most numerous class of X-ray binaries. They constitute an excellent tracer of star formation and can be used to study several aspects of astrophysics, from mass loss in massive stars to binary evolution. This short review, intended for the non-specialist, presents a summary of their basic observational properties and outlines the physical mechanisms giving rise to these characteristics. 1. What are we calling a Be/X-ray binary? A Be/X-ray binary can be trivially defined as a binary system containing a Be star, which, for some reason, produces X-ray emission. Modern reviews of the properties of Be/X-ray binaries can be found in Nagase (2001, concentrating on X-ray properties) and Coe (2000, mainly optical observations). A Be star is trivially defined as “a non-supergiant B-type star whose spectrum has, or had at some time, one or more Balmer lines in emission” (Collins 1987). However, such trivial definitions are necessarily too broad. If we want to define a class of objects with common physical characteristics, these definitions need some qualification. For a start, we concentrate on “classical Be stars”, early-type (mostly Btype, but also late O-type) stars which show emission lines because they are surrounded by a disk of material lost from their equator (see Porter & Rivinius 2003 for a recent review; see also Balona 2000; Slettebak 1988). The mass loss in a classical Be star is due to causes intrinsic to the star itself (though binary companions, when present, may have some triggering effect; cf. Miroshnichenko et al. 2003). In a Be/X-ray binary, emission lines should be associated with a classical Be star and come from such a decretion disk (Okazaki 2001). A system like the black hole candidate LMC X-3 (Cowley et al. 1983) is not a Be/X-ray binary, as the emission lines most likely come from an accretion disk around the black hole. The case of the bright transient A0538−66 is less clear, as it looks like a Be star during quiescence states, but has a spectrum completely different from a Be star when in outburst (Charles et al. 1983) and displays optical variability unprecedented in a Be star (McGowen & Charles 2003). At present, we know the optical counterparts of > 20 Be/X-ray binaries in the Galaxy and > 10 in the Large Magellanic Cloud (LMC). A relatively up-to-date list of massive X-ray binaries, with their properties, is given by Liu et al. (2000) and a recent list of Be/X-ray binaries and candidate is provided by Popov & Raguzova (2004). All the counterparts have spectral type earlier than B2 (Negueruela 1998). As a matter of fact, the spectral distribu-