Annu. Rev. Astron. Astrophys. 1999. 37: 409-443
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While the first evidence of jet-like features emanating from the nuclei of galaxies goes back to the discovery by Curtis (1918) of the optical jet from the elliptical galaxy M87 in the Virgo cluster, the finding that jets can also be produced in smaller scale by binary stellar systems is much more recent. The d etection by Margon et al (1979) of large, periodic Doppler drifts in the optical lines of SS 433 resulted in the proposition of a kinematic model (Fabian & Rees 1979, Milgrom 1979) consisting of two precessing jets of collimated matter with velocity of 0.26c. High angular radio imaging as a function of time showed the presence of outflowing radio jets and fully confirmed the kinematic model (Spencer 1979, Gilmore & Seaquist 1980, Gilmore et al 1981, Hjellming & Johnston 1981). The early history of SS 433 has been reviewed by Margon (1984).

Since the detection of Sco X-1 at radio wavelengths (Ables 1969), some X-ray binaries had been known to be strong, time-variable non-thermal emitters. Ejection of synchrotron-emitting clouds was suspected from those days, but the actual confirmation of radio jets came only with the observations of SS 433. At present, there are about 200 known galactic X-ray binaries (van Paradijs 1995), of which about 10 percent are radio-loud (Hjellming & Han 1995). Of these radio-emitting X ray binaries, 10 have shown evidence of relativistic jets of synchrotron emission, and this review focuses on this set of objects. After the definition of Bridle & Perley (1984) for extragalactic jets, we use the term "jets" to designate collimated ejecta that have opening angles leq 15°.

In the last years it has become clear that collimated ejecta can be produced in several stellar environments when an accretion disk is present. Jets with terminal velocities in the order of a few hundred to a few thousand km s-1 are now known to emanate from objects as diverse as very young stars (Reipurth & Bertout 1997), nuclei of planetary nebulae (López 1997), and accreting white dwarfs that appear as supersoft X-ray sources (Motch 1998, Cowley et al 1998). These types of stellar jets have, however, non-relativistic velocities (~ 100-10000 km s-1) and their associated emission is dominantly thermal (i.e. free-free continuum emission in the radio as well as characteristic near-IR, optical and UV lines). Interestingly, in all known types of jet sources a disk is believed to be present. This review concentrates on synchrotron jets with velocities that can be considered relativistic (v geq 0.1c), which are observed in X-ray binaries that contain a compact object, that is, a neutron star or a black hole. Our emphasis is on the radio characteristics of these sources. For detailed reviews of the X-ray properties of these sources we refer the reader to the reviews by Tanaka & Shibazaki (1996), Zhang et al (1997).

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