Annu. Rev. Astron. Astrophys. 1999. 37: 409-443
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In X-ray binaries there is a general correlation between the X-ray properties and the jet properties. The time interval and flux amplitude of the variations in radio waves seems to correspond to the time and amplitude variations in the X-ray flux. More specifically, persistent X-ray sources are also persistent radio sources, and the transient X-ray sources produce at radio waves sporadic outburst/ejection events. Persistent sources of hard X-rays (e.g. 1E1740.7-2942, GRS 1758-258) are usually associated to faint, double-sided radio structures that have sizes of several arcmin (parsec scales). The radio core of these two persistent sources are weak (leq 1 mJy) and do not exhibit high amplitude variability. On the contrary, rapidly variable hard X-ray transients (e.g. GRS 1915+105, GRO J1655-40, XTE J1748-288) may exhibit variations in the X-rays and radio fluxes of several orders of magnitude in short intervals of time. Because these black-hole X-ray transients produce sporadic ejections of discrete, bright plasma clouds, the proper motions of the ejecta can be measured.

Probably all hard X-ray sources that accrete at super-Eddington rates produce relativistic jets. However, the observational study of these jets presents in practice several difficulties. Persistent hard X-ray sources like Cygnus X-1 are surrounded by faint non-thermal radio features extending several arcmin (Martí et al 1996), and even in the cases where they are well aligned with the variable compact radio counterpart it is very difficult to prove conclusively that the faint and extended radio features are actually associated with the X-ray source. This was the case of Sco X-1, where possible large-scale radio "lobes" were found to be extragalactic sources symmetrically located in the plane of the sky with respect to Sco X-1 (Fomalont & Geldzahler 1991). On the other hand, in transient black hole binaries one may observe transient sub-arcsec jets, but unless the interferometric observations are conveniently scheduled, the evolution is too rapid and it may not be possible to follow up the proper motions of discrete clouds. This may have been the case in the radio observations of the X-ray sources Nova Oph 93 (Dela Valle et al 1994) and Nova Muscae (Ball et al 1995), among others.

We list in Table 1 the sources of relativistic jets in the Galaxy known so far. The first six are transients, whereas the next four are persistent X-ray sources. Proper motions of the relativistic ejecta have been determined with accuracy in GRS 1915+105, GRO J1655-40, XTE J1748-288, and SS 433. Besides these four sources, proper motions were also measured - but with less accuracy - for moving features in Cygnus X-3 (Schalinski et al 1995, Martí et al 1999), Scorpius X-1 (Fomalont 1999), Circinus X-1 (Fender et al 1998), and CI Cam (XTE J0421+560; Hjellming & Mioduszewski 1998, Mioduszewski et al 1998). Jet structures have been reported to be associated to Cygnus X-1, but these results are still uncertain.

Table 1. Sources of Relativistic Jets in the Galaxy (1)

Source Compact object Vapp (2) Vint (3) theta (4) References

GRS 1915+105 black hole 1.2c-1.7c 0.92c-0.98c 66°-70° MR94; F+99; DMR99
GRO J1655-40 black hole 1.1c 0.92c 72°-85° T+95; HR95; OB97
XTE J1748-288 black hole 0.9c-1.5c > 0.9c H+98
SS 433 neutron star ? 0.26c 0.26c 79° M84; S84
Cygnus X-3 neutron star ? ~ 0.3c ~ 0.3c >70° S+93; M+99
CI Cam neutron star ? ~ 0.15c ~ 0.15c >70° M+98; G+98
Circinus X-1 neutron star geq 0.1c geq 0.1c >70° S+93; F+98
1E1740.7-2942 black hole M+92; RM99c
GRS 1758-258 black hole R+94
Sgr A* black hole L+98

(1) Sources reported as of December 1998.
(2) Vapp is the apparent speed of the highest velocity component of the ejecta.
(3) Vint is the intrinsic velocity of the ejecta.
(4) theta is the angle between the direction of motion of the ejecta with the line of sight.

It is interesting that the ejecta from the black hole binaries GRS 1915+105, GRO J1655-40, and probably also XTE J1748-288 have velocities greater than 0.9c, while the ejecta from the four sources believed to be neutron star binaries have velocities leq 0.5c. From their models of magnetically driven jets, Kudoh & Shibata (1995) have proposed that jet velocities such as those listed in Table 1 are comparable to the Keplerian rotational velocities expected at the base of the jets, close to neutron stars and black holes, respectively. Livio (1997) has also stressed the similarity between the velocity of jets and the escape velocity of the gravitational well from where they were ejected. If this notion is confirmed, jet velocities could then be used to discriminate between neutron stars and black holes, with jet velocities close to the speed of light been produced only in black hole binaries.

Another possible source of relativistic jets in the Galaxy is, of course, Sgr A*, the presumed black hole of 2.5 million solar masses at the galactic center (Eckart & Genzel 1997). The radio source is always present at about the 1 Jy level and exhibits a flat spectrum with relative small variations, a behavior similar to that of the faint compact mJy radio sources associated with Cygnus X-1 (Martí et al 1996) and GRS 1915+105 in its plateau state at times when no strong outburst/ejection events take place, a state that in the latter source can last from days to weeks (Pooley & Fender 1997). This type of radio emission could arise from a jet in a coupled jet-disk system (Falcke et al 1993), from electrons in an advection dominated flow (Narayan et al 1998, Mahadevan 1998), or from shocks in massive winds (Blandford & Begelman 1999). Despite heavy interstellar scattering at radio wavelengths, recent VLBA observations at 7-mm may have resolved Sgr A* in an elongated radio source of 72 Schwarzschild radii suggesting the presence of a jet (Lo et al 1998).

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