Some 200 -- 250 X-ray binaries, systems in which a neutron star or black hole is accreting material from a companion star, are known. Approximately 20% of these sources have been detected at radio wavelengths, and in several cases high resolution radio observations have resolved this emission into jet-like structures, sometimes with components moving at relativistic velocities. All of these relativistic jets emit primarily via incoherent synchrotron emission from very high energy electrons spiralling in magnetic fields (although other emission mechanisms may contribute to the weak, flat spectral components which are sometimes observed).
It may seem paradoxical that relativistic jets were first discovered in the nuclei of galaxies and distant quasars and that for more than a decade SS433 was the only known object of its class in our Galaxy (Margon 1984). The reason for this is that disks around supermassive black holes emit strongly at optical and UV wavelengths. Indeed, the more massive the black hole, the cooler the surrounding accretion disk is. For a black hole accreting at the Eddington limit, the characteristic black body temperature at the last stable orbit in the surrounding accretion disk will be given approximately by T ~ 2*107M-1/4 (Rees 1984), with T in K and the mass of the black hole, M, in solar masses.
Then, while accretion disks in AGNs have strong emission in the
optical and ultraviolet with distinct
broad emission lines, black hole and neutron star
binaries usually are identified for the first time by their X-ray emission.
Among these sources, SS 433 is unusual given its broad optical emission lines
and its brightness in the visible.
Observations in the two extremes of the electromagnetic
spectrum, in the domain of the hard X-rays on one hand
(Sunyaev et al. 1991; Paul et al. 1991), and in the domain
of radio wavelengths on the other hand, revealed the existence of new
stellar sources of relativistic jets known as microquasars
(Mirabel et al. 1992; Mirabel and Rodriguez 1998). These are
stellar-mass black holes in our Galaxy that mimic, on a smaller scale, many
of the phenomena seen in quasars.
The microquasars combine two relevant aspects of relativistic astrophysics: accreting black holes (of stellar origin) which are a prediction of general relativity and are identified by the production of hard X-rays and gamma-rays from surrounding accretion disks, and relativistic jets of particles that are understood in terms of special relativity and are observed by means of their synchrotron emission.
Multi-wavelength studies of the X-ray and gamma-ray sources in
the galactic center region led in the year 1992 to the discovery of two
microquasars: 1E1740.7-2942 and GRS 1758-258 (Mirabel et al. 1992;
Rodriguez, Mirabel, and Marti 1992).
1E1740.7-2942 and GRS 1758-258 seem to be persistent sources of both X-rays and relativistic jets. Mirabel et al. (1993) have argued why it would be unlikely that the radio sources are radio galaxies accidentally superposed on the X-rays sources. For 1E1740.7-2942 no counterpart in the optical or near infrared wavelengths has been found so far, although there is a report of a marginal detection at 3.8mum by Djorgovski et al. (1992). GRS 1758-258 has two possible faint candidate counterparts (Marti et al. 1998). In these binaries of stellar-mass are found the three basic ingredients of quasars; a black hole, an accretion disk heated by viscous dissipation, and collimated jets of high energy particles. But in microquasars the black hole is only a few solar masses instead of several millon solar masses; the accretion disk has mean thermal temperatures of several million degrees instead of several thousand degrees; and the particles ejected at relativistic speeds can travel up to distances of a few light years only, instead of several millon light years as in giant radio galaxies (Mirabel and Rodriguez (1998). Indeed, simple scaling laws govern the physics of flows around black holes, with length and time scales being proportional to the mass of the black holes (Sams et al. 1996; Rees 1998).
The word microquasar was chosen to suggest that the analogy with quasars is more than morphological, and that there is an underlying unity in the physics of accreting black holes over an enormous range of scales, from stellar-mass black holes in binary systems, to supermassive black holes at the center of distant galaxies.
Strictly speaking and not being for the historical circumstances described above, the acronym quasar (`quasi-stellar-radio-source') would have suited better the stellar mass versions rather than their super-massive analogs at the centers of galaxies.
CygX-3 , for which an expansion velocity of ~0.3-0.99c has been repeatedly measured during outbursts but in which no observations have to date been able to resolve the motion of individual plasmons.
SS433 famous X-ray, optical, IR, radio variable source, close binary, indisputably possesses collimated outflows, visible in optical and X-rays ranges.
GRS 1915+105 a spectacular X-ray and radio transient which mapping with VLA has revealed to possess relativistic jets with apparent superluminal motion, implying true velocities of ~0.9c (Mirabel 1994).
LSI+61 303 a periodic radio flaring source with measured expansion following outburst.
GRO J1655-40 a second superluminal X-ray transient, with ~0.9c jets resolved by southern hemisphere VLBI (Tingay+ 1995) and the VLA (Hjellming 1995)
SAX J1819-254 == V4641 Sgr, detected in the radio observations of the 1999 September event in the very fast and power X-ray transient XTE J1819-254 were carried out (Hjellming et al., ApJ, 2000, v.544, 977-992). This event was extremely rapid in its rise and decay at radio, optical and X-ray wavelengths with radio jet ejection associated with a 12 Crab X-ray outburst on 1999 September 15 event The radio source was resolved as a 0.25''-source at the first VLA observation on Sept. 16.027 UT, with apparent proper motions of 0.2''-0.5'' per day, and decayed by a factor of 04 over the first day. Finite jet segment models for a relativistic jet with v=0.88c. At a probably distance of ~0.5 kpc, XTE J1819-254 is the closest relativistic jet source yet observed - with 1.5c and 0.6c for the apparent velocities for the approaching and receding jets.
XTE J1859+22 , the X-ray transient (IAUC 7274). On Oct. 16 it was ~1.4 Crab in XTE band. It was detected in optical and radio ranges.
1E1740.7-2942 which while not definitely established as X-ray binary, are bright hard X-ray source near the galactic centre with spectacular arcmin-scale radio jets.
GRS 1758-258 is recently esblished as X-ray binary, bright hard X-ray source with Chandra.
Cir X-1 another periodic flaring source which is embedded in a synchrotron structure with associated radio jets (Stewart 1993).
XTE J1550-564 was recently detected as a black hole candidate with XTE. It is variable radio source. QPOs were detected with XTE.
LS5039 is new microquasar, the 11.2 magnitude star, probably EGRET gamma-ray source 3EG J1824-1514. The jet-like structure was detected with VLA/VLBA mapping at 5 GHz.