Microquasars are scaled-down versions of quasars and both are believed to be powered by spinning black holes with masses of up to a few tens that of the Sun (see Figure 1). The word microquasar was chosen by Mirabel et al. (1992) to suggest that we could learn about microquasars from previous decades of studies on AGN. In fact, the analogy with quasars is more than morphological, because 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 stellar systems, to super-massive black holes at the centre of distant galaxies (Rees, 1998). A major difference is that the linear and time scales of the phenomena are proportional to the black hole mass.
Figure 1. Diagram illustrating current ideas concerning microquasars, quasars and gamma-ray bursts (not to scale). It is proposed that a universal mechanism may be at work in all sources of relativistic jets in the universe. Synergism between these three areas of research in astrophysics should help to gain a more comprehensive understanding of the relativistic jet phenomena observed everywhere in the universe.
In quasars and microquasars are found the following three basic ingredients: 1) a spinning black hole, 2) an accretion disk heated by viscous dissipation, and 3) collimated jets of relativistic particles. However, in microquasars the black hole is only a few solar masses instead of several million 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 the several million light-years as in some giant radio galaxies. In quasars matter can be drawn into the accretion disk from disrupted stars or from the interstellar medium of the host galaxy, whereas in microquasars the material is being drawn from the companion star in the binary system. In quasars the accretion disk has sizes of ~ 109 km and radiates mostly in the ultraviolet and optical wavelengths, whereas in microquasars the accretion disk has sizes of ~ 103 km and the bulk of the radiated energy comes out in the X-rays. It is believed that part of the spin energy of the black hole can be tapped to power the collimated ejection of magnetized plasma at relativistic speeds.
Because of the relative proximity and shorter time scales, in microquasars it is possible to firmly establish the relativistic motion of the sources of radiation, and to better study the physics of accretion flows and jet formation near the horizon of black holes. Jets in microquasars are easier to follow because their apparent motions in the sky are 103 faster than in quasars. Because microquasars are found in our Galaxy the two-sided moving jets are more easily seen than in AGN (Mirabel & Rodríguez, 1994). However, to know how the jets are collimated in units of length of the black hole's horizon, AGN up to distances of a few Mpc may present an advantage. Biretta et al. (2002) find that the initial collimation of the non-thermal jet in the galaxy M87 of the Virgo cluster takes place on a scale of 30-100 RS, which is consistent with poloidal collimation by an accretion disk.
At first glance 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). This is because that disks around super-massive 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 × 107 M-1/4 (Rees 1984), with T in K and the mass of the black hole, M, in solar masses. Then, while accretion disks in AGN 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 this class of sources, SS433 is unusual given its broad optical emission lines and its brightness in the visible. Therefore, it is understandable that there was an impasse in the discovery of new stellar sources of relativistic jets until the development of X-ray astronomy that started with the discovery of the first extra-solar X-ray source by Giacconi et al. (1962). Strictly speaking, if it had not been for the historical circumstances described above, the acronym quasar would have suited better the stellar mass versions rather than their super-massive analogs at the centers of galaxies.