Annu. Rev. Astron. Astrophys. 1999. 37: 409-443 Copyright © 1999 by Annual Reviews. All rights reserved

2. MICROQUASARS

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 SS 433 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 × 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 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. Therefore, it is understandable that there was an impasse in the discovery of new stellar sources of relativistic jets until the recent developments in X-ray astronomy.

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 & Rodríguez 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, Rodríguez et al 1992). The X-ray luminosity, the photon spectrum, and the time variability of these two sources are comparable to those of the black hole binary Cygnus X-1 (Churazov et al 1994, Kuznetsov et al 1997), and it is unlikely that they are extragalactic since no such persistent hard X-ray ultraluminous AGNs are observed (Mirabel et al 1993). In Figure 1 we show the radio counterpart of 1E1740.7-2942. As in Cygnus X-1, the centimeter radio counterpart of 1E1740.7-2942 is a weak core source that exhibits flux variations of the order of ~ 50% which at epochs appear anticorrelated with the X-ray flux (Mirabel et al 1992). At radio wavelengths these two X-ray persistent sources located near the galactic center have a striking morphological resemblance with distant radio galaxies; they consist of compact components at the center of two-sided jets that end in weak, extended lobes with no significant radio flux variations observed in the last 6 years (Rodríguez & Mirabel 1999b). 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 3.8 µm by Djorgovski et al (1992). GRS 1758-258 has two possible faint candidate counterparts (Martí et al 1998).

Figure 1. Contour map of the 6-cm emission from the radio counterpart of 1E1740.7-2942, as observed with the Very Large Array (Mirabel et al 1992, Rodríguez & Mirabel 1999c). The error circle of the ROSAT position (Heindl et al 1995), that includes the core source, is also shown. At a distance of 8 kpc the length of the jet structure would be ~ 5 pc. The half power contour of the beam is shown in the top left corner. Contours are -4, 4, 5, 6, 8, 10, 12, 15, and 20 times 28 µJy beam-1.

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 millon 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 & Rodríguez 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 if it had not been 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.