Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA ASP Conference Series, Vol. ***, 2003

I partially review our current knowledge on microquasars, making special emphasis on radio interferometric observations, and I comment on emerging trends in this field of astrophysics. 1. X-ray binaries, radio emission and microquasars An X-ray binary is a binary system containing a compact object, either a neutron star or a stellar-mass black hole accreting matter from the companion star. The accreted matter carries angular momentum on its way to the compact object, around which, in most cases, forms an accretion disk responsible for the X-ray emission. The most recent catalogs contain a total of 280 X-ray binaries (Liu, van Paradijs, & van den Heuvel 2000; Liu, van Paradijs, & van den Heuvel 2001). In 131 of such systems the companion star has a spectral type O or B, and they are classified as High Mass X-ray Binaries (HMXBs), where mass transfer takes place via a decretion disk (in Be stars) or via a strong stellar wind or Roche-lobe overflow (in OB supergiants). In 149 X-ray binaries the optical companion has a spectral type later than B, and they are called Low Mass X-ray Binaries (LMXBs), where mass transfer occurs via Roche-lobe overflow. Among them, this author has found 43 Radio Emitting X-ray Binaries (REXBs), of which 8 are HMXBs that usually show persistent radio emission and 35 are transient LMXBs. Since the detected radio emission (in nearly all cases) displays nonthermal spectra, shows some degree of polarization, and implies high brightness temperatures, it is interpreted as produced by the synchrotron radiation mechanism, which takes place when we have charged particles accelerated in the presence of magnetic fields. A microquasar is simply a REXB displaying relativistic radio jets that can be imaged at a variety of angular scales using different interferometers. Due to the lack of space, I refer the reader to Mirabel & Rodríguez (1999) and to Fender (2004) for extended reviews on the topic (including discussion on emission mechanisms, relativistic effects, observed radio/X-ray correlations, energet-ics, jet formation, etc.). The name microquasar was given not only because of the observed morphological similarities between these sources and the distant quasars but also because of physical similarities, since when the compact object is a black hole, some parameters appear to scale with the mass of the central object. In this context, the temperatures of the inner parts of the accretion disks are of the order of ∼10 7 K in the case …