Radio emission and jets from microquasars

Abstract

To some extent, all Galactic binary systems hosting a compact object are potential "microquasars", so much as all Galactic nuclei may have been quasars, once upon a time. The necessary ingredients for a compact object of stellar mass to qualify as a microquasar seem to be: accretion, rotation, and magnetic field. The presence of a black hole may help, but is not strictly required, since neutron star X-ray binaries and dwarf novae can be powerful jet sources as well. The above issues are broadly discussed throughout this chapter, with a rather trivial question in mind: Why do we care? In other words: are jets a negligible phenomenon in terms of accretion power, or do they contribute significantly to dissipating gravitational potential energy? How do they influence their surroundings? The latter point is especially relevant in a broader context, as there is mounting evidence that outflows powered by supermassive black holes in external galaxies may play a crucial role in regulating the evolution of cosmic structures. Microquasars can also be thought of as a form of quasars for the impatient: what makes them appealing, despite their low number statistics with respect to quasars, are the fast variability timescales. In the first approximation, the physics of the jet-accretion coupling in the innermost regions should be set by the mass/size of the accretor: stellar mass objects vary by 105-108 times shorter timescales, making it possible to study variable accretion modes and related ejection phenomena over average Ph.D. timescales. At the same time, allowing for a systematic comparison between different classes of compact objects - black holes, neutron stars, and white dwarfs - microquasars hold the key to identify and characterize properties that may be unique to, e.g., the presence (or the lack) of an event horizon. © 2010 Springer-Verlag Berlin Heidelberg.