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A microquasar (or radio emitting X-ray binary) is the smaller cousin of a quasar. Microquasars are named after quasars, as they have some common characteristics: strong and variable radio emission, often resolvable as a pair of radio jets, and an accretion disk surrounding a compact object which is either a black hole or a neutron star. In quasars, the black hole is supermassive (millions of solar masses); in microquasars, the mass of the compact object is only a few solar masses. In microquasars, the accreted mass comes from a normal star, and the accretion disk is very luminous in the optical and X-ray regions. Microquasars are sometimes called radio-jet X-ray binaries to distinguish them from other X-ray binaries. A part of the radio emission comes from relativistic jets, often showing apparent superluminal motion.
Microquasars are very important for the study of relativistic jets. The jets are formed close to the compact object, and timescales near the compact object are proportional to the mass of the compact object. Therefore, ordinary quasars take centuries to go through variations a microquasar experiences in one day.
Noteworthy microquasars include SS 433, in which atomic emission lines are visible from both jets; GRS 1915+105, with an especially high jet velocity and the very bright Cygnus X-1, detected up to the High Energy gamma rays (E> 60 MeV). Extremely high energies of particles emitting in the VHE band might be explained by several mechanisms of particle acceleration (see Fermi acceleration and Centrifugal mechanism of acceleration). No microquasar has been detected in the Very High Energy gamma-ray band (E>100 GeV). LS I +61 303, detected in this wavelength, was proposed as a microquasar, but after VLBA observations, the pulsar wind scenario is the most consistent one with this source.