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Microquasars are smaller cousins of quasars. They are named after quasars, as they have some common characteristics: strong and variable radio emission often seen as radio jets, and an accretion disk surrounding a black hole. In quasars, the black hole is supermassive (millions of solar masses) as in microquasars, the black hole mass is a few solar masses. In microquasars, the accreted mass comes from a normal star and the accretion disk is very luminous in optical regions and X-rays. 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 black hole, and timescales near the black hole are proportional to the mass of the black hole. Therefore, ordinary quasars take centuries to go through variations a microquasar experiences in one day.
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