A microquasar is an X-ray binary system which launches and collimates relativistic jets. These binary systems are formed by a normal star and a compact object, a black hole or a neutron star, which accretes mass from the star. Other components conform a microquasar: an accretion disk, a hot plasma corona, and relativistic jets. A brief description of these components is given below. Black hole X-ray binaries with relativistic jets mimic, on a much smaller scale, many of the phenomena seen in quasars and are thus called microquasars ,.
Star: If the star is a massive star (usually of spectral type O,B) the system is called a high-mass microquasar (HMMQ). On the other hand, if the star is not an early-type star (spectral types A, F, G) the system is said to be a low-mass microquasar (LMMQ).
Accretion disk: the accretion disk is formed from the infalling material from the star which is accreted by the compact object. This material has high angular momentum, therefore a disk structure is formed. The disk has differential rotation, and generates shear which heats the gas. The disk is mainly detected in X-rays produced by viscous dissipation .
Corona: it is an ultra hot plasma which surrounds the compact object. In the innermost part of the accretion disk the temperature is so high that the material evaporates thus forming the corona. In systems where the neutron star is highly magnetized this component is not present. The corona is thought to be responsible for the non-thermal radiation detected at gamma ray energies.
Relativistic jets: the jets are two-sided high collimated outflows of particles and electromagnetic fields. Jet formation is highly associated with the accretion disk and with the large-scale magnetic field. The jets are mainly detected through their synchrotron radio emission  .
The spectral energy distribution of these sources is very complex because it is the result of the sum of different radiative processes which take place in different spatial scales. Microquasars can emit through the entire electromagnetic spectrum. The donor star can emit radiation from the infrared to the ultraviolet. The accretion disk produces soft X-rays, and the corona is responsible for the hard X-rays and soft gamma rays. The jet emits from radio up to gamma rays in some cases.
Microquasars present different spectral states. The most characteristic ones are the thermal dominated state known as high-soft, and the non-thermal dominated state known as low-hard. In both states the spectrum is the sum of two components: a blackbody spectral distribution and a power law with an exponential cutoff.
During the high-soft state, the spectral energy distribution is dominated by a blackbody contribution with a temperature 0.5-1 Kev. The non-thermal contribution is weak, it has a power law index 2.5-3 and an exponential cutoff with energies greater than 200 KeV.
During the low-hard state the thermal component is diminished and shifted to lower energies. The power law distribution has a harder index 1.6 and an exponential cutoff at 100 keV. During this state steady relativistic jets are launched.
These two components of the spectrum come from two physically related regions: the accretion disk responsible for the thermal component, and the corona, responsible for the non-thermal one.
SS 433 is an unusual X-ray binary. This microquasar consists of a donor star , possible an A-type supergiant and a very extended disk around a black hole. The jets from SS 433 precess, with a period of 13 days. These jets have been observed at radio wavelengths since the seventies .
Cygnus X-1 is the best established candidate for a black hole in the Galaxy. This system is the most studied microquasar . It is located at 2 kpc, and is a HMMQ. The companion star is a O9.7 Iab. This system is the first black hole microquasar detected at gamma-ray energies .
Cygnus X-3 is also a very well studied microquasar. This system is a high-mass X-ray binary with a WN Wolf-Rayet companion star .
LS I +61 303
The X-ray binary LS I +61 303 is formed by a BO-BO.5 Ve star and a dense disk. The compact object is of unknown nature. This system exhibits jet-like features at radio frequencies which may indicate that is a microquasar. However, some authors claim that the high-resolution radio observations is evidence for a pulsar.
An example of a neutron star microquasar is Scorpius X-1. This system is a low-mass microquasar where the donor star has a mass of 0.48 solar masses, and the compact object is a neutron star of 1.4 solar masses.
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