Blazars are a subset of the Active Galactic Nuclei (AGN or active galaxies). This radio-loud extragalactic objects are optically violent variable quasars, flat-spectrum quasars, high polarized quasars, and BL Lacertae (BL Lac) type objects, which display extremely intense, broad and rapidly varing electromagnetic emission, from radio to gamma-rays in some case. This emission is thought to originate in a relativistic plasma jet which is probably to be powered and accelerated by a billion solar mass black hole in gravitational accretion. Blazars show intense, flat-spectrum radio-loud emission, and their relativistic jet point nearly straight toward us. The Compton Gamma-Ray Observatory-EGRET found that many blazars are intense gamma-ray sources. This has been one of the most exciting astrophysical discovery of past decade. They are called classic BL Lac type objects if the optical continuum emission dominates compared to any line emission.
Briefly a blazar, is an object thet have the following characteristics:
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It appear optically point-like on the sky, i.e. not appear widespread like a galaxy or a nebula. Some blazars have nebulae around them (are fuzzy), but most of the light comes from a point source.
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Their spectra appear to be smooth (i.e. no strong absorption lines that a star might have) and flatter than a star. These two properties by themselves would make them a quasar.
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Their visible light is often partially polarized.
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Their output in all wavelength bands varies more rapidly, and by a larger amount than a classical quasar, with a flare-like behaviour.
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Many blazar emits a significant fraction of their radiation at energy above 100 MeV. Their flux is peaked in high bands around 10Mev-1Gev for the LBL (red-blazar) class, around 1GeV-100GeV for the HBL (blue-blazar) class and around 200Gev-1TeV for the few TeV-blazar.
