Blazars

A blazar (alternatively blasar) is a galaxy with a very compact and highly variable energy source at its core. Blazars are among the most violent phenomena in the universe and are an important topic in extragalactic astronomy.

Blazars are members of a larger group of Active Galaxies, also termed Active Galactic Nuclei (AGN). However, blazars are not a homogeneous group and can be divided into two: highly variable quasars, sometimes called Optically Violently Variable (OVV) quasars (these are a small subset of all quasars) and BL Lacertae objects ("BL Lac objects" or simply "BL Lacs"). A few rare objects may be "intermediate blazars" that appear to have a mixture of properties from both OVV quasars and BL Lac objecs.

Blazars are AGN with a relativistic jet that is pointing in the general direction of the Milky Way Galaxy. We observe "down" the jet, or nearly so, and this accounts for the rapid variability and compact features of both types of blazars. Many blazars have apparent superluminal features within the first few parsecs of their jets, probably due to relativistic shock fronts.

The generally accepted picture is that OVV quasars are intrinsically powerful radio galaxies while BL Lac objects are intrinsically weak radio galaxies. In both cases the host galaxies are giant ellipticals.

Alternative models, for example gravitational microlensing may account for a few observations of some blazars but are not consistent with the general properties.