Probing Extremes of Speed and Energy
One of radio astronomy and very long baseline interferometry’s great successes was its discovery of the extremely fast and apparently faster-than-light or "superluminal" motions in the nuclei of some galaxies.
The most extreme of these sources are the blazars, each of which likely harbors a supermassive black hole and relativistic jets that emanate in opposite directions from near its poles. These sources are unique probes of the most extreme speeds and energies known in the Universe.
A blazar is a very compact and highly variable energy source associated with a supermassive black hole at the center of a host galaxy. 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 and BL Lacertae objects ("BL Lac objects" or simply "BL Lacs"). The name "blazar" was originally coined in 1978 by astronomer Ed Spiegel to denote the combination of these two classes.
Blazars are AGN with a relativistic jet that is pointing in the general direction of the Earth. 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.
VLBA radio images of these sources reveal the physical conditions and, when combined with X-ray and gamma-ray observations, constrain the models for these most powerful particle accelerators in the Universe.