Probing Extremes of Speed and EnergyOne 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.  Apparent superluminal motion in the quasar 3C279 is illustrated by these five NRAO Very Long Baseline Array (VLBA) images made over seven years. The stationary core is the bright red spot to the left of each image. The observed location of the rightmost radio source (blue-green) seems to move 25 light years in just 7 years (1991 to 1998). Hence, the changes appear to an observer to be faster than the speed of light or "superluminal". 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 movie illustrates what astronomers believe is happening outside the black hole of the radio source called "3C 120". The supermassive black hole has a mass about 30 millions times that of our Sun and is just a tiny black dot (center). Surrounding it is a disk of gas and dust that is slowly falling into the black hole while rotating around it in nearly circular orbits. The disk contains a magnetic field that is twisted by the rotation, which is faster closer to the black hole (Boston University). 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. |
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