Relativistic Jets and Blazars
These jets are linear features originating very close to the super massive black hole (SMBH) at the center of some quasars and radio galaxies. The commonly accepted model consists of two oppositely directed jets, although in many cases only one side is easily detected. There is still some uncertainty as to their basic composition; all we really know is that they act as conduits for transporting energy over intergalactic distances which in some cases exceeds a million light years. We also are convinced that when we detect radiation from jets, it means there are relativistic electrons producing the emission (see below). These jets are called 'Relativistic' because the energy is being transported at a velocity very close to the velocity of light, and they are 'Jets' because they are well collimated and we know that power is flowing away from the SMBH. A picture of the very innermost part of the jet in one of the nearest radio galaxies is shown here.
Blazars are a subclass of quasars and radio galaxies and we believe that their extreme brightness and variability are explained by the notion that one of their jets is pointed almost exactly towards the Earth. Whenever an emitting volume is moving close to the speed of light, its radiated energy is squeezed into the forward direction, producing a narrow cone of emission. We call this 'relativistic beaming', and this effect not only explains why blazars are so bright, but also explains why in many cases, only one jet of a radio galaxy or quasar is detected.
The term "Thermal emission" is used for the natural radiation from any material which can be characterized as having some temperature. The sun is close to 6000 deg K and emits most of its energy in the optical band whereas the Earth is close to 300 deg K and radiates mostly in the far IR or millimeter wavebands. Hot gas in clusters of galaxies is generally over a million degrees and radiates mostly in the X-ray. See: Clusters & Galaxies.
Non-thermal Emission is a general term which describes radiation from a body which can not be assigned any temperature. In astrophysics, this usually arises from a power law distribution of relativistic electrons interacting with a magnetic field and with ambient photons. The two most common sorts of non-thermal radiation are synchrotron emission and inverse Compton emission.
Synchrotron emission arises from relativistic electrons spiraling in the ambient magnetic field. It was first observed on Earth as a bluish light coming from an electron accelerator called a Synchrotron. Synchrotron emission is responsible for most of the celestial radio sources we observe. However, for many jets, we observe synchrotron emission in the optical and X-ray bands as well.
Inverse Compton emission results when a high energy electron scatters off a lower energy photon and transfers most of its energy to the photon, thereby producing a much higher frequency photon than the original. Except for a few classes of objects (e.g. pulsars), we believe that most of the X-ray emission we observe is either thermal, synchrotron, or inverse Compton.
By measuring the characteristics of jets with all available wavebands we seek to deduce the basic properties of these enigmatic features: how and why the jet is formed, how it propagates over such large distances, the composition, and just how close to the speed of light are the velocities describing the transport of power and the motion of the emitting regions associated with the jets.
Dan Harris, Ralph Kraft, Aneta Siemiginowska, Dan Schwartz, Martin Elvis, Tom Aldcroft
There are many more who have been associated with the occasional jet paper...