Blazar
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A blazar is a galaxy with a very compact and highly variable energy source at the center of the 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 homogenous group and can be divided into two groups of galaxies: 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 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. 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.
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Structure
Galaxies_AGN_Jet_Line-of-Sight.jpg
Blazars, like all AGN, are ultimately powered by material falling onto a supermassive black hole at the center of the host galaxy. Gas, dust and the occasional star are captured and spiral into this central black hole creating a hot accretion disk which generates enormous amounts of energy in the form of photons, electrons, positrons and other elementary particles. This region is quite small, approximately 10−3 parsecs in size.
There is also a larger opaque torus extending several parsecs from the central black hole, containing a hot gas with embedded regions of higher density. These "clouds" can absorb and then re-emit energy from regions closer to the black hole. On Earth the clouds are detected as emission lines in the blazar spectrum.
Perpendicular to the accretion disk, a pair of relativistic jets carry a highly energetic plasma away from the AGN. The jet is collimated by a combination of intense magnetic fields and power winds from the accretion disk and torus. Inside the jet, high energy photons and particles interact with each other and the strong magnetic field.These relativistic jets can extend as far as many tens of kiloparsecs from the central black hole.
All of these regions can produce a variety of observed energy, mostly in the form of a nonthermal spectrum ranging from very low frequency radio to extremely energetic gamma rays, with a high polarization (typically a few percent) at some frequencies. The nonthermal spectrum consists of synchrotron radiation in the radio to X-ray range, and inverse Compton emission in the X-ray to gamma-ray region. A thermal spectrum peaking in the ultraviolet region and faint optical emission lines are also present in OVV quasars, but faint or non-existent in BL Lac objects.
Relativistic Beaming
Galaxies_AGN_Jet_Properties-with-LoS.jpg
The observed emission from a Blazar is greatly enhanced by relativistic effects in the jet, a process termed relativistic beaming.The bulk speed of the plasma that constitues the jet can be in the range of 95%–99% of the speed of light. (This bulk velocity is not the speed of a typical electron or proton in the jet. The individual particles move in many directions with the result being that the net speed for the plasma is in the range mentioned.)
The relationship between the luminosity emitted in the rest frame of the jet and the luminosity observed from Earth depends on the characteristics of the jet. These include whether the luminosity arises from a shock front or a series of brighter blobs in the jet, as well as details of the magnetic fields within the jet and their interaction with the moving particles.
A simple model of beaming however, illustrates the basic relativistic effects connecting the luminosity emitted in the rest frame of the jet, Se and the luminosity observed on Earth, So. These are connected by a term referred to in astrophysics as the doppler factor, D, where So is proportional to Se × D2.
When looked at in much more detail than shown here, three relativistic effects are at involved:
- Relativistic Aberration contributes a factor of D2. Aberration is a consequence of special relativity where directions which appear isotropic in the rest frame (in this case, the jet) appear pushed towards the direction of motion in the observer's frame (in this case, the Earth).
- Time Dilation contributed a factor of D+1. This effect speeds up the apparent release of energy. If the jet emits a burst of energy every minute in its own rest frame this may be observed on Earth as being a much faster release, perhaps one burst every ten seconds.
- Blueshifting contributes a factor of D−1 and works to decrease the amount of beaming.
An Example
Consider a jet with an angle to the lines of sight θ = 5 degrees and a speed of 98% of the speed of light. On Earth the observed luminiosity is 70 times that of the emitted luminosity. However if θ is at the minimum value of 0 degrees the jet will appear 600 times brighter from Earth.
Beaming Away
Relativistic beaming also has another critical consequence. The jet which is not approaching Earth will appear dimmer because of the same relativistic effects. Therefore two intrinsically identical jets will appear significantly asymmetric. Indeed, in the example giving above any jet where θ < 35 degrees will be observed on Earth as less luminious than it would be from the rest frame of the jet.
A further consequence is that a population of intrinsically identical AGN scattered in space with random jet orientations will look like a very inhomogeneous population on Earth. The few objects where θ is small will have very bright jets, while the rest will have considerably weaker and asymmetric jets.
This is the essence behind the connection between blazars and radio galaxies. AGN which have jets oriented close to the line of sight with Earth can appear extremely different from other AGN even if they are intrinsically identical.
Discovery
Many of the brighter blazars were first identified, not as powerful distant galaxies, but as irregular variable stars in our own galaxy. These blazars, like genuine irregular variable stars, changed in brightness on periods of days or years, but with no pattern.
The early development of radio astronomy had shown that there are numerous bright radio sources in the sky. By the end of the 1950s the resolution of radio telescopes was sufficient to be able to identify specific radio sources with optical counterparts, leading to the discovery of quasars. Blazars were highly represented among these early quasars, and indeed the first redshift was found for 3C273 a highly variable quasar which is also a blazar.
In 1968 a similar connection between the "variable star" BL Lacertae and a powerful radio source was made. BL Lacertae shows many of the characteristics of quasars, but the optical spectrum was devoid of the spectral lines used to detemine redshift. Faint indications of an underlying galaxy — proof that BL Lacertae was not a star — was found in 1974.
The extragalactic nature of BL Lacertae was not a surprise. In 1972 a few variable optical and radio sources were grouped together and proposed as a new class of galaxy: BL Lacertae-type objects. This terminology was soon shortened to "BL Lacertae object," "BL Lac object," or simply "BL Lac." (Note that the latter term can also mean the original blazar and not the entire class.)
As of 2003, a few hundreds of BL Lac objects are known.
Current vision
Blazars are thought to be active galaxy nuclei, with relativistic jets oriented close to the line of sight with the observer.
The special jet orientation explains the general peculiar characteristics: high observed luminosity, very rapid variation, high polarization (when compared with non-blazar quasars), and the apparent superluminal motions detected along the first few parsecs of the jets in most blazars.
A Unified Scheme or Unified Model has become generally accepted where highly variable quasars are related to intrinsically powerful radio galaxies, and BL Lac objects are related to intrinsically weak radio galaxies. The distinction between these two connected populations explains the difference in emission line properties in Blazars.
Alternate explanations for the relativistic jet/unified scheme approach have been proposed include gravitational microlensing and coherent emission from the relativistic jet. Neither of these explain the overall properties of blazars. For example microlensing is achromatic, that is all parts of a spectrum will rise and fall together. This is very clearly not observed in blazars. However it is possible that these processes, as well as more complex plasma physics can account for specific observations or some details.
Some examples for blazars include 3C273, BL Lacertae, Markarian 421, Markarian 501. The latter two are also called TeV Blazars for their high energy X-ray emission.hu:Blazár