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. Supermassive Black Hole is a song by English rock band Muse and is featured on their 2006 album, Black Holes and Revelations. ... A host galaxy is one with an AGN (Active Galactic Nuclei or Active galaxy) at its core. ... Extragalactic astronomy is the branch of astronomy concerned with objects outside our own Milky Way Galaxy (the study of all astronomical objects which are not covered by galactic 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. The name "blazar" was originally coined in 1978 by astronomer Ed Spiegel to denote the combination of these two classes. An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... Artists impression of quasar GB1508 A quasar (contraction of QUASi-stellAR radio source) is an astronomical source of electromagnetic energy, including light, which shows a very high redshift. ... An OVV Quasar, or an Optically Visually Violent Quasar, is a type of highly variable quasar. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ... A BL Lac object or BL Lacertae object or BL Lac is a type of galaxy with an active galactic nucleus (AGN), making it a type of active galaxy. ...

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.^[1] In astronomy, superluminal motion is the apparently faster-than-light motion seen in some radio galaxies, quasars and recently also in some galactic sources called microquasars. ...

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. An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... A host galaxy is one with an AGN (Active Galactic Nuclei or Active galaxy) at its core. ... An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ...

Alternative models, for example, gravitational microlensing, may account for a few observations of some blazars which are not consistent with the general properties. This article is in need of attention from an expert on the subject. ...

Structure

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. A diagram of the angle to the line-of-sight (los) for a relativistic jet. ... Supermassive Black Hole is a song by English rock band Muse and is featured on their 2006 album, Black Holes and Revelations. ... The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, microwaves, radio waves, and visible light are all forms of light. ... The electron is a fundamental subatomic particle that carries an electric charge. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... In particle physics, an elementary particle is a particle of which other, larger particles are composed. ... Stellar parallax motion The parsec (symbol pc) is a unit of length used in astronomy. ...

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. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ... Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultra high frequency VHF = Very high frequency HF = High...

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. Artists conception of a binary star system with one black hole and one main sequence star An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... Relativistic Jet. ...

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. Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultra high frequency VHF = Very high frequency HF = High... In electrodynamics, polarization (also spelled polarisation) is the property of electromagnetic waves, such as light, that describes the direction of their transverse electric field. ... Synchrotron radiation is electromagnetic radiation, similar to cyclotron radiation, but generated by the acceleration of ultrarelativistic (i. ... In physics, Compton scattering or the Compton effect, is the decrease in energy (increase in wavelength) of an X-ray or gamma ray photon, when it interacts with matter. ...

Relativistic Beaming

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.) An illustration of how jet orientation effects observed properties in Active Galaxies. ... The special theory of relativity was proposed in 1905 by Albert Einstein in his article On the Electrodynamics of Moving Bodies. Some three centuries earlier, Galileos principle of relativity had stated that all uniform motion was relative, and that there was no absolute and well-defined state of rest... Relativistic beaming is the process by which the relativistic effect modifies the apparent luminosity of a relativistic jet. ...

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, S_e and the luminosity observed on Earth, S_o. These are connected by a term referred to in astrophysics as the doppler factor, D, where S_o is proportional to S_e × D². Relativistic beaming is the process by which the relativistic effect modifies the apparent luminosity of a relativistic jet. ... Relativistic beaming is the process by which the relativistic effect modifies the apparent luminosity of a relativistic jet. ...

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.
• Windowing can contribute a factor of D⁻¹ and then works to decrease the amount of boosting. This happens for a steady flow, because there are then D fewer elements of fluid within the observed window, as each element has been expanded by factor D. However, for a freely propagating blob of material, the radiation is boosted by the full D⁺³.

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 given 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 one very bright jet, while the rest will apparently have considerably weaker jets. Those where θ varies from 90° will appear to have 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. An irregular variable is a type of variable star in which variations in brightness show no regular periodicity. ...

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 3C 273 — a highly variable quasar which is also a blazar. Microwave image of 3C353 galaxy at 8. ... The 1950s was the decade spanning from the 1st of January, 1950 to the 31st of December, 1959. ... Angular resolution describes the resolving power of any optical device such as a telescope, a microscope, a camera, or an eye. ... The 64 metre radio telescope at Parkes Observatory The Very Large Array, an interferometric array formed from many smaller telescopes, like many larger radio telescopes. ... Artists impression of quasar GB1508 A quasar (contraction of QUASi-stellAR radio source) is an astronomical source of electromagnetic energy, including light, which shows a very high redshift. ... 3C273 is a quasar located in the constellation Virgo. ...

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. 1968 (MCMLXVIII) was a leap year starting on Monday (the link is to a full 1968 calendar). ... BL Lacertae is a highly variable, extragalactic AGN (Active Galactic Nuclei or Active galaxy) originally thought to be an irregular variable star in our own galaxy and so was given a variable star designation. ... Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultra high frequency VHF = Very high frequency HF = High... 1974 (MCMLXXIV) was a common year starting on Tuesday. ...

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.) A BL Lac object or BL Lacertae object or BL Lac is a type of galaxy with an active galactic nucleus (AGN), making it a type of active galaxy. ...

As of 2003, a few hundreds of BL Lac objects are known. 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ...

Current vision

Blazars are thought to be active galaxy nuclei, with relativistic jets oriented close to the line of sight with the observer. An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ...

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. In astronomy, superluminal motion is the apparently faster-than-light motion seen in some radio galaxies, quasars and recently also in some galactic sources called microquasars. ...

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 3C 273, BL Lacertae, PKS 2155-304, Markarian 421, and Markarian 501. The latter two are also called "TeV Blazars" for their high energy (Tera electron volt range) gamma-ray emission. 3C273 is a quasar located in the constellation Virgo. ... BL Lacertae is a highly variable, extragalactic AGN (Active Galactic Nuclei or Active galaxy) originally thought to be an irregular variable star in our own galaxy and so was given a variable star designation. ... Markarian 421 is a blazar located in the constellation Ursa Major. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ...

Notes

1. ^ Biretta, John (1999-01-06). HUBBLE DETECTS FASTER-THAN-LIGHT MOTION IN GALAXY M87. Space Telecsope Science Institute.