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Ablazar is anactive galactic nucleus (AGN) with arelativistic jet (a jet composed ofionized matter traveling at nearly thespeed of light) directed very nearly towards an observer.Relativistic beaming ofelectromagnetic radiation from the jet makes blazars appear much brighter than they would be if the jet were pointed in a direction away from Earth.[1] Blazars are powerful sources of emission across theelectromagnetic spectrum and are observed to be sources of high-energygamma rayphotons. Blazars are highly variable sources, often undergoing rapid and dramatic fluctuations in brightness on short timescales (hours to days). Some blazar jets appear to exhibitsuperluminal motion, another consequence of material in the jet traveling toward the observer at nearly the speed of light.
The blazar category includesBL Lac objects andoptically violently variable (OVV) quasars. The generally accepted theory is that BL Lac objects are intrinsically low-powerradio galaxies while OVV quasars are intrinsically powerful radio-loudquasars. The name "blazar" was coined in 1978 by astronomerEdward Spiegel to denote the combination of these two classes.[2]
In visible-wavelength images, most blazars appear compact and pointlike, but high-resolution images reveal that they are located at the centers ofelliptical galaxies.[3]
Blazars are important topics of research inastronomy andhigh-energy astrophysics. Blazar research includes investigation of the properties ofaccretion disks andjets, the centralsupermassive black holes and surrounding hostgalaxies, and the emission of high-energyphotons,cosmic rays, andneutrinos.
In July 2018, theIceCube Neutrino Observatory team traced aneutrino that hit itsAntarctica-based detector in September 2017 to its point of origin in a blazar 3.7 billionlight-years away. This was the first time that aneutrino detector was used to locate an object in space.[4][5][6]
Blazars, like all active galactic nuclei (AGN), are thought to be powered by material falling into asupermassive black hole in thecore of the host galaxy. Gas, dust and the occasional star are captured and spiral into this central black hole, creating a hotaccretion disk which generates enormous amounts of energy in the form ofphotons,electrons,positrons and otherelementary particles. This region is relatively small, approximately 10−3parsecs in size.
There is also a larger opaquetoroid extending several parsecs from the black hole, containing a hot gas with embedded regions of higher density. These "clouds" can absorb and re-emit energy from regions closer to the black hole. On Earth, the clouds are detected asemission lines in the blazarspectrum.
Perpendicular to the accretion disk, a pair ofrelativistic jets carries highly energeticplasma away from the AGN. The jet iscollimated by a combination of intense magnetic fields and powerful winds from the accretion disk and toroid. 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 ofkiloparsecs 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 highpolarization (typically a few percent) at some frequencies. The nonthermal spectrum consists ofsynchrotron radiation in the radio to X-ray range, andinverse 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.
The observed emission from a blazar is greatly enhanced byrelativistic effects in the jet, a process calledrelativistic beaming.[7] The bulk speed of the plasma that constitutes the jet can be 99.5% of the speed of light, although individual particles move at higher speeds in various directions.[8]
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 ofbeaming illustrates the basic relativistic effects connecting the luminosity in the rest frame of the jet,Se, and the luminosity observed on Earth,So:So is proportional toSe × D2, whereD is thedoppler factor.
When considered in much more detail, three relativistic effects are involved:
Consider a jet with an angle to the line of sight θ = 5° and a speed of 99.9% of the speed of light. The luminosity observed from Earth is 70 times greater than the emitted luminosity. However, if θ is at the minimum value of 0° the jet will appear 600 times brighter from Earth.
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. In the example given above any jet where θ > 35° will be observed on Earth as less luminous 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.
Many of the brighter blazars were first identified, not as powerful distant galaxies, but asirregular 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 ofradio astronomy had shown that there are many bright radio sources in the sky. By the end of the 1950s, theresolution ofradio telescopes was sufficient to identify specific radio sources with optical counterparts, leading to the discovery ofquasars. Blazars were highly represented among these early quasars, and the first redshift was found for3C 273, a highly variable quasar which is also a blazar.
In 1968, a similar connection was made between the "variable star"BL Lacertae and a powerful radio source VRO 42.22.01.[9] BL Lacertae shows many of the characteristics of quasars, but the opticalspectrum was devoid of the spectral lines used to determine redshift. Faint indications of an underlying galaxy—proof that BL Lacertae was not a star—were 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". (The latter term can also mean the original individual blazar and not the entire class.)
As of 2003[update], a few hundred BL Lac objects were known. One of the closest blazars is 2.5 billion light years away.[10][11]
Blazars are thought to beactive galactic 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 (compared to non-blazar quasars), and the apparentsuperluminal 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.[12] The distinction between these two connected populations explains the difference in emission line properties in blazars.[13]
Other explanations for the relativistic jet/unified scheme approach which have been proposed include gravitational microlensing and coherent emission from the relativistic jet. Neither of these explains the overall properties of blazars. For example, microlensing is achromatic. That is, all parts of a spectrum would rise and fall together. This is 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.
Examples of blazars include3C 454.3,3C 273,BL Lacertae,PKS 2155-304,Markarian 421,Markarian 501,4C +71.07,PKS 0537-286 (QSO 0537-286) andS5 0014+81. Markarian 501 and S5 0014+81 are also called "TeV Blazars" for their high energy (teraelectron-volt range) gamma-ray emission.
In July 2018, a blazar calledTXS 0506+056[14] was identified as a source of high-energy neutrinos by theIceCube project.[5][6][15]
