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Artist's impression of a blazar. Blazar-illustration.jpg
Artist's impression of a blazar.

A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionized matter traveling at nearly the speed of light) directed very nearly towards Earth. Relativistic beaming of electromagnetic radiation from the jet makes blazars appear much brighter than they would be if the jet were pointed in a direction away from the Earth. [1] Blazars are powerful sources of emission across the electromagnetic spectrum and are observed to be sources of high-energy gamma ray photons. Blazars are highly variable sources, often undergoing rapid and dramatic fluctuations in brightness on short timescales (hours to days). Some blazar jets exhibit apparent superluminal motion, another consequence of material in the jet traveling toward the observer at nearly the speed of light.

Active galactic nucleus Compact region at the center of a galaxy that has a much higher than normal luminosity

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess non-stellar emission has been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an "active galaxy". The radiation from an AGN is believed to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.

An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets become relativistic jets as they show effects from special relativity.

Plasma (physics) State of matter

Plasma is one of the four fundamental states of matter, and was first described by chemist Irving Langmuir in the 1920s. Plasma can be artificially generated by heating or subjecting a neutral gas to a strong electromagnetic field to the point where an ionized gaseous substance becomes increasingly electrically conductive, and long-range electromagnetic fields dominate the behaviour of the matter.


The blazar category includes BL Lac objects and optically violently variable (OVV) quasars. The generally accepted picture is that BL Lac objects are intrinsically low-power radio galaxies while OVV quasars are intrinsically powerful radio-loud quasars. The name "blazar" was originally coined in 1978 by astronomer Edward Spiegel to denote the combination of these two classes.

BL Lacertae object

A BL Lacertae object or BL Lac object is a type of active galactic nucleus (AGN) or a galaxy with such an AGN, named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large-amplitude flux variability and significant optical polarization. Because of these properties, the prototype of the class was originally thought to be a variable star. When compared to the more luminous active nuclei (quasars) with strong emission lines, BL Lac objects have spectra dominated by a relatively featureless non-thermal emission continuum over the entire electromagnetic range. This lack of spectral lines historically hindered BL Lac's identification of their nature and proved to be a hurdle in the determination of their distance.

OVV quasar type of highly variable quasar or a subtype of blazar, whose visible light output can change by 50% in a day

An optically violent variable quasar is a type of highly variable quasar. It is a subtype of blazar that consists of a few rare, bright radio galaxies, whose visible light output can change by 50% in a day. OVV quasars have essentially become unified with highly polarized quasars (HPQ), core-dominated quasars (CDQ), and flat-spectrum radio quasars (FSRQ). Different terms are used but the term FSRQ is gaining popularity effectively making the other terms archaic.

Radio galaxy

Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy nuclei that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process. The observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming. The host galaxies are almost exclusively large elliptical galaxies. Radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology. Recently, much work has been done on the effects of these objects on the intergalactic medium, particularly in galaxy groups and clusters.

In visible-wavelength images, most blazars appear compact and pointlike, but high-resolution images reveal that they are located at the centers of elliptical galaxies. [2]

Elliptical galaxy Galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile

An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.

Blazars are important topics of research in astronomy and high-energy astrophysics. Blazar research includes investigation of the properties of accretion disks and jets, the central supermassive black holes and the surrounding host galaxies, and the emission of high-energy photons, cosmic rays, and neutrinos.

Astronomy natural science that deals with the study of celestial objects

Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets; the phenomena also includes supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject is physical cosmology, which is the study of the Universe as a whole.

High energy astronomy is the study of astronomical objects that release electromagnetic radiation of highly energetic wavelengths. It includes X-ray astronomy, gamma-ray astronomy, and extreme UV astronomy, as well as studies of neutrinos and cosmic rays. The physical study of these phenomena is referred to as high-energy astrophysics.

Accretion disk structure formed by diffuse material in orbital motion around a massive central body

An accretion disk is a structure formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.

In July 2018, the IceCube Neutrino Observatory announced that they have traced a neutrino that hit their Antarctica-based detector in September 2017 back to its point of origin in a blazar 3.7 billion light-years away. This is the first time that a neutrino detector has been used to locate an object in space. [3] [4] [5]

IceCube Neutrino Observatory neutrinodetector in Antarktika

The IceCube Neutrino Observatory is a neutrino observatory constructed at the Amundsen–Scott South Pole Station in Antarctica. Its thousands of sensors are located under the Antarctic ice, distributed over a cubic kilometre.

Antarctica Polar continent in the Earths southern hemisphere

Antarctica is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14,200,000 square kilometres, it is the fifth-largest continent. For comparison, Antarctica is nearly twice the size of Australia. About 98% of Antarctica is covered by ice that averages 1.9 km in thickness, which extends to all but the northernmost reaches of the Antarctic Peninsula.

Light-year unit of length that light travels within one Earthyear; equal to just under 10 trillion kilometres (or about 6 trillion miles)

The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 1012 km) or 5.88 trillion miles (5.88 x 1012 mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.


Sloan Digital Sky Survey image of blazar Markarian 421, illustrating the bright nucleus and elliptical host galaxy. SDSS Mrk 421.jpg
Sloan Digital Sky Survey image of blazar Markarian 421, illustrating the bright nucleus and elliptical host galaxy.

Blazars, like all AGNs, are thought to be 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 relatively small, approximately 10−3 parsecs in size.

Supermassive black hole Largest type of black hole; usually found at the centers of galaxies

A supermassive black hole is the largest type of black hole, containing a mass of the order of hundreds of thousands, to billions of times, the mass of the Sun (M). Black holes are a class of astronomical object that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that nearly all large galaxies contain a supermassive black hole, located at the galaxy's center. In the case of the Milky Way, the supermassive black hole corresponds to the location of Sagittarius A* at the Galactic Core. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering quasars and other types of active galactic nuclei.

The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force. The photon has zero rest mass and always moves at the speed of light within a vacuum.

Electron subatomic particle with negative electric charge

The electron is a subatomic particle, symbol
, whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure. The electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum (spin) of a half-integer value, expressed in units of the reduced Planck constant, ħ. As it is a fermion, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of both particles and waves: they can collide with other particles and can be diffracted like light. The wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a longer de Broglie wavelength for a given energy.

There is also a larger opaque toroid 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 carries a highly energetic plasma away from the AGN. The jet is collimated 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 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

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 constitutes 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 involved:

An example

Consider a jet with an angle to the line of sight θ = 5° and a speed of 99.9% of the speed of light. On Earth, the observed luminosity is 70 times that of the emitted luminosity. However, if θ is at the minimum value of 0° 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. 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 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 the first redshift was found for 3C 273, 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 VRO 42.22.01 [6] was made. BL Lacertae shows many of the characteristics of quasars, but the optical spectrum 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". (Note that the latter term can also mean the original blazar and not the entire class.)

As of 2003, a few hundred BL Lac objects are known.

Current view

Blazars are thought to be active 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 (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. [7] The distinction between these two connected populations explains the difference in emission line properties in blazars. [8]

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 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 of blazars include 3C 454.3, 3C 273, BL Lacertae, PKS 2155-304, Markarian 421, Markarian 501 and S5 0014+81. Markarian 501 and S5 0014+81 are also called "TeV Blazars" for their high energy (teraelectron-volt range) gamma-ray emission. S5 0014+81 is also notable for the most massive black hole ever observed, at 40 billion solar masses.

In July 2018, a blazar called TXS 0506+056 [9] was found as the identified source of high-energy neutrinos by the IceCube project. [4] [5] [10]

See also


  1. Urry, C. M.; Padovani, P. (1995). "Unified Schemes for Radio-Loud Active Galactic Nuclei". Publications of the Astronomical Society of the Pacific. 107: 803. arXiv: astro-ph/9506063 . Bibcode:1995PASP..107..803U. doi:10.1086/133630.
  2. Urry, C. M.; Scarpa, R.; O'Dowd, M.; Falomo, R.; Pesce, J. E.; Treves, A. (2000). "The Hubble Space Telescope Survey of BL Lacertae Objects. II. Host Galaxies". The Astrophysical Journal. 532 (2): 816. arXiv: astro-ph/9911109 . Bibcode:2000ApJ...532..816U. doi:10.1086/308616.
  3. Overbye, Dennis (12 July 2018). "It Came From a Black Hole, and Landed in Antarctica - For the first time, astronomers followed cosmic neutrinos into the fire-spitting heart of a supermassive blazar". The New York Times . Retrieved 13 July 2018.
  4. 1 2 "Neutrino that struck Antarctica traced to galaxy 3.7bn light years away". The Guardian. 12 July 2018. Retrieved 12 July 2018.
  5. 1 2 "Source of cosmic 'ghost' particle revealed". BBC. 12 July 2018. Retrieved 12 July 2018.
  6. Schmitt J. L. (1968): "BL Lac identified as radio source", Nature 218, 663
  7. "Black Hole 'Batteries' Keep Blazars Going and Going" . Retrieved 2015-05-31.
  8. Ajello, M.; Romani, R. W.; Gasparrini, D.; Shaw, M. S.; Bolmer, J.; Cotter, G.; Finke, J.; Greiner, J.; Healey, S. E. (2014-01-01). "The Cosmic Evolution of Fermi BL Lacertae Objects". The Astrophysical Journal. 780 (1): 73. arXiv: 1310.0006 . Bibcode:2014ApJ...780...73A. doi:10.1088/0004-637X/780/1/73. ISSN   0004-637X.
  9. "SIMBAD query result". Retrieved 2018-07-13.
  10. "IceCube Neutrinos Point to Long-Sought Cosmic Ray Accelerator". Retrieved 2018-07-13.

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Relativistic beaming is the process by which relativistic effects modify the apparent luminosity of emitting matter that is moving at speeds close to the speed of light. In an astronomical context, relativistic beaming commonly occurs in two oppositely-directed relativistic jets of plasma that originate from a central compact object that is accreting matter. Accreting compact objects and relativistic jets are invoked to explain the following observed phenomena: x-ray binaries, gamma-ray bursts, and, on a much larger scale, active galactic nuclei (AGN).

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