View source for Blazar

{{short description|Very compact quasi-stellar radio source}}
{{distinguish|Blazer|Blazor}}

[[File:M87 jet.jpg|thumb|upright=1.5|The [[elliptical galaxy M87]] emitting a relativistic jet, as seen by the [[Hubble Space Telescope]]. An active galaxy is classified as a blazar when its jet is pointing close to the line of sight. In the case of M87, because the angle between the jet and the line of sight is not small, its nucleus is not classified as a blazar, but rather as radio galaxy.<ref>{{cite journal | title=The unique case of the active galactic nucleus core of M87: a misaligned low-power blazar? | last1=Lucchini | first1=M. | last2=Krauß | first2=F. | last3=Markoff | first3=S. | journal=Monthly Notices of the Royal Astronomical Society | volume=489 | issue=2 | pages=1633–1643 | date=October 2019 | doi=10.1093/mnras/stz2125 | doi-access=free | arxiv=1907.13408 | bibcode=2019MNRAS.489.1633L }}</ref>]]
A '''blazar''' is an [[active galactic nucleus]] (AGN) with a [[Astrophysical jet|relativistic jet]] – a jet composed of [[Plasma (physics)|ionized matter]] traveling at nearly the [[speed of light]] – directed very nearly towards an observer. [[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 Earth.<ref>{{Cite journal |last1=Urry |first1=C. Megan |last2=Padovani |first2=Paolo |date=September 1995 |title=Unified Schemes for Radio-Loud Active Galactic Nuclei  |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=107 |pages=803 |doi=10.1086/133630 |issn=0004-6280|arxiv=astro-ph/9506063 |bibcode=1995PASP..107..803U }}</ref> Blazars are powerful sources of emission across the [[electromagnetic spectrum]] and are observed to be sources of high-energy [[gamma ray]] [[photon]]s. Blazars are highly variable sources, often undergoing rapid and dramatic fluctuations in brightness on short timescales (hours to days). Some blazar jets appear to exhibit [[superluminal motion]], another consequence of material in the jet traveling toward the observer at nearly the speed of light.

The blazar category is sub-divided into [[BL Lacertae object|BL Lac objects]] and [[flat-spectrum radio quasar]]s (FSRQ), with the former having weak or no emission lines and the latter showing strong emission lines.<ref name=Chen_et_al_2015/> The generally accepted theory is that BL Lac objects are intrinsically low-power [[radio galaxy|radio galaxies]] while FSRQ quasars are intrinsically powerful radio-loud [[quasar]]s. The name "blazar" was coined in 1978 by astronomer [[Edward Spiegel]] to denote the combination of these two classes.<ref>{{cite journal | title=Variability of Blazars and Blazar Models over 38 Years | last=Marscher | first=Alan P. | journal=Galaxies | volume=4 | issue=4 | at=id. 37 | date=September 2016 | doi=10.3390/galaxies4040037 | doi-access=free | bibcode=2016Galax...4...37M }}</ref> In visible-wavelength images, most blazars appear compact and pointlike, but high-resolution images reveal that they are located at the centers of [[Elliptical galaxy|elliptical galaxies]].<ref>{{Cite journal |last1=Urry |first1=C. Megan |last2=Scarpa |first2=Riccardo |last3=O’Dowd |first3=Matthew |last4=Falomo |first4=Renato |last5=Pesce |first5=Joseph E. |last6=Treves |first6=Aldo |date=April 2000 |title=The Hubble Space Telescope Survey of BL Lacertae Objects. II. Host Galaxies |journal=The Astrophysical Journal |language=en |volume=532 |issue=2 |pages=816–829 |doi=10.1086/308616 |arxiv=astro-ph/9911109 |bibcode=2000ApJ...532..816U |issn=0004-637X}}</ref>

Blazars are important topics of research in [[astronomy]] and [[high-energy astronomy|high-energy astrophysics]]. Blazar research includes investigation of the properties of [[accretion disk]]s and [[astrophysical jet|jets]], the central [[supermassive black hole]]s and surrounding host [[galaxy|galaxies]], and the emission of high-energy [[photons]], [[cosmic ray]]s, and [[neutrino]]s.

==Structure==
[[File:SDSS Mrk 421.jpg|thumb|[[Sloan Digital Sky Survey]] image of blazar [[Markarian 421]] (center), illustrating the bright nucleus and elliptical host galaxy,<ref>{{cite journal | title=The TeV blazar Markarian 421 at the highest spatial resolution | last1=Blasi | first1=M. G. | last2=Lico | first2=R. | last3=Giroletti | first3=M. | last4=Orienti | first4=M. | last5=Giovannini | first5=G. | last6=Cotton | first6=W. | last7=Edwards | first7=P. G. | last8=Fuhrmann | first8=L. | last9=Krichbaum | first9=T. P. | last10=Kovalev | first10=Y. Y. | last11=Jorstad | first11=S. | last12=Marscher | first12=A. | last13=Kino | first13=M. | last14=Paneque | first14=D. | last15=Perez-Torres | first15=M. A. | last16=Piner | first16=B. G. | last17=Sokolovsky | first17=K. V. | journal=Astronomy & Astrophysics | volume=559 | at=id. A75 | date=November 2013 | doi=10.1051/0004-6361/201321858 | arxiv=1310.4973 | bibcode=2013A&A...559A..75B }}</ref> with a spiral galaxy companion to the upper left of center<ref>{{cite journal | title=Markarian 421'S Unusual Satellite Galaxy | last1=Gorham | first1=Peter W. | last2=van Zee | first2=Liese | last3=Unwin | first3=Stephen C. | last4=Jacobs | first4=Christopher | journal=The Astronomical Journal | volume=119 | issue=4 | pages=1677–1686 | date=April 2000 | doi=10.1086/301289 | arxiv=astro-ph/9908077 | bibcode=2000AJ....119.1677G }}</ref>]]
Blazars, like all active galactic nuclei (AGN), are thought to be powered by material falling into a [[supermassive black hole]] in the [[Galactic Core|core]] of the host galaxy. Gas, dust and the occasional star are captured and spiral into this central black hole, creating a hot [[accretion disc|accretion disk]]<ref>{{cite conference | title=Exploring the AGN Accretion Disks Using Continuum Reverberation Mapping | last1=Jha | first1=Vivek Kumar | last2=Joshi | first2=Ravi | last3=Saraswat | first3=Jayesh | last4=Chand | first4=Hum | last5=Barway | first5=Sudhanshu | last6=Mandal | first6=Amit Kumar | conference=Proceedings of the 3rd BINA Workshop on the Scientific Potential of the Indo-Belgian Cooperation | journal=Bulletin de la Société Royale des Sciences de Liège | volume=93 | issue=2 | pages=766–779 | date=June 2024 | doi=10.25518/0037-9565.11871 | arxiv=2307.16568 | bibcode=2024BSRSL..93..766J }}</ref> which generates enormous amounts of energy in the form of [[photon]]s, [[electron]]s, [[positron]]s and other [[elementary particle]]s.<ref>{{cite journal | title=Effects of High-energy Particles on Accretion Flows onto a Supermassive Black Hole | last1=Kimura | first1=Shigeo S. | last2=Toma | first2=Kenji | last3=Takahara | first3=Fumio | journal=The Astrophysical Journal | volume=791 | issue=2 | at=id. 100 | date=August 2014 | doi=10.1088/0004-637X/791/2/100 | arxiv=1407.0115 | bibcode=2014ApJ...791..100K }}</ref> This region is relatively small, approximately 10<sup>−3</sup> [[parsec]]s in size.<ref name=Beckmann_Shrader_2013>{{cite book | title=Active Galactic Nuclei | first1=Volker | last1=Beckmann | first2=Chris | last2=Shrader | publisher=John Wiley & Sons | year=2013 | isbn=978-3-527-66680-5 | pages=75–76 | url=https://books.google.com/books?id=0WOoAAAAQBAJ&pg=PA76 }}</ref>

There is a larger opaque [[toroid]] extending several parsecs from the black hole,<ref name=Beckmann_Shrader_2013/> containing a hot gas with embedded regions of higher density.<ref>{{cite journal | title=Forming Circumnuclear Disks and Rings in Galactic Nuclei: A Competition Between Supermassive Black Hole and Nuclear Star Cluster | last1=Trani | first1=Alessandro A. | last2=Mapelli | first2=Michela | last3=Ballone | first3=Alessandro | journal=The Astrophysical Journal | volume=864 | issue=1 | at=id. 17 | date=September 2018 | doi=10.3847/1538-4357/aad414 | doi-access=free | arxiv=1807.09780 | bibcode=2018ApJ...864...17T }}</ref> These "clouds" can absorb and re-emit energy from regions closer to the black hole. On Earth, the clouds are detected as [[spectral line|emission lines]] in the blazar [[electromagnetic spectrum|spectrum]].<ref>{{cite journal | title=Electron-beam interaction with emission-line clouds in blazars | last1=Wendel | first1=Christoph | last2=Becerra González | first2=Josefa | last3=Paneque | first3=David | last4=Mannheim | first4=Karl | journal=Astronomy & Astrophysics | volume=646 | at=id. A115 | date=February 2021 | doi=10.1051/0004-6361/202038343 | arxiv=2012.05215 | bibcode=2021A&A...646A.115W }}</ref>

Perpendicular to the accretion disk, a pair of [[relativistic jet]]s carries highly energetic [[Plasma (physics)|plasma]] away from the AGN. The jet is [[Collimated beam|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.<ref name=Oukacha_Becherini_2025>{{cite journal | title=Towards a unified scheme of blazar evolution | last1=Oukacha | first1=E. | last2=Becherini | first2=Y. | journal=Astronomy & Astrophysics | volume=700 | at=id. A260 | date=August 2025 | doi=10.1051/0004-6361/202555303 | arxiv=2507.03088 | bibcode=2025A&A...700A.260O }}</ref> These relativistic jets can extend as far as many tens of [[parsec|kiloparsecs]] from the central black hole.<ref name=Beckmann_Shrader_2013/>

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 (waves)|polarization]] (typically a few percent) at some frequencies. The nonthermal spectrum consists of [[synchrotron radiation]] in the radio to X-ray range, and [[Compton scattering|inverse Compton emission]] in the X-ray to gamma-ray region.<ref name=Oukacha_Becherini_2025/> A thermal spectrum peaking in the ultraviolet region and faint optical emission lines are also present in FSRQ, but faint or non-existent in BL Lac objects.<ref>{{cite journal | title=Spectral Energy Distribution variation in BL Lacs and FSRQs | last1=Bindu | first1=Rani | last2=Alok C. | first2=Gupta | last3=R. | first3=Bachev | last4=A. | first4=Strigachev | last5=E. | first5=Semkov | last6=F. | first6=D’Ammando | last7=P. J. | first7=Wiita | last8=M. A. | first8=Gurwell | last9=E. | first9=Ovcharov | last10=B. | first10=Mihov | last11=S. | first11=Boeva | last12=S. | first12=Peneva | journal=Monthly Notices of the Royal Astronomical Society | volume=417 | issue=3 | pages=1881–1890 | date=November 2011 | doi=10.1111/j.1365-2966.2011.19373.x | doi-access=free | arxiv=1107.0597 | bibcode=2011MNRAS.417.1881R }}</ref>

==Relativistic beaming==
{{main|Relativistic beaming}}
[[File:Relativistic Beaming.png|right|thumb|upright=1.3|Light from a relativistic source becomes more directed and blue-shifted in the direction of motion with increasing velocity ''v''. &beta;&nbsp;=&nbsp;v/c]]
The observed emission from a blazar is greatly enhanced by [[special relativity|relativistic effects]] in the jet, a process called relativistic beaming.<ref>{{cite book | chapter=Variability of Nonthermal Continuum Emission in Blazars | display-authors=1 | first1=Alan P. | last1=Marscher | first2=Walter K. | last2=Gear | first3=John P. | last3=Travis | title=Variability of Blazars | editor1-first=Esko | editor1-last=Valtoja | editor2-first=Mauri | editor2-last=Valtonen | publisher=Cambridge University Press | year=1992 | isbn=9780521413510 | page=86 | chapter-url=https://books.google.com/books?id=UNLNKJraXgYC&pg=PA86 }}</ref> 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.<ref>{{cite web | title=Attack of the Blobs | first=Mitch | last=Begelman | date=July 9, 2009 | publisher=JILA Physics Frontier Center | url=https://jila-pfc.colorado.edu/news-events/articles/attack-blobs | access-date=2025-08-13 }}</ref>

Relativistic jets emit most of their energy via [[synchrotron emission]]. The luminosity emitted in the rest frame of the jet depends on the physical characteristics of the jet. These include whether the luminosity arises from a shock front<ref>{{cite journal | title=Turbulent, Extreme Multi-zone Model for Simulating Flux and Polarization Variability in Blazars | last=Marscher | first=Alan P. | journal=The Astrophysical Journal | volume=780 | issue=1 | at=id. 87 | date=January 2014 | doi=10.1088/0004-637X/780/1/87 | arxiv=1311.7665 | bibcode=2014ApJ...780...87M }}</ref> or a series of brighter blobs in the jet,<ref>{{cite journal | title=A multizone view on the multiwavelength emission of blazars | last1=Liu | first1=Ruo-Yu | last2=Xue | first2=Rui | last3=Wang | first3=Ze-Rui | last4=Tan | first4=Hong-Bin | last5=Böttcher | first5=Markus | journal=Monthly Notices of the Royal Astronomical Society | volume=526 | issue=4 | pages=5054–5071 | date=December 2023 | doi=10.1093/mnras/stad2911 | doi-access=free | arxiv=2309.12171 | bibcode=2023MNRAS.526.5054L }}</ref> as well as details of the magnetic fields within the jet and their interaction with the moving particles.<ref>{{cite journal | title=Magnetization of jets in luminous blazars | last1=Janiak | first1=M. | last2=Sikora | first2=M. | last3=Moderski | first3=R. | journal=Monthly Notices of the Royal Astronomical Society | volume=449 | issue=1 | pages=431–439 | date=May 2015 | doi=10.1093/mnras/stv200 | doi-access=free | arxiv=1411.7331 | bibcode=2015MNRAS.449..431J }}</ref>

A simple model of [[relativistic beaming|beaming]] illustrates the basic relativistic effects connecting the luminosity in the rest frame of the jet, ''S''<sub>e</sub>, and the luminosity observed on Earth, ''S''<sub>o</sub>:  ''S''<sub>o</sub> is proportional to ''S''<sub>e</sub>&nbsp;×&nbsp;''D''<sup>2</sup>, where ''D is'' the [[relativistic beaming|doppler factor]].<ref>{{cite book | chapter=Interpretation of large scale extragalactic jets | first=J. P. | last=Leahy | title=Beams and Jets in Astrophysics | volume=19 | series=Cambridge Astrophysics | editor-first=Philip A. | editor-last=Hughes | publisher=Cambridge University Press | year=1991 | isbn=978-0-521-33576-8 | pages=112–113 | chapter-url=https://books.google.com/books?id=dPKru7hptR4C&pg=PA112 }}</ref>

When considered in much more detail, three relativistic effects are involved:

* Relativistic aberration contributes a factor of ''D''<sup>2</sup>. 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, Earth).
* Time dilation contributes a factor of ''D''<sup>+1</sup>. 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 release would be observed on Earth as much more frequent, perhaps every ten seconds.
* Windowing can contribute a factor of ''D''<sup>−1</sup> and then works to decrease 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''<sup>+3</sup>.

For example, consider a jet with an angle to the line of sight θ&nbsp;=&nbsp;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.{{cn|date=November 2025}}

===Receding beam===
Relativistic beaming has another critical consequence. A counter-jet that is receding from Earth will appear dimmer because of the same relativistic effects. Therefore, intrinsically identical bipolar jets will appear significantly asymmetric.<ref name=Liodakis_2017>{{cite journal | title=Detecting the elusive blazar counter-jets | display-authors=1 | last1=Liodakis | first1=I. | last2=Pavlidou | first2=V. | last3=Angelakis | first3=E. | journal=Monthly Notices of the Royal Astronomical Society | volume=465 | issue=1 | pages=180–191 | date=February 2017 | doi=10.1093/mnras/stw2723 | doi-access=free | arxiv=1610.06561 | bibcode=2017MNRAS.465..180L }}</ref> In the example given above any jet where θ&nbsp;>&nbsp;35° will be observed on Earth as less luminous than it would be from the rest frame of the jet.{{cn|date=November 2025}}

A further consequence is that, due to "Doppler favouritism",<ref name=Liodakis_2017/> 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.<ref name=Dick_2013>{{cite book | title=Discovery and Classification in Astronomy: Controversy and Consensus | first=Steven J. | last=Dick | publisher=Cambridge University Press | year=2013 | isbn=978-1-107-03361-0 | pages=147–148 | url=https://books.google.com/books?id=IT8oAAAAQBAJ&pg=PA148 }}</ref>

The early development of [[radio astronomy]] had shown that there are many bright radio sources in the sky. By the end of the 1950s, the [[Angular resolution|resolution]] of [[radio telescope]]s was sufficient to identify specific radio sources with optical counterparts, leading to the discovery of [[quasar]]s. Blazars were highly represented among these early quasars, and the first redshift was found for [[3C 273]],<ref>{{cite book | title=Quasars and Active Galactic Nuclei: An Introduction | first1=Ajit K. | last1=Kembhavi | first2=Jayant V. | last2=Narlikar | publisher=Cambridge University Press | year=1999 | isbn=978-0-521-47989-9 | pages=2–5 | url=https://books.google.com/books?id=vO7KDjwzUK4C&pg=PA3 }}</ref> a highly variable quasar which is also a blazar.<ref>{{cite journal | title=Constraining X-Ray Variability of the Blazar 3C 273 Using XMM-Newton Observations over Two Decades | last1=Dinesh | first1=Adithiya | last2=Bhatta | first2=Gopal search by orcid | last3=Adhikari | first3=Tek P. | last4=Mohorian | first4=Maksym | last5=Dhital | first5=Niraj | last6=Chaudhary | first6=Suvas C. | last7=Pánis | first7=Radim | last8=Góra | first8=Dariusz | journal=The Astrophysical Journal | volume=955 | issue=2 | at=id. 121 | date=October 2023 | doi=10.3847/1538-4357/acf316 |doi-access=free | arxiv=2309.00406 | bibcode=2023ApJ...955..121D }}</ref>

In 1968, a similar connection was made between the "variable star" [[BL Lacertae]] and a powerful radio source VRO 42.22.01.<ref>{{Cite journal |last=Schmitt |first=John L. |date=May 1968 |title=BL Lac identified as a Radio Source |journal=Nature |language=en |volume=218 |issue=5142 |pages=663 |doi=10.1038/218663a0 |bibcode=1968Natur.218..663S |issn=0028-0836}}</ref> BL Lacertae shows many of the characteristics of quasars, but the optical [[electromagnetic spectrum|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.<ref>{{cite journal | last1=Oke | first1=J. B. | last2=Gunn | first2=J. E. | year=1974 | page=L5 | title=The Distance of BL Lacertae | journal=Astrophysical Journal Letters | volume=189 | bibcode=1974ApJ...189L...5O | doi=10.1086/181450 | doi-access=free }}</ref>

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 Lac object|BL Lacertae-type objects]].<ref>{{cite journal | first1=W. A. | last1=Stein | last2=O'Dell | first2=S. L. | last3=Strittmatter | first3=P. A. | date=September 1976 | title=The BL Lacertae Objects | journal=Annual Review of Astronomy and Astrophysics | volume=14 | pages=173–195 | bibcode=1976ARA&A..14..173S | doi=10.1146/annurev.aa.14.090176.001133 | url=http://james.as.arizona.edu/~psmith/Blazars/stein.pdf | archive-date=2018-05-18 | access-date=2011-12-09 | archive-url=https://web.archive.org/web/20180518030043/http://james.as.arizona.edu/~psmith/Blazars/stein.pdf | url-status=dead }}</ref><ref name=Dick_2013/> 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|2015}}, over three thousand sources have been confirmed as BL Lac objects, or exhibit similar characteristics.<ref>{{cite journal | title=The 5th edition of the Roma-BZCAT. A short presentation | last1=Massaro | first1=E. | last2=Maselli | first2=A. | last3=Leto | first3=C. | last4=Marchegiani | first4=P. | last5=Perri | first5=M. | last6=Giommi | first6=P. | last7=Piranomonte | first7=S. | journal=Astrophysics and Space Science | volume=357 | issue=1 | at=id. 75 | date=May 2015 | doi=10.1007/s10509-015-2254-2 | arxiv=1502.07755 | bibcode=2015Ap&SS.357...75M }}</ref> One of the closest blazars, 3C 273, is 2.5 billion light years away.<ref>{{Cite web |last=Bichell |first=Rae Ellen |date=4 January 2017 |title=Some Bizarre Black Holes Put On Light Shows |url=https://www.npr.org/2017/01/03/507594456/some-bizarre-black-holes-put-on-light-shows |access-date=2020-07-12 |website=[[NPR]] |language=en}}</ref><ref>{{Cite journal |last1=Uchiyama |first1=Yasunobu |last2=Urry |first2=C. Megan |last3=Cheung |first3=C. C. |last4=Jester |first4=Sebastian |last5=Van Duyne |first5=Jeffrey |last6=Coppi |first6=Paolo |last7=Sambruna |first7=Rita M. |last8=Takahashi |first8=Tadayuki |last9=Tavecchio |first9=Fabrizio |last10=Maraschi |first10=Laura |date=10 September 2006 |title=Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope |journal=The Astrophysical Journal |language=en |volume=648 |issue=2 |pages=910–921 |doi=10.1086/505964 |issn=0004-637X|arxiv=astro-ph/0605530 |bibcode=2006ApJ...648..910U }}</ref> The nearest BL Lac object is [[Centaurus A]].<ref>{{cite journal | bibcode=2001MNRAS.324L..33C | title=The BL Lac heart of Centaurus A | last1=Chiaberge | first1=M. | last2=Capetti | first2=A. | last3=Celotti | first3=A. | journal=Monthly Notices of the Royal Astronomical Society | date=2001 | volume=324 | issue=4 | pages=L33–L37 | doi=10.1046/j.1365-8711.2001.04642.x | doi-access=free | arxiv=astro-ph/0105159 }}</ref>

==Current view==
[[File:Quasar with jet and magnetic lines.jpg|right|thumb|Illustration of a prototypical quasar showing the supermassive black hole at center with its accretion disk and magnetically-confined, bipolar jets<ref>{{cite web | title=The Recipe for Powerful Quasar Jets | publisher=NASA | url=https://www.nasa.gov/image-article/recipe-powerful-quasar-jets/ | access-date=2025-11-13 }}</ref>]]
Blazars are thought to be [[active galaxy|active galactic nuclei]], with relativistic jets oriented close to the line of sight with the observer. They are sub-divided into BL Lac objects and flat-spectrum radio quasars (FSRQ), with the former having weak or no emission lines and the latter showing strong emission lines.<ref name=Chen_et_al_2015>{{cite journal | title=Black Hole Mass, Jet Power, and Accretion in AGNs | last1=Chen | first1=Yong-Yun | last2=Zhang | first2=Xiong | last3=Xiong | first3=Dingrong | last4=Yu | first4=Xiaoling | journal=The Astronomical Journal | volume=150 | issue=1 | at=id. 8 | date=July 2015 | doi=10.1088/0004-6256/150/1/8 | arxiv=1504.05413 | bibcode=2015AJ....150....8C }}</ref> The FSRQ are alternatively defined as optically violently variable (OVV) quasars, highly polarized quasars (HPQ), or core-dominated quasars (CDQ).<ref>{{cite journal | title=Are the X-ray spectra of flat-spectrum radio quasars and BL Lacertae objects different? | first1=Paolo | last1=Padovani | first2=Paolo | last2=Giommi | first3=Fabrizio | last3=Fiore | journal=Monthly Notices of the Royal Astronomical Society | volume=284 | issue=3 | date=January 1997 | pages=569–575 | doi=10.1093/mnras/284.3.569 | arxiv=astro-ph/9610093 |doi-access=free | bibcode=1997MNRAS.284..569P }}</ref> The term FSRQ comes from the distinction between steep spectrum and flat spectrum radio-loud quasars, based on the overall shape of their radio continuum (after disregarding emission features).<ref>{{cite book | chapter=Observed properties and empirical classification of AGN | title=Unified schemes for radio-loud Active Galactic Nuclei | first1=C. Megan | last1=Urry | first2=Paolo | last2=Padovani | series=Publications of the Astronomical Society of the Pacific | volume=107 | pages=803–845 | date=September 1995 | doi=10.1086/133630 | arxiv=astro-ph/9506063 | chapter-url=https://ned.ipac.caltech.edu/level5/Urry1/UrryP2.html | access-date=2025-11-16 }}</ref>

The special jet orientation explains the general peculiar characteristics: high observed luminosity, very rapid variation, high polarization (compared to non-blazar quasars), and the apparent [[superluminal jet|superluminal motions]] detected along the first few parsecs of the jets in most blazars.<ref>{{cite book | chapter=Quasar Classes and their Relationships, § 4.1 BL Lacs and Blazars | first=Deborah | last=Dultzin | title=Fifty Years of Quasars: From Early Observations and Ideas to Future Research | volume=386 | series=Astrophysics and Space Science Library | editor1-first=Mauro | editor1-last=D'Onofrio | editor2-first=Paola | editor2-last=Marziani | editor3-first=Jack W. | editor3-last=Sulentic | publisher=Springer Science & Business Media | year=2012 | isbn=978-3-642-27563-0 | page=220 | chapter-url=https://books.google.com/books?id=J1n31K0WfQMC&pg=PA220 }}</ref>

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.<ref>{{Cite web |last=Reddy |first=Francis |date=3 June 2014 |title=Black Hole 'Batteries' Keep Blazars Going and Going |url=http://www.nasa.gov/content/goddard/black-hole-batteries-keep-blazars-going-and-going |access-date=2015-05-31 |website=[[NASA]]}}</ref> The distinction between these two connected populations explains the difference in emission line properties in blazars.<ref>{{Cite journal |last1=Ajello |first1=M. |last2=Romani |first2=R. W. |last3=Gasparrini |first3=D. |last4=Shaw |first4=M. S. |last5=Bolmer |first5=J. |last6=Cotter |first6=G. |last7=Finke |first7=J. |last8=Greiner |first8=J. |last9=Healey |first9=S. E. |last10=King |first10=O. |last11=Max-Moerbeck |first11=W. |last12=Michelson |first12=P. F. |last13=Potter |first13=W. J. |last14=Rau |first14=A. |last15=Readhead |first15=A. C. S. |date=2013-12-13 |title=The Cosmic Evolution of Fermi BL Lacertae Objects |journal=The Astrophysical Journal |volume=780 |issue=1 |pages=73 |doi=10.1088/0004-637X/780/1/73 |issn=0004-637X|arxiv=1310.0006 }}</ref>

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 include [[3C 454.3]], [[3C 273]], [[BL Lacertae]], [[PKS 2155-304]], [[Markarian 421]], [[Markarian 501]], [[4C +71.07]], [[PKS 0537-286]] (QSO 0537-286),<ref>{{cite journal | title=Roma-BZCAT: a multifrequency catalogue of blazars | display-authors=1 | last1=Massaro | first1=E. | last2=Giommi | first2=P. | last3=Leto | first3=C. | last4=Marchegiani | first4=P. | last5=Maselli | first5=A. | last6=Perri | first6=M. | last7=Piranomonte | first7=S. | last8=Sclavi | first8=S. | journal=Astronomy and Astrophysics | volume=495 | issue=2  | pages=691–696 | date=February 2009 | doi=10.1051/0004-6361:200810161 | arxiv=0810.2206 | bibcode=2009A&A...495..691M }}</ref> and [[S5 0014+81]].<ref>{{cite journal | title=The blazar S5 0014+813: a real or apparent monster? | display-authors=1 | last1=Ghisellini | first1=G. | last2=Foschini | first2=L. | last3=Volonteri | first3=M. | last4=Ghirlanda | first4=G. | last5=Haardt | first5=F. | last6=Burlon | first6=D. | last7=Tavecchio | first7=F. | journal=Monthly Notices of the Royal Astronomical Society: Letters | volume=399 | issue=1 | pages=L24–L28 | date=October 2009 | doi=10.1111/j.1745-3933.2009.00716.x | doi-access=free | arxiv=0906.0575 | bibcode=2009MNRAS.399L..24G }}</ref> Markarian 501 and [[1ES 1959+650]] are called "TeV Blazars" for their high energy (teraelectron-volt range) gamma-ray emission.<ref>{{cite journal | title=X-Ray Spectral Variability of 13 TeV High-energy-peaked Blazars with XMM-Newton | display-authors=1 | last1=Devanand | first1=P. U. | last2=Gupta | first2=Alok C. | last3=Jithesh | first3=V. | last4=Wiita | first4=Paul J. | last5=Gupta | first5=Archana | journal=The Astrophysical Journal Supplement Series | volume=278 | issue=1 | at=id. 20 | date=May 2025 | doi=10.3847/1538-4365/adc10d |doi-access=free | arxiv=2503.08386 | bibcode=2025ApJS..278...20D }}</ref>

In July 2018, the [[IceCube Neutrino Observatory]] team traced a [[neutrino]] that hit its [[Antarctica]]-based detector in September 2017 to its point of origin in the blazar [[TXS 0506+056]], located 3.7 billion [[light-year]]s away.<ref>{{Cite web |title=SIMBAD query result |url=https://simbad.u-strasbg.fr/simbad/sim-id?Ident=TXS+0506%2B056 |access-date=2018-07-13 |website=SIMBAD Astronomical Database}}</ref> This was the first time that a [[neutrino detector]] was used to locate an object in space.<ref name="NYT-2018">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=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. |url=https://www.nytimes.com/2018/07/12/science/space-neutrinos-blazar.html |date=12 July 2018 |work=[[The New York Times]] |access-date=13 July 2018 }}</ref><ref name="Guardian-2018">{{cite web |last=Sample |first=Ian |date=12 July 2018 |title=Neutrino that struck Antarctica traced to galaxy 3.7bn light years away |url=https://www.theguardian.com/science/2018/jul/12/neutrino-that-struck-antarctica-traced-to-galaxy-37bn-light-years-away |access-date=12 July 2018 |work=[[The Guardian]]}}</ref><ref name="BBC-2018">{{cite web |last=Halton |first=Mary |date=12 July 2018 |title=Source of cosmic 'ghost' particle revealed |url=https://www.bbc.co.uk/news/science-environment-44786125 |access-date=12 July 2018 |work=[[BBC]]}}</ref><ref name="BBC-2018" /><ref>{{Cite web |date=12 July 2018 |title=IceCube Neutrinos Point to Long-Sought Cosmic Ray Accelerator |url=https://icecube.wisc.edu/news/view/586 |access-date=2018-07-13 |website=IceCube Neutrino Observatory |language=en}}</ref>

==See also==
* [[Galaxy formation and evolution]]
* [[Seyfert galaxy]]

==Notes==
{{Reflist}}

==External links==
{{commons category|Blazars}}
* [https://www.aavso.org/blazars AAVSO High Energy Network]
* [http://www.physics.purdue.edu/astro/MOJAVE/blazarlist.html Blazar Monitoring List, Purdue University] {{Webarchive|url=https://web.archive.org/web/20201027174220/http://www.physics.purdue.edu/astro/MOJAVE/blazarlist.html |date=2020-10-27 }}
* [http://www.perseus.gr/Astro-DSO-Quasars-Blazars.htm Expanding Gallery of Hires Blazar Images]
* [https://www.youtube.com/watch?v=DK9TMnC7n8E NASA:Blazars Artist Conception Video]
* [http://www.nasa.gov/mission_pages/GLAST/news/cosmic-fog.html NASA Cosmic Fog]
* [http://www.nasa.gov/mission_pages/GLAST/news/gamma-ray-census.html NASA Gamma Ray Census]
* [https://www.youtube.com/watch?t=106&v=1J-LQqs_rYk&ab_channel=NASA.gov Video May 13 2013, NASA's Fermi Shows How Active Galaxies Can Be-Blazars]
* [https://www.ted.com/talks/jedidah_isler_how_i_fell_in_love_with_quasars_blazars_and_our_incredible_universe TED talk on blazars by Jedidah Isler]
* [https://www.sciencealert.com/astronomers-just-found-a-radio-galaxy-that-turned-into-a-blazar Astronomers Just Found a Radio Galaxy That Turned Into a Blazar]

{{black holes}}
{{Portal bar|Astronomy|Stars|Outer space}}

[[Category:Blazars| ]]
[[Category:Active galaxy types]]