Theelliptical galaxy M87 emitting a relativistic jet, as seen by theHubble 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.[1]
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.[2] 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 is sub-divided intoBL Lac objects andflat-spectrum radio quasars (FSRQ), with the former having weak or no emission lines and the latter showing strong emission lines.[3] The generally accepted theory is that BL Lac objects are intrinsically low-powerradio galaxies while FSRQ quasars are intrinsically powerful radio-loudquasars. The name "blazar" was coined in 1978 by astronomerEdward Spiegel to denote the combination of these two classes.[4] In visible-wavelength images, most blazars appear compact and pointlike, but high-resolution images reveal that they are located at the centers ofelliptical galaxies.[5]
Sloan Digital Sky Survey image of blazarMarkarian 421 (center), illustrating the bright nucleus and elliptical host galaxy,[6] with a spiral galaxy companion to the upper left of center[7]
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[8] which generates enormous amounts of energy in the form ofphotons,electrons,positrons and otherelementary particles.[9] This region is relatively small, approximately 10−3parsecs in size.[10]
There is a larger opaquetoroid extending several parsecs from the black hole,[10] containing a hot gas with embedded regions of higher density.[11] 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.[12]
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.[13] These relativistic jets can extend as far as many tens ofkiloparsecs from the central black hole.[10]
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.[13] 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.[14]
Light from a relativistic source becomes more directed and blue-shifted in the direction of motion with increasing velocityv. β = v/c
The observed emission from a blazar is greatly enhanced byrelativistic effects in the jet, a process called relativistic beaming.[15] 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.[16]
Relativistic jets emit most of their energy viasynchrotron 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[17] or a series of brighter blobs in the jet,[18] as well as details of the magnetic fields within the jet and their interaction with the moving particles.[19]
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.[20]
When considered in much more detail, three relativistic effects are involved:
Relativistic aberration contributes a factor ofD2. 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 ofD+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 release would be observed on Earth as much more frequent, perhaps every ten seconds.
Windowing can contribute a factor ofD−1 and then works to decrease boosting. This happens for a steady flow because there are thenD fewer elements of fluid within the observed window, as each element has been expanded by factorD. However, for a freely propagating blob of material, the radiation is boosted by the fullD+3.
For 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. 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.[citation needed]
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.[21] 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.[citation needed]
A further consequence is that, due to "Doppler favouritism",[21] 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.[22]
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,[23] a highly variable quasar which is also a blazar.[24]
In 1968, a similar connection was made between the "variable star"BL Lacertae and a powerful radio source VRO 42.22.01.[25] 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.[26]
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.[27][22] 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[update], over three thousand sources have been confirmed as BL Lac objects, or exhibit similar characteristics.[28] One of the closest blazars, 3C 273, is 2.5 billion light years away.[29][30] The nearest BL Lac object isCentaurus A.[31]
Illustration of a prototypical quasar showing the supermassive black hole at center with its accretion disk and magnetically-confined, bipolar jets[32]
Blazars are thought to beactive 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.[3] The FSRQ are alternatively defined as optically violently variable (OVV) quasars, highly polarized quasars (HPQ), or core-dominated quasars (CDQ).[33] 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).[34]
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.[35]
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.[36] The distinction between these two connected populations explains the difference in emission line properties in blazars.[37]
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.
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^Blasi, M. G.; Lico, R.; Giroletti, M.; Orienti, M.; Giovannini, G.; Cotton, W.; Edwards, P. G.; Fuhrmann, L.; Krichbaum, T. P.; Kovalev, Y. Y.; Jorstad, S.; Marscher, A.; Kino, M.; Paneque, D.; Perez-Torres, M. A.; Piner, B. G.; Sokolovsky, K. V. (November 2013). "The TeV blazar Markarian 421 at the highest spatial resolution".Astronomy & Astrophysics.559. id. A75.arXiv:1310.4973.Bibcode:2013A&A...559A..75B.doi:10.1051/0004-6361/201321858.
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^Uchiyama, Yasunobu; Urry, C. Megan; Cheung, C. C.; Jester, Sebastian; Van Duyne, Jeffrey; Coppi, Paolo; Sambruna, Rita M.; Takahashi, Tadayuki; Tavecchio, Fabrizio; Maraschi, Laura (10 September 2006). "Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope".The Astrophysical Journal.648 (2):910–921.arXiv:astro-ph/0605530.Bibcode:2006ApJ...648..910U.doi:10.1086/505964.ISSN0004-637X.
^Ajello, M.; Romani, R. W.; Gasparrini, D.; Shaw, M. S.; Bolmer, J.; Cotter, G.; Finke, J.; Greiner, J.; Healey, S. E.; King, O.; Max-Moerbeck, W.; Michelson, P. F.; Potter, W. J.; Rau, A.; Readhead, A. C. S. (2013-12-13). "The Cosmic Evolution of Fermi BL Lacertae Objects".The Astrophysical Journal.780 (1): 73.arXiv:1310.0006.doi:10.1088/0004-637X/780/1/73.ISSN0004-637X.
