Blazar
Encyclopedia
A blazar is a very compact quasar
(quasi-stellar object) associated with a presumed supermassive black hole
at the center of an active, giant elliptical galaxy. Blazars are among the most energetic phenomena in the universe and are an important topic in extragalactic astronomy
.
Blazars are members of a larger group of active galaxies that host active galactic nuclei (AGN). A few rare objects may be "intermediate blazars" that appear to have a mixture of properties from both optically violent variable (OVV) quasar
s and BL Lac object
s. The name "blazar" was originally coined in 1978 by astronomer Edward Spiegel
to denote the combination of these two classes.
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.
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
.
Alternative models, for example, gravitational microlensing
, may account for a few observations of some blazars which are not consistent with the general properties.
Blazars, like all AGN, 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 photon
s, electron
s, positron
s and other elementary particles. This region is quite small, approximately 10−3 parsec
s in size.
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 jet
s 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.
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 at involved:
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.
s 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 telescope
s was sufficient to be able to identify specific radio sources with optical counterparts, leading to the discovery of quasar
s.
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.
In 1968 a similar connection between the "variable star" BL Lacertae
and a powerful radio source VRO 42.22.01 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.
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. 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 which 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 of blazars include 3C 454.3
, 3C 273, BL Lacertae
, PKS 2155-304
, Markarian 421
, and Markarian 501. The latter two are also called "TeV Blazars" for their high energy (teraelectron-volt range) gamma-ray emission.
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
(quasi-stellar object) associated with a presumed supermassive black hole
Supermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...
at the center of an active, giant elliptical galaxy. Blazars are among the most energetic phenomena in the universe and are an important topic in extragalactic astronomy
Extragalactic astronomy
Extragalactic astronomy is the branch of astronomy concerned with objects outside our own Milky Way Galaxy. In other words, it is the study of all astronomical objects which are not covered by galactic astronomy, the next level of galactic astronomy....
.
Blazars are members of a larger group of active galaxies that host active galactic nuclei (AGN). A few rare objects may be "intermediate blazars" that appear to have a mixture of properties from both optically violent variable (OVV) quasar
OVV quasar
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. They are similar in appearance to BL Lacs but generally have a stronger broad emission...
s and BL Lac object
BL Lac object
A BL Lacertae object or BL Lac object is a type of active galaxy with an active galactic nucleus and is 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...
s. The name "blazar" was originally coined in 1978 by astronomer Edward Spiegel
Edward Spiegel
Edward A. Spiegel is the Rutherfurd Professor of Astronomy at Columbia University and highly regarded for his work on convection theory and on the application of fluid dynamics to astrophysics.-Career:...
to denote the combination of these two classes.
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
Superluminal motion
In astronomy, superluminal motion is the apparently faster-than-light motion seen in someradio galaxies, quasars and recently also in some galactic sources called microquasars...
within the first few parsecs of their jets, probably due to relativistic shock fronts.
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
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
.
Alternative models, for example, gravitational microlensing
Gravitational lens
A gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...
, may account for a few observations of some blazars which are not consistent with the general properties.
Structure
Supermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...
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 photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s, electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
s and other elementary particles. This region is quite small, approximately 10−3 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s in size.
There is also a larger opaque toroid
Toroid
Toroid may refer to*Toroid , a doughnut-like solid whose surface is a torus.*Toroidal inductors and transformers which have wire windings on circular ring shaped magnetic cores.*Vortex ring, a toroidal flow in fluid mechanics....
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
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
in the blazar spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
.
Perpendicular to the accretion disk
Accretion disc
An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...
, a pair of relativistic jet
Relativistic jet
Relativistic jets are extremely powerful jets of plasma which emerge from presumed massive objects at the centers of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years...
s carries a highly energetic plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
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
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
from the central black hole.
All of these regions can produce a variety of observed energy, mostly in the form of a nonthermal spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
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
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
in the radio to X-ray range, and inverse Compton emission
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
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
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
in the jet, a process termed relativistic beaming
Relativistic beaming
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...
.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
Relativistic beaming
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...
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
Relativistic beaming
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...
, D, where So is proportional to Se × D2.
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° and a speed of 99.9% 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° 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° 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.
Discovery
Many of the brighter blazars were first identified, not as powerful distant galaxies, but as irregular variable starIrregular variable
An irregular variable is a type of variable star in which variations in brightness show no regular periodicity. There are two main sub-types of irregular variable: eruptive and pulsating.Eruptive irregular variables are divided into three categories:...
s 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
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
had shown that there are numerous bright radio sources in the sky. By the end of the 1950s the resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
of radio telescope
Radio telescope
A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...
s was sufficient to be able to identify specific radio sources with optical counterparts, leading to the discovery of quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s.
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.
In 1968 a similar connection between the "variable star" BL Lacertae
BL Lacertae
BL Lacertae or BL Lac is a highly variable, extragalactic AGN . It was first discovered by Cuno Hoffmeister in 1929, but was originally thought to be an irregular variable star in the Milky Way galaxy and so was given a variable star designation...
and a powerful radio source VRO 42.22.01 was made. BL Lacertae shows many of the characteristics of quasars, but the optical spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
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
BL Lac object
A BL Lacertae object or BL Lac object is a type of active galaxy with an active galactic nucleus and is 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...
. 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 vision
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. 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 which 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 of blazars include 3C 454.3
3C 454.3
3C 454.3 is a quasar/blazar located off the galactic plane. It lies some 7.1 billion light-years away in Pegasus and is currently undergoing a flaring episode that makes it especially bright, especially in the gamma-ray part of the spectrum.- History :...
, 3C 273, BL Lacertae
BL Lacertae
BL Lacertae or BL Lac is a highly variable, extragalactic AGN . It was first discovered by Cuno Hoffmeister in 1929, but was originally thought to be an irregular variable star in the Milky Way galaxy and so was given a variable star designation...
, PKS 2155-304
PKS 2155-304
PKS 2155-304 is one of the brightest active galaxies in the sky.It is a strong emitter from radio to high energy frequencies.PKS 2155-305 is at redshift z = 0.116 and it is one of the brightest and most studied BL Lacs and is often considered the prototype of X-ray selected BL Lacs....
, Markarian 421
Markarian 421
Markarian 421 is a blazar located in the constellation Ursa Major. The object is an active galaxy and a BL Lacertae object, and is a strong source of gamma rays. It is about 397 million light-years to 434 million light-years from the Earth...
, and Markarian 501. The latter two are also called "TeV Blazars" for their high energy (teraelectron-volt range) gamma-ray emission.
External links
- AAVSO High Energy Network
- Expanding Gallery of Hires Blazar Images (with animation)