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| Fig. 1: (a) Dipole radiation pattern for non-relativistic electron. (b) Synchrotron radiation pattern for relativistic electron. Emission is beamed into a cone in the direction of the electron's velocity. |
Named from the conflation of quasi-stellar because they looked like stars,the discovery of quasars in the 1960s by Greenstein and Schmidt led to a watershed moment in astrophysics. [1] Extragalactic in origin and appearing to possess luminosities 1013 times greater than that of the sun, quasars presented astronomers with the problem of explaining the emission properties of these incredibly powerful and distant astronomical sources. [2] Continued observation led to the conlusion that supermassive black holes at the centers of other galaxies were responsible and this linked quasars to other types of what became known as active galactic nuclei (AGN), such as Seyfert and radio galaxies. However, astrophysicists have yet to explain the precise mechanisms for the many properties of the different AGN types.
This note will address a few of the emission properties of the Blazar subclass of AGN and currently accepted explanations for those properties. Blazars are named for their their extreme luminosity variability. They are differentiated from other AGN because they are believed to possess a relativistic jet of particles pointing in the direction of the Earth which leads to a continuum spectrum dominated by high-energy radiative processes like synchrotron and inverse compton. [2,3] Blazars are typically divided into two groups: flat spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). FSRQs generally have spectra with lower frequency peaks and larger bolometric luminosities than BL Lacs. BL Lacs are also characterized by very weak spectral lines, being almost completely dominated by continuum emission and making it very difficult to measure redshift, whereas FSRQs have normal strong line emission [4]. Whether these are phenomenological or inherent differences between these two types of blazars is yet a controversial subject, but most efforts to study any such distinction begin with the modelling of emission processes.
We begin with an explanation of the synchrotron emission process. In 1955, Iosef Shlovsky recognized that the radiation emitted by man-made electron accelerators called synchrotrons was the same as non-thermal radio and optical emission observed from astrophysical objects. [2] Synchrotron emission originates from the interaction of a relativistic electron with a magnetic field. While special relativity is important in this process, we begin with the classical description of a charged particle in a magnetic field. The force from the magnetic field B on an electron of charge e and velocity v is
This force causes the electron to rotate and the rotational force can also be written as
where r is the orbital radius and me is the electron mass. Assuming that v and B are perpendicular, r can be calculated by equating the forces. This gives
The classical frequency of this orbitting electron is then given by
For weak magnetic fields and nonrelativistic electrons, this would result in radio emission and would have a dipole pattern (Fig. 1a). For relativistic electrons, however, we must modify the classical framework somewhat. The emitted synchrotron frequency is then given by
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| Fig. 2: Blazar spectral energy distribution (SED) of the leptonic jet model. Four different components are represented: Synchrotron emission (Sy) in red, synchrotron self-compton (SSC) emission in blue, External Comptonization of Direct disk radiation (ECD) in green, and External Comptonization of radiation from the clouds (ECC) in yellow. |
where the factor γ is
The radiation is now beamed in a cone in the instantaneous direction of the electron's motion (Fig. 2b). While the electron sweeps the cone during a complete revolution, an observer sees a passing flash as the electron sweeps by. The observed frequency increases by γ2 because of the relativistic transformation of the electric field vector of the electron. Since the electron energy E is given by E=γmc2, the synchrotron frequency can be written as
Since we care about ensembles of electrons radiating, we can think of an energy spectrum expressed by the number of electrons per energy range, N(E)dE, which is given by N(E)dE α E-s, where s is the index of the electron power-law distribution. The energy lost by the electrons as they are accelerated in the magnetic field is converted to a continuum of photons with intensity I(ν)dν which also goes as a power-law.
where α is the spectral index of the radiation spectrum.
If synchrotron radiation comes from the interaction of a relativistic electron with a magnetic field, one can think of inverse compton radiation resulting from the interaction of a relativistic electron with a photon field. It is the electric analog to the synchrotron process. Inverse compton emission occurs when a relativistic electron scatters off a photon, with the electron donating some of its energy to the photon. Photons undergoing inverse compton scattering are boosted to higher energies as
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| Fig. 3: Leptonic jet model geometry for blazar emission. Supermassive black hole surrounded by an accretion disk. Electrons are bulk accelerated at relativistic speeds in the jet and emit photons through a variety of high-energy radiative processes, including synchrotron and inverse compton emission, which involve photon and gas fields surrounding the accretion disk. |
where ν0 is the photon's original frequency and ν is the boosted frequency. [2] For example, if the original photons had a frequency of 300 GHz and the electron's possessed gamma factors of 1000, the boosted photons have a frequency of 3x1017 Hz, in the keV energy range. Similarly, optical photons would be boosted to MeV energies, and x-ray photons would reach TeV energies. Inverse compton processes occur only in regions with relativistic electrons, a field of photons with which the electrons interact, and significant energy densities for the both photons and electrons. This explains why we only see inverse compton and synchrotron photons from highly energetic regions like active galactic nuclei.
The spectral energy distribution (SED) of blazar emission is made up of several components caused by synchrotron and inverse compton processes. Blazar SEDs generally have two peaks, a lower energy peak coming from synchrotron emission and a higher energy peak from inverse compton emission (Fig. 2). The synchrotron emission generally peaks at radio or infrared frequencies. The inverse compton emission peaks in the MeV to TeV energy range. Several different inverse compton processes contribute to the high-energy peak, each depending on the source of seed photons encountering the relativistic electrons in the jet. Fig. 3 shows blobs of electrons being accelerated away from the black hole at relativistic bulk velocities and emitting synchrotron photons (Sy). Relativistic electrons interact with these synchrotron photons via the inverse compton process producing the synchrotron self-compton (SSC) peak. Thermal photons from the accretion disk also interact with electrons in the jet to produce the External Comptonization of Direct disk radiation (ECD) peak. Photons also come from the clouds of gas in the broad line region (BLR) and interact with jet electrons to produce the External Comptonization of radiation from the Clouds (ECC) peak. [3]
While blazars continue to leave questions for astronomers and astrophysicists, such as how they produce the relativistic jets of particles that power their emission, how they relate to other types of AGN like quasars and Seyfert galaxies and why there appear to be two types of blazars, the leptonic model of blazar jet emission processes offers a salient explanation of the features we do see in observed blazar SEDs.
© 2008 Neville Eclov. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. L. Greenstein and M. Schmidt, "The Quasi-stellar Radio Sources 3C 48 and 3C 273," Astrophys. J. 140, 1 (1964).
[2] I. Robson, Active Galactic Nuclei, (Wiley-Praxis, 1996).
[3] M. Böttcher, "Physics Input from Multiwavelength Observations of AGN," Bull. Astron. Soc. India, 30, 115 (2002).
[4] P. Padovani and C. M. Urry, "Issues in Blazar Research," in Blazar Demographics and Physics, ed. by P. Padovani and C. M. Urrey, ASP Conference Series 227 (Astron. Soc. Pac., 2001).