Active Galaxies are galaxies characterized by certain properties: (1)
High Luminosity, (2) Nonthermal Spectra that do not look like the sum
of many stellar spectra, (3) Most of the luminosity is in a region of
the spectrum other than optical (e.g., radio, UV, Infrared), (4)
bright, star-like nucleus, (5) strong emission lines (most), (6) rapid
variability, and sometimes (7) radio jets.
The central region of an active galaxy is called an Active Galactic Nucleus (AGN). This is where the energtic activity is concentrated. The active galactic nuclei are believed to contain supermassive black holes that power the nonstellar phenomena associated with active galaxies.
Active galaxies are galaxies which have a small core of emission embedded in an otherwise typical galaxy. This core may be highly variable and very bright compared to the rest of the galaxy. Models of active galaxies concentrate on the possibility of a supermassive black hole which lies at the center of the galaxy. The dense central galaxy provides material which accretes onto the black hole releasing a large amount of gravitational energy. Part of the energy in this hot plasma is emitted as x-rays and gamma rays. For "normal" galaxies, we can think of the total energy they emit as the sum of the emission from each of the stars found in the galaxy. For the "active" galaxies, this is not true. There is a great deal more emitted energy than there should be... and this excess energy is found in the infrared, radio, UV, and X-ray regions of the electromagnetic spectrum. The energy emitted by an active galaxy (or AGN) is anything but "normal". So what is happening in these galaxies to produce such an energetic output?
There are several types of active galaxies: Seyferts, quasars, and blazars. Most scientists believe that, even though these types look very different to us, they are really all the same thing viewed from different directions! Quasars are active galaxies which are all very, very, very far away from us. Some of the quasars we have seen so far are 12 billion light-years away! Blazars are very bright in the radio band, which results from looking directly down a jet which is emitting in synchrotron radiation. On the other hand, if the jet is not pointing toward you at all, and the dusty disk of material which lies in the plane of the galaxy is in the way, you would see just what we see from the Seyferts. By measuring their redshifts, we find that Seyferts are much closer to us than quasars or blazars.
Active galaxies are intensely studied at all wavelengths. Since they can change their behavior on short timescales, it is useful to study them simultaneously at all energies. X-ray and gamma-ray observations have proven to be important parts of this multiwavelength approach since many high-energy quasars emit a large fraction of their power at such energies. X-rays can penetrate outward from very near the center of a galaxy. Since that is where the "engines" of AGN are located, X-rays provide scientists with unique insights into the physical processes occurring there. In addition, gamma-ray observations alone can provide valuable information on the nature of particle acceleration in the quasar jet, and clues as to how the particles interact with their surroundings.
Observations of Seyfert galaxies in gamma rays are also important for studies of the cosmic gamma-ray background. Even in regions of the sky where there are no point sources, a faint gamma-ray glow is detectable. It may be that this glow is the sum of many faint galaxies or perhaps a more exotic process. Studies of individual Seyfert galaxies can be combined with a model of how such objects are distributed in the Universe to compare to the diffuse gamma-ray background. In this way, astronomers not only learn about the interesting AGN phenomena, but learn more about the general nature of the Universe as a whole.
Blazars and quasars are both subclasses of active galactic nuclei (AGN).Blazars and quasars are intrinsically the same object — a supermassive black hole with a surrounding accretion disk, producing a jet — but seen at different orientation angles with respect to the jet’s axis.
Somewhere, the jet’s kinetic energy must be dissipated. There are three likely culprits: shock waves, magnetic reconnection, and instabilities. Shocks form when one fluid plows into another quickly enough that there’s no time for the second fluid to get out of the way (an example of this occurs when a gun is fired). Magnetic reconnection occurs when magnetic field lines break and reform in a lower-energy configuration, thus releasing energy (such as when a coronal loop pinches off and forms a solar flare,). Instabilities arise, as an example, at interfaces between the jet and environment around it (like the Kelvin-Helmholtz instability, which occurs between two fluids of different velocities).
The Model
Reconfinement shocks are shocks that occur as a result of the jet plowing into the environment around it and being forced to re-close by the pressure of that environment . The question the author seeks to address is whether a reconfinement shock could dissipate enough energy to explain the jet luminosities that we observe.
Using the equations that describe how a fluid behaves when it crosses a shock front, the author obtains an expression for the energy dissipation efficiency in terms of the pressure of the external environment, the jet’s opening and closing angles (Θj , Θr ), and the jet’s speed, which is measured by its Lorentz factor (Γj).
Results
Comparison to blazars
According to the author, observations of AGN jets indicate that they satisfy the relation Γj Θj <~ 1, and that the radiative efficiencies of the brightest blazars are around 10%. The author’s calculations show that, for a product of the jet’s Lorentz factor and its closing angle of less than 1 (Γj Θr < 1), the radiative efficiency of reconfinement shocks is well-described by the expression εdiss = 8% (Γj Θr) 2, regardless of the external pressure. Thus, this model is in keeping with blazar observations.
Comparison to GRBs
GRBs have to break out of a star before they can become free. Once they have done so, however, they are expected to spread out with a wide opening angle such that Γj Θj >> 1. In this limit, the author’s model predicts that the radiative efficiency is roughly 80-90% depending on the external pressure, which consistent with the anticipated efficiencies of nearly 90% based on observations of GRBs.
The central region of an active galaxy is called an Active Galactic Nucleus (AGN). This is where the energtic activity is concentrated. The active galactic nuclei are believed to contain supermassive black holes that power the nonstellar phenomena associated with active galaxies.
Active galaxies are galaxies which have a small core of emission embedded in an otherwise typical galaxy. This core may be highly variable and very bright compared to the rest of the galaxy. Models of active galaxies concentrate on the possibility of a supermassive black hole which lies at the center of the galaxy. The dense central galaxy provides material which accretes onto the black hole releasing a large amount of gravitational energy. Part of the energy in this hot plasma is emitted as x-rays and gamma rays. For "normal" galaxies, we can think of the total energy they emit as the sum of the emission from each of the stars found in the galaxy. For the "active" galaxies, this is not true. There is a great deal more emitted energy than there should be... and this excess energy is found in the infrared, radio, UV, and X-ray regions of the electromagnetic spectrum. The energy emitted by an active galaxy (or AGN) is anything but "normal". So what is happening in these galaxies to produce such an energetic output?
There are several types of active galaxies: Seyferts, quasars, and blazars. Most scientists believe that, even though these types look very different to us, they are really all the same thing viewed from different directions! Quasars are active galaxies which are all very, very, very far away from us. Some of the quasars we have seen so far are 12 billion light-years away! Blazars are very bright in the radio band, which results from looking directly down a jet which is emitting in synchrotron radiation. On the other hand, if the jet is not pointing toward you at all, and the dusty disk of material which lies in the plane of the galaxy is in the way, you would see just what we see from the Seyferts. By measuring their redshifts, we find that Seyferts are much closer to us than quasars or blazars.
Active galaxies are intensely studied at all wavelengths. Since they can change their behavior on short timescales, it is useful to study them simultaneously at all energies. X-ray and gamma-ray observations have proven to be important parts of this multiwavelength approach since many high-energy quasars emit a large fraction of their power at such energies. X-rays can penetrate outward from very near the center of a galaxy. Since that is where the "engines" of AGN are located, X-rays provide scientists with unique insights into the physical processes occurring there. In addition, gamma-ray observations alone can provide valuable information on the nature of particle acceleration in the quasar jet, and clues as to how the particles interact with their surroundings.
Seyfert Galaxies
Of the two types of Active Galactic Nuclei (AGN) which emit gamma rays, Seyfert galaxies are the low-energy gamma-ray sources. Seyfert galaxies typically emit most of their gamma rays up to energies of about 100 keV and then fade as we observe them at higher energies. Early gamma-ray observations of Seyfert galaxies indicated that photons were detected up to MeV energies, but more sensitive observations have cast doubt on this possibility. At these low gamma-ray energies, the emission is usually a smooth continuation of the X-ray emission from such objects. This generally indicates that the physical processes creating the gamma rays are thermal processes similar to those responsible for emission from galactic black hole sources. As a result, gamma-ray studies of the high-energy spectrum and variability can give scientists important information about the physical environment in the AGN.Observations of Seyfert galaxies in gamma rays are also important for studies of the cosmic gamma-ray background. Even in regions of the sky where there are no point sources, a faint gamma-ray glow is detectable. It may be that this glow is the sum of many faint galaxies or perhaps a more exotic process. Studies of individual Seyfert galaxies can be combined with a model of how such objects are distributed in the Universe to compare to the diffuse gamma-ray background. In this way, astronomers not only learn about the interesting AGN phenomena, but learn more about the general nature of the Universe as a whole.
Quasars
One of the most remarkable trends in gamma-ray astronomy in recent years has been the emergence of high-energy gamma-ray quasars as an important component of the gamma-ray sky. At gamma-ray energies, these active galaxies are bright; they are highly variable at all energies. Unlike the Seyfert type AGN, most of these sources are preferentially detected at high energies, usually 100 MeV or more. In fact, they have been detected above 1 GeV, and some up to several TeV! Given the large distances to these objects and the strong emission of high-energy gamma rays, these are the most powerful particle accelerators in the Universe. Over 50 high-energy quasars are known at this time. Some appear as fuzzy stars that can be seen with large amateur telescopes. Many astronomers believe that Seyfert galaxies and high-energy quasars are basically the same type of objects, but we are simply viewing them differently. Radio observations of AGN often show powerful jets, streams of particles coming from the central source -- like water from a spigot. Charged particles are accelerated to nearly the speed of light in these jets. In the unified view of active galaxies, high-energy quasars are being viewed with the jet pointed towards us which allows us to see the resulting energetic radiation. With Seyfert galaxies, we are viewing from the side and do not see the very high-energy radiation which is traveling down the jet.Blazars
The AGNs observed at higher energies form a subclass of AGNs known as blazars; a blazar is believed to be an AGN which has one of its relativistic jets pointed toward the Earth so that what we observe is primarily emission from the jet region. They are thus similar to quasars, but are not observed to be as luminous. The visible and gamma-ray emission from blazars is variable on timescales from minutes to days. Although theories exist as to the causes of this variability, the sparse data do not yet allow any of the ideas to be tested. To date more than 60 blazars have been detected by the EGRET experiment aboard the Compton Gamma-Ray Observatory. All these objects appear to emit most of their bolometric luminosity at gamma-ray energies and, in addition, are strong extragalactic radio sources.Blazars and quasars are both subclasses of active galactic nuclei (AGN).Blazars and quasars are intrinsically the same object — a supermassive black hole with a surrounding accretion disk, producing a jet — but seen at different orientation angles with respect to the jet’s axis.
gamma-ray bursts and blazars
Two of the brightest phenomena in the Universe, namely gamma-ray bursts and blazars (active galactic nuclei with jets pointed toward us), are both thought to be powered by relativistic jets. We know that relativistic Doppler-beaming of the radiation toward us helps to explain the enormous luminosities observed (GRBs clock in at up to ~1053 erg/s, blazars at up to ~1050 erg/s), but even accounting for this, the jets must be radiating huge amounts of energy. A relativistic jet inherently has a great deal of kinetic energy, so the relevant question is: how does this get turned into radiation?Somewhere, the jet’s kinetic energy must be dissipated. There are three likely culprits: shock waves, magnetic reconnection, and instabilities. Shocks form when one fluid plows into another quickly enough that there’s no time for the second fluid to get out of the way (an example of this occurs when a gun is fired). Magnetic reconnection occurs when magnetic field lines break and reform in a lower-energy configuration, thus releasing energy (such as when a coronal loop pinches off and forms a solar flare,). Instabilities arise, as an example, at interfaces between the jet and environment around it (like the Kelvin-Helmholtz instability, which occurs between two fluids of different velocities).
The Model
Reconfinement shocks are shocks that occur as a result of the jet plowing into the environment around it and being forced to re-close by the pressure of that environment . The question the author seeks to address is whether a reconfinement shock could dissipate enough energy to explain the jet luminosities that we observe.
Using the equations that describe how a fluid behaves when it crosses a shock front, the author obtains an expression for the energy dissipation efficiency in terms of the pressure of the external environment, the jet’s opening and closing angles (Θj , Θr ), and the jet’s speed, which is measured by its Lorentz factor (Γj).
Results
Comparison to blazars
According to the author, observations of AGN jets indicate that they satisfy the relation Γj Θj <~ 1, and that the radiative efficiencies of the brightest blazars are around 10%. The author’s calculations show that, for a product of the jet’s Lorentz factor and its closing angle of less than 1 (Γj Θr < 1), the radiative efficiency of reconfinement shocks is well-described by the expression εdiss = 8% (Γj Θr) 2, regardless of the external pressure. Thus, this model is in keeping with blazar observations.
Comparison to GRBs
GRBs have to break out of a star before they can become free. Once they have done so, however, they are expected to spread out with a wide opening angle such that Γj Θj >> 1. In this limit, the author’s model predicts that the radiative efficiency is roughly 80-90% depending on the external pressure, which consistent with the anticipated efficiencies of nearly 90% based on observations of GRBs.
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