This illustration is from the 15 June 2001 issue of Science magazine.
What are AGNs?
Gamma-ray Loud Surveillance Program
Seyfert Galaxy Monitoring Program
AGN stands for Active Galactic Nuclei. Such objects include a broad range of categories of objects such as blazars, quasars, and radio galaxies that appear different from "normal" galaxies. These differences all arise from activity in the nuclear regions of the galaxy. For nearby objects, a bright stellar appearing core appears to be superimposed on an underlying and otherwise normal galaxy. For distant objects, only the core appears and these objects become indistinguishable from stars. Active galaxies are more luminous than normal galaxies. AGNs can be hundreds or thousands of times brighter than normal galaxies. AGNs often have unusually strong or unusually broad emission lines. These emission lines are often used to discover these objects. The emission lines reveal extremely large red shifts that betray the extragalactic nature of these objects. Many AGNs appear to be variable on time scales of years and months. Some are known to flare or decrease in brightness on time scales of weeks or even days. The variability does not appear to be periodic. Some of these objects are known to vary in brightness by up to 2 or even 4 magnitudes or more. It has been suggested that all AGN may be variable at some level. A sample of galaxies from the Hubble Deep Field (HDF) were observed over a time span of several years and nearly all objects exhibited some variability.
Some AGN are strong radio sources. In general, radio sources that have a non-thermal spectrum (decrease in flux toward higher frequencies) tend to be extragalactic. Some of the nearer AGN radio sources have extended radio lobes and sharply focused jets of radio emission. Not all AGN are radio-loud, and some are not detected as radio sources. Virtually all AGN appear to be x-ray sources. Indeed, it appears likely that x-ray emission from AGNs is the dominant source of the diffuse x-ray emission that is observed from the entire sky. AGNs that are strong x-ray sources have also been detected as gamma-ray sources. AGNs also appear to be variable at radio frequencies and at x-ray and gamma-ray energies.
Examples of types of AGNs include quasars (QSOs), blazars (BL Lac objects, or BLLs), radio galaxies, and Seyfert galaxies.
A unified model for AGNs has been proposed in which a single physical phenomenon is the source for all categories of AGNs. In the unified model, accretion onto a supermassive black hole at the core of a galaxy produces all the categories of objects we refer to as AGNs. The accretion process will proceed to form an accretion disk with jets of accelerated particles normal to the disk. In this model, the different forms of AGNs we observe are merely different views of this process from different orientations with respect to the axis of the jet and the accretion disk. In this model a radio-loud QSO with radio lobes is a view nearly along the accretion disk, while a blazar is a view nearly along the axis of the jet.
This illustration is from the 15 June 2001 issue of Science magazine.
Seyfert galaxies are likely to be less extreme examples of this phenomenon with lower luminosity. For such objects the supermassive black hole may have less mass. This may also be true for some lower luminosity, less active qusars. For the radio-quiet AGNs, the jet may not have developed. The jet is fundamentally charged particles that have been accelerated to relativistic speeds. It may be that the relativistic acceleration that produces the jet only occurs above some critical mass limit for the central supermassive black hole.
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While nearly all AGNs appear to be variable, the variability appears to be fundamentally irregular. There has never been a convincing demonstration of periodicity for any AGN variability. AGN variability consists of slow irregular fluctuations upon which are superimposed occasional outbursts and declines. The larger outbursts and declines have longer time spans. Outbursts can occur over a rather large range in activity. Outbursts can last for years or even decades, while smaller outbursts can last for months or days. Smnaller outbursts have even been observed to last for an hour or less. Larger outbursts can amount to 4 magnitudes or more, while the small outbursts can amount to only 0.1 magnitude. The most active objects are known as blazars or Optically Violent Variables (OVVs). These objects can have outbursts of 4 magnitudes or more and are more variable on all time scales. Normal quasars are generally more quiescent, although there may be a continuous variation between the violent objects and the quiscent objects.
A more extended description of AGN Variability is available which includes illustrations of various types of lightcurves plus recommendations for the type of photometric data that can be most helpful in improving our understanding of the AGN phenomenon.
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In support of the GLAST mission, SSUO will be monitoring several gamma-ray loud AGNs. This will establish a baseline for variability for these objects during the time that these objects will be observed by GLAST. This surveillance will be undertaken in conjunction with the GLAST Telescope Network (GTN).
SSUO will survey the bright northern hemisphere gamma-ray loud AGNs. It is our goal to obtain a minimum of one observation each month beginning summer 2001 and extending through the GLAST mission. Anyone interested in contributing to this program should contact Gordon G. Spear ( spear@sonoma.edu ). This includes students at SSU, observers at other institutions, and amateur astronomers with access to a telescope with a CCD system.
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SSUO is now monitoring a few Seyfert galaxies to reliably document both short-term and long-term variability. Information about this program will be available here soon.
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