Active Galactic Nuclei
When astronomers first recognized in the 1920s that galaxies are huge stellar islands comprising up to one
hundred billion stars, they also realized that some galaxies are much brighter than others at certain wavelengths.
These were called "active galaxies." As observations expanded into the radio, X-ray, and gamma-ray wavelength
bands, astronomers noticed that certain active galaxies emit huge amounts of radiation outside the visible
spectrum. Over time, these objects were classified according to their observed properties.
Some have strong radio emission extending over broad "lobes" (radio galaxies); others are very luminous and are
physically compact (quasars and blazars); still others have intensely bright nuclei (Seyfert galaxies).
Nowadays we think that all kinds of active galaxies -- whether radio galaxies, blazars, or Seyferts --
function according to the same physical principles. In the center of each active galactic nucleus (AGN) sits a
supermassive black hole. The black hole siphons matter from nearby stars, spinning it into a rapidly rotating
accretion disk. The material in the accretion disk is heated to extremely high energies, and the high-energy
matter and radiation are eventually ejected along the rotational axis of the disk in jets that extend far
into space.
The unified model of active galactic nuclei (AGN). Surrounding the supermassive
black hole is a large accretion disk of orbiting dust and gas. There is often a jet (or two back-to-back
jets) of accelerated
particles aligned with the axis of rotation of the black hole. The type of AGN seen from Earth depends
on the viewing angle. Image credit: NASA.
Hence, the radiation observed from an AGN is driven by the "black hole engine" at its center.
Depending on our viewing angle with respect to the galaxy, we may observe a different class of object.
Viewing the jet edge-on, we see extended radio and Seyfert galaxies. If we observe the jet face-on,
we observe a compact, intense source (quasars and blazars).
Cosmic ray acceleration in AGNs
Cosmic rays are charged particles of extraordinarily high energies. They are accelerated when they cross
shock waves in the hot gas of interstellar space. Because cosmic rays are charged, their trajectories will be
bent by magnetic fields. Therefore, a strong magnetic field can "confine" a cosmic ray to a small region of
space. If a cosmic ray is confined to a region near a shock wave, it can cross the shock repeatedly and gain a
tremendous amount of energy. At some point, the particle gains so much energy that it escapes the magnetic field;
and eventually, we detect it at earth as a cosmic ray.
There is considerable evidence that the accretion disks and jets in AGNs contain very large magnetic fields
and very intense shocks. Hence, AGNs are strong candidates for cosmic ray acceleration. However, the exact
mechanism of cosmic ray acceleration in AGNs is still not known.
In addition, the environment around an AGN is thick with visible, X-ray, and gamma-ray radiation. Charged
particles are expected to lose considerable energy as they move out of the radiation field and into space.
Understanding how cosmic rays are accelerated in such an environment remains a considerable technical problem.
The identification of AGNs as sites of charged particle acceleration raises many fascinating and important
questions. These are among the many we still have to answer before we can truly solve the mystery of
ultrahigh energy cosmic rays.
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