Sunday, March 22, 2015

Cosmic Rays 3

This is the final post in a series on cosmic rays. For the first, see here.
We began by showing another graph of the cosmic ray flux at various energies.

It so happens that this graph very nearly follows an inverse cube relationship (which appears straight on log-log graphs). That is, cosmic rays with double the energy are about eight times rarer in the cosmos. The above diagram also shows the small deviations from the inverse cube relationship that occur at high densities, known as the "knee" and the "ankle".

The first major deviation from the inverse cube law, the so-called "knee", occurs around 1015 eV. Theoretically, this is near the maximum energy that a charged particle could have and still be contained in the Milky Way by the galaxy's galactic halo. The Earth receives a slightly larger flux of these energies than expected because particles slowed to this energy remain in the Milky Way while higher energy particles do not. The transition to extragalactic particle origins forms the "ankle" of the graph.

Extragalactic Cosmic Rays

Cosmic rays with energies more than about 1015 electron volts seldom, if ever, originate in our galaxy. Such cosmic rays could originate from particularly powerful supernovae, or, accelerated by the same mechanism of magnetic shock waves, from two colliding galaxies. However, it is likely that most cosmic rays originate in active galactic nuclei, such as quasars.

The above is an artist's rendering of a quasar. As the name suggests, quasars are very similar to microquasars, except much larger (similar features include the accretion disk and relativisitic jets, both illustrated above). The compact object involved, rather than being a neutron star or stellar black hole, is a supermassive black hole, the kind found at the center of most galaxies (including the Milky Way). Such black holes often exceed one million solar masses.

The characteristic property of active galactic nuclei is that they consume matter at an extraordinarily rapid rate. As a result, the energy and radiation released far outshine the rest of the galaxy. In fact, many quasars are hundreds or thousands of times more luminous than our entire Milky way, corresponding to a luminosity sometimes exceeding a trillion suns. However, over time, the supermassive black hole consumes all nearby matter, and the galaxy becomes dormant, becoming in many respects like our own. For this reason, quasars tend to be young galaxies, and thus the ones we see tend to be quite distant (that is, we see them as they were a long time ago). Even the nearest quasars are still billions of light-years away.

Cosmic rays from these distant quasars may have energies up to about 5x1019 eV (50 million trillion eV). However, this value is the theoretical upper-limit for the energy of cosmic rays from distant sources, also known as the Greisen-Zatsepin-Kuzmin Limit (GZK limit). Computed independently in the 1960's by Kenneth Greisen, Georgiy Zatsepin, and Vadim Kuzmin, the GZK limit comes about from the interactions between cosmic ray particles and the cosmic background radiation (see also here), the leftover radiation from shortly after the Big Bang which permeates the Universe. In theory, if cosmic rays with higher energy travel a sufficient distance, interactions with the cosmic background radiation will reduce them to the GZK limit.

However, this is not the end of the story. It has been confirmed that cosmic rays with even higher energies have hit the Earth, though such events are rare: cosmic rays with energy exceeding the GZK limit hit a given square kilometer of the atmosphere only about once per decade. The most commonly accepted explanation for these particles is that they have not yet traveled far enough for the cosmic background radiation to slow them down, and therefore come from sources within about 200 million light-years.

Recently, scientists have proposed that quasar remnants (galactic nuclei that were formerly active) could eject such particles, though being relatively quiet in electromagnetic emissions. Unlike quasars, so-called "retired quasars" are very common in our region of the cosmos, some within 100 million light-years. Very massive, rotating supermassive black holes are the most probable candidates for the origin of these cosmic rays.

The elliptical galaxy M60 (above) is one plausible source for cosmic rays with energies exceeding the GZK limit. The galaxy is only about 55 million light-years away, and harbors a supermassive black hole of 4.5 billion solar masses, among the largest known.

Likely the current record-holder for the most energetic cosmic ray ever observed was aptly named "Oh-My-God" particle. This particle (most likely a proton) was observed in Utah on October 15, 1991. It had an energy of approixmately 3x1020 eV, roughly six times the GZK limit. Since this energy is about 50 J, it is comparable to the kinetic energy of a pitched baseball, all in a single particle! Upon Earth impact, this particle was traveling at roughly 99.99999999999999999999951% of the speed of light. However, the Oh-My-God particle, and other similar cosmic rays, are no threat to Earth or its indigineous life, due to their infrequency and their interactions with the atmosphere before reaching Earth's surface.

Nevertheless, studying the composition and energy of cosmic rays is very important to astrophysics. By observing cosmic rays in addition to collecting images using electromagneitc radiation, we obtain a more complete picture of our Universe.