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30 May 2008 Physics Today www.physicstoday.org
Active galactic nucleus
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Figure 2. Four neutrino sources. (a) An accelerator proton beam collides with nuclei in a target to produce mesons. Their decay products include neutrinos of various flavors (see box 1). (b) Shock fronts in a relativistic jet emerging from an active galactic nucleus or other high-energy astrophysical source accelerate nuclei that then create mesons when they hit surrounding radiation or gas. (c) An air shower initiated by a cosmic-ray nucleus hitting Earth’s atmos- phere produces mesons and their decay products. (d) A cosmic-ray proton with energy above the Greisen-Zatsepin- Kuzmin threshold can produce a π+ simply by colliding with a cosmic-microwave-background photon. A resulting decay neutrino can have energy as high as 1020 eV.
other evidence points clearly to the existence of that primor- dial neutrino background. Fortunately for the enterprise of neutrino astronomy, the neutrino’s cross section for interaction with detector materials increases with its energy.
Neutrinos with MeV energies are produced by nuclear burning in stars and supernova explosions. Going to higher energies in figure 1, the atmospheric neutrinos are the decay products of π and K mesons created in showers of hadrons engendered by high-energy cosmic-ray protons and nuclei hitting the top of the atmosphere (box 1 and figure 2). The atmospheric-neutrino flux has been measured up to 1014 eV. Although they are locally produced, atmospheric neutrinos are important to our story because they are the dominant background that searches for extraterrestrial neutrinos have to contend with. Happily, the flux of atmospheric neutrinos falls dramatically with increasing energy; events above 1014 eV are very rare, which leaves a relatively clear field of view for extraterrestrial sources.
The highest-energy neutrinos in figure 1 are the decay products of pions produced in the interactions of ultra-high- energy cosmic rays with microwave photons. Above a thresh- old of about 4 × 1019 eV, the so-called Greisen-Zatsepin- Kuzmin (GZK) cutoff, cosmic-ray protons interact with the cosmic microwave background to produce pions (see PHYSICS TODAY, May 2007, page 17). The upshot is that the range of extragalactic cosmic rays is limited to roughly 250 million light-years. High-energy gammas are also restricted in how far they can travel. They lose energy by colliding with the cosmic background of infrared photons to create electron–positron pairs.
That leaves neutrinos as the only known probes of the high-energy universe at larger distances. What they will re- veal is a matter of speculation. In the November 2003 issue of PHYSICS TODAY (page 38), Martin Harwit reminded his readers that each time astronomers have opened a new win-
dow in the sky, they’ve made major discoveries. One should expect no less from neutrinos.
High-energy neutrinos and cosmic rays
At energies above 1 GeV, cosmic rays, rather than neutrinos or photons, dominate the sky. Up to about 1015 eV, cosmic rays are believed to originate in our own galaxy. Above 1018 eV, extragalactic sources are thought to dominate.
The trajectories of galactic cosmic rays are governed by diffusion in the galaxy’s magnetic fields. For a typical cosmic ray, say a 1012-eV (TeV) proton, the confinement lifetime in the galaxy is on the order of a million years. The steady-state en- ergy density of cosmic rays in the galaxy is about 10–12 ergs (10 MeV) per cubic centimeter. One supernova explosion somewhere in the galaxy every 30 years, contributing 1050 ergs to the creation of cosmic rays, provides just enough energy to maintain that steady state. Furthermore, the supernova ejecta also match the nuclear composition of cosmic rays.
The supernova shock wave, expanding into the inter- stellar medium over about 1000 years, builds magnetic fields that provide an environment in which cosmic rays could be accelerated to 1015 eV. Atmospheric Cherenkov telescopes have observed TeV photons associated with supernova rem- nants, perhaps produced where the expanding shock fronts collide with molecular clouds (see PHYSICS TODAY, January 2005, page 19). Cosmic rays in the expanding shock wave could interact with those clouds and produce neutral pions, which decay into TeV gamma rays, and charged pions, whose decays create neutrinos.
The origin of galactic cosmic rays is not settled. The TeV photons might come from inverse Compton scattering of low-energy photons off energetic electrons, a mechanism fa- miliar from other nonthermal photon sources. The detection of high-energy neutrinos from the shock front would be clear evidence that supernova remnants are, indeed, the sources
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