Black Holes Produce the Most Energetic Cosmic Radiation

Blasts of radiation brighter than a trillion suns. Charged particles with the energy of a well-thrown baseball. Jets of magnetized plasma streaming across intergalactic space. What do these have in common? Their energy is derived from black holes.

Today, at the 2009 Fermi Symposium in Washington, DC, Charles Dermer, an astrophysicist in the Space Science Division of the Naval Research Laboratory (NRL), and Govind Menon, a professor of physics at Troy University, Troy, Alabama, are describing black holes as the sources of the highest energy photon and particle radiations. In particular, rapidly rotating black holes could make the radio and gamma-ray luminous supermassive black-hole blazars. Newly born black holes could accelerate the highest energy cosmic rays and make the brief bright gamma-ray emissions known as gamma-ray bursts. Black holes, so-named because not even light can escape their inner regions, are actually extraordinarily luminous, "and have little in common with their proverbial counterpart," says Professor Menon.

Space science hasn't been the same since Victor Hess discovered cosmic rays in 1912 by measuring discharge rates of an electroscope in a balloon experiment. Hess discovered particle radiations in space, namely the cosmic rays, but many different types of radiations are known to exist besides cosmic rays, including trapped particles in the radiation belts, solar energetic particles from flares and coronal mass ejections, and cosmic X-rays. At energies well above one billion (Giga-) electron Volts (GeV), cosmic ray particles cannot be trapped in the Sun's magnetized plasma cavity, nor are they easily made by solar events. These GeV to million GeV cosmic rays, having been stripped of their electrons, follow a tangled trajectory in the magnetic field of the Milky Way Galaxy and do not point back to their sources. Consequently, the origin of cosmic rays remains an unsolved problem nearly 100 years after their discovery.

At still higher energies, the magnetic field of the Galaxy cannot contain ultrahigh-energy cosmic rays (UHECRs) with energies exceeding a billion GeV or so. At these extreme energies, particles move along nearly straight paths through the vast expanse of intergalactic space. The highest energy cosmic ray ever detected, weighing in at about 3 x 10^20 eV, was so powerful that this single atomic particle or nucleus carried the kinetic energy of a 60 mph baseball. But even at the very highest energies, where the deflections of UHECRs by the intergalactic magnetic field are least, there is no population of objects -- whether astrophysical or astroparticle -- that available data establishes as the solution to the puzzle of the origin, acceleration site, and power source of UHECRs.

The newly launched Fermi Gamma ray Space Telescope, the South Pole IceCube Neutrino Experiment, ground-based TeV (1000 GeV) gamma-ray detectors, and the Pierre Auger Cosmic Ray Observatory in Argentina are now poised to provide crucial evidence on this and many other features of the high-energy space radiation environment. According to Dermer, "This is a decade of incredible scientific discovery in high-energy astronomy and astroparticle physics."

Dermer and Menon anticipate advances over the coming decade in their new book "High Energy Radiation from Black Holes: Gamma Rays, Cosmic Rays, and Neutrinos," published this week by Princeton University Press as part of the Princeton Series in Astrophysics, an international set of monographs for researchers and students. "Because the high-energy cosmic ray, neutrino, and gamma-ray fields have such extensive overlap, I wanted to write about the problem of the sources of the highest energy radiations in a common language. And in my work, all roads lead to the black hole." Menon adds, "We studied a battery mechanism to extract the energy of a spinning black hole, and it provides a compelling way to power jets in high-energy gamma ray sources."

"High Energy Radiation from Black Holes" gives an extensive theoretical framework involving the radiation physics and strong-field gravity of black holes, and pulls together Einstein's general theory of relativity with electrodynamics using a new space-time approach that simplifies the excessive mathematical formalism found in past treatments. A detailed description of fundamental astrophysical radiation processes is treated. Several subjects, for example, astrophysics of gamma-ray attenuation by photons and relativistic blast-wave physics developed to understand the major advances in gamma-ray burst research since the 1990s, appear for the first time in monograph form.

The book provides a basis for graduate students and researchers in the field to interpret the latest results from high-energy observatories, and helps answer whether energy released by rotating black holes powers the highest-energy radiations in nature. Black-hole research, no less esoteric than uranium physics was in the 1920s, finds defense applications in radiation effects, neutral and charged beam physics, explosion physics, and autonomous telescopes and rapid-response science.

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The Naval Research Laboratory is the Department of the Navy's corporate laboratory. NRL conducts a broad program of scientific research, technology, and advanced development. The Laboratory, with a total complement of nearly 2,500 personnel, is located in southwest Washington, DC, with other major sites at the Stennis Space Center, MS; and Monterey, CA.

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