Extragalactic Variable Sources and Cosmic-Ray Acceleration Near Massive Black Holes

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The rapid variations in the luminosity of active galactic nuclei imply that their energy is emitted from a highly compact regions with dimensions of the order of light hours to light days. Yet, the rate of energy output from such a compact region in some galactic nuclei exceeds taht of the Sun by about a factor of a trillion. The energy is emitted over a broad band of frequencies, ranging from the radio to the x-ray and γ-ray parts of the spectrum. The effects of the powerful, compact source can be seen at distances that considerably exceed the dimensions of the parent galaxy: radio lobes, jets, and high-velocity clouds are expelled from the center. Various models have been proposed to explain the central power source; these invoke an ultramassive black hole or, alternatively, a spinar. We examine here the acceleration processes near a black hole. An accretion disk around a supermassive black hole in the center of an active galactic nucleus is shown to be a likely site of particle acceleration. Relativistic electrons can be generated by various electromagnetic processes as well as by purely gravitational processes (e.g. Penrose pair production). Protons and heavier nuclei are accelerated to very high energies by the following mechanisms: stochastic Fermi acceleration, betatron acceleration and possible shock-wave acceleration. We find that, depending on the confinement time for the protons, different galactic nuclei would contribute to different parts of the rigidity spectrum. Examining the magnetic field strengths in different disk models, we find that the betatron process can boost particle moments by some four orders of magnitude. Finally, if shock waves can be sustained in an accretion disk (for example if the disk has a corona around it, or if supersonic turbulence can be sustained), such shock waves would also accelerate particles to cosmic-ray energies. Observations of high-energy neutrinos together with γ-ray observations would provide a useful experimental probe of these models.


This article was originally published in Comments on Astrophysics, volume 9, issue 4, in 1981.

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Taylor & Francis