E9 High Energy Astrophysics

Topic

High-energy astrophysics studies processes involving energies far exceeding ordinary thermal energies—specifically, X-rays and gamma rays.

Synchrotron emission occurs when electrons, accelerated to speeds approaching that of light, spiral while trapped within a magnetic field; the power radiated by these electrons in this manner increases with the square of their relativistic energy and with the strength of the surrounding magnetic field, making synchrotron emission the dominant process in supernova remnants, jets from active galactic nuclei, and pulsars.

Cosmic rays, meanwhile, are particles accelerated to extraordinary energies by mechanisms operating within the shocks produced by both supernovae and active galactic nuclei.

The spectrum of these cosmic rays follows a power law that bends—changing its slope—at two characteristic points known as the "knee" and the "ankle"; at each of these points, the change in slope signals a shift in the physical regime of the acceleration mechanism dominating that energy range. Space- and ground-based observatories specializing in X-rays and gamma rays collectively cover all these windows of the high-energy spectrum. Thus, spanning the range from the supra-thermal energies mentioned at the beginning of this text to the spectral windows discussed at its conclusion—and encompassing synchrotron emission as well as cosmic rays accelerated in supernova and active galactic nucleus shocks—high-energy astrophysics reveals how the universe's most violent events leave a signature that becomes visible only outside the range of light perceived by the human eye.

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