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The Question

(Submitted March 13, 1997)

Please send me some information about the mechanism behind X-ray radiation in the interstellar medium. For example, the plasma mechanism.

The Answer

X-rays in space come from a variety of sources. These include objects, such as supernova remnant, active galactic nuclei (including quasars), stars, and compact objects (black holes or neutron stars) in binary orbits with more normal stars. In addition, X-rays are likely to be emitted by diffuse gas in the interstellar medium. The relative contributions or these various sources to the total X-ray flux received at earth is a subject of some debate, and it varies with the X-ray energy.

It is customary to divide the emission mechanisms for X-rays into "thermal" and "non-thermal", according to whether the velocity distribution of the emitting electrons is Maxwellian. Among thermal mechanisms, the most common is almost certainly bremsstrahlung, in which radiation occurs as the result of coulomb collisions between electrons and nuclei in an ionized gas. This mechanism is likely to be operating in virtually all X-ray sources, and dominates the emission from many of them. One of the most common non-thermal mechanisms is synchrotron emission, in which electrons radiate as the result of their gyroscopic motion in a magnetic field. This mechanism, and variation, called synchrotron-self Compton, is likely to dominate in some supernova remnants and in some quasars. Both of these mechanisms are described in electricity and magnetism texts, such as Jackson's "Classical Electrodynamics". More details can be found in a "Radiative Processes in Astrophysics" by Rybicki and Lightman.

I hope this helps!

Tim Kallman
for the Ask an Astrophysicist team

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