Masers
Astronomical masers are another source of non-thermal radiation. “Maser” is short for microwave-amplified stimulated emission of radiation. Masers are very compact sites within molecular clouds where emission from certain molecular lines can be enormously amplified. The interstellar medium contains only a smattering of molecular species such as water (H₂O), hydroxl radicals (OH), silicon monoxide (SiO), and methanol (CH₃OH). Normally, because of the scarcity of these molecules, their line emissions would be very difficult to detect with anything but very crude resolution. However, because of the phenomenon of “massing,” these clouds can be detected in other galaxies!
In simplified terms, massing occurs when clouds of these molecules encounter an intense radiation field, such as that from a nearby source such as a luminous star, or when they collide with the far more abundant H2 molecules. What is called a “population inversion” occurs, in which there are more molecules in an excited state (that is, their electrons have “jumped” to a higher energy level), than in a stable, ground state. This phenomenon is called pumping. As the radiation causing the pumping travels through the cloud, the original ray is amplified exponentially, emerging at the same frequency and phase as the original ray, but greatly amplified. Some masers emit as powerfully as stars! This phenomenon is related to that of spectral line emissions.
Incidentally, this same principle is used in a device called a maser amplifier, which is installed as part of some radio telescopes (not in the GAVRT, however) to amplify the signal received by the antenna.