Astrophysical maser

An astrophysical maser is a naturally occurring source of stimulated spectral line emission. These emissions usually appear in the microwave portion of the electromagnetic spectrum. They can arise in many places, such as molecular clouds, comets, or planetary atmospheres. They also occur in stellar atmospheres and throughout interstellar space. Masers are vital because they act as beacons for astronomers. They provide data about the physical conditions of distant cosmic environments.

To understand a maser, we must look at the process of stimulated emission. This begins with a pumping process that moves molecules into a non-thermal population distribution. This state is called a population inversion, where more molecules exist in a high-energy state than a low-energy state. When a wave passes through this gain medium, it triggers the molecules to release energy. This creates a single pass of amplified radiation. Unlike laboratory masers, astrophysical masers lack an engineered resonant cavity. Because they lack this cavity, they often lack spatial coherence and mode purity.

There are several ways to categorize these celestial emitters. Some scientists use the term megamaser to describe sources with extreme luminosity. In 1982, an extra-galactic source was discovered with a luminosity 10^6 times larger than previous sources. Another distinction involves the frequency of the light. Some researchers use the term iraser for masers emitting at wavelengths of a few micrometres. In the laboratory, these are often called lasers. Other terms like taser refer to the terahertz regime. In space, astronomers might simply call these sub-millimeter masers.

The history of these discoveries began with a mystery. In 1965, Weaver et al. detected unexpected emission lines at 1665 MHz. At the time, many believed molecules could not exist in space. Because of this, researchers initially thought the signal came from a hypothetical substance called "mysterium." They soon discovered the emission actually came from hydroxide molecules in molecular clouds. This led to a wave of new findings. Water masers were found in 1969, methanol in 1970, and silicon monoxide in 1974.

Masers exhibit unique physical characteristics due to exponential gain. As radiation passes through the cloud, the amplification grows exponentially. This causes the radiation to undergo beaming, where most light emerges along the longest path length. This process makes maser spots appear much smaller than their parent clouds. Exponential gain also causes line narrowing. This means the emission line becomes taller but not much wider. Furthermore, the output can show rapid variability. This happens because any small change in the population inversion results in an exponential change in output.

We can observe masers in many different environments. In our own Solar System, comet halos can produce them. When comet Shoemaker-Levy 9 hit Jupiter in 1994, it caused water maser emissions at 22 GHz. Scientists also study masers in the atmospheres of gas giants. For example, cyclotron masers have been detected at the north pole of Jupiter.

In 2009, water masers were also detected in the plumes of Saturnian moons like Enceladus and Titan. These different locations show how diverse maser conditions can be.

Studying these emissions provides deep insights into the universe. Masers allow scientists to measure temperature, density, and magnetic fields. They are especially useful in studying stellar birth, stellar death, and the centers of galaxies. The brightness temperature of a maser can be incredibly high. Some masers reach 10^9 K, while others reach 10^12 K or even 10^14 K. This temperature is a measure of brightness rather than actual heat. By analyzing polarization and spectral lines, astronomers can refine their theoretical models of the cosmos.