Astrophysical maser

An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar atmospheres, or various other conditions in interstellar space.

Background

Discrete transition energy

Like a laser, the emission from a maser is stimulated and monochromatic, having the frequency corresponding to the energy difference between two quantum-mechanical energy levels of the species in the gain medium which have been pumped into a non-thermal population distribution. However, naturally occurring masers lack the resonant cavity engineered for terrestrial laboratory masers. The emission from an astrophysical maser is due to a single pass through the gain medium and therefore generally lacks the spatial coherence and mode purity expected from a laboratory maser.

Nomenclature

It is often stated that astrophysical masers are not "true" masers because they lack these oscillation cavities. However, the distinction between oscillator-based lasers and single-pass lasers was intentionally disregarded by the laser community in the early years of the technology.
This fundamental incongruency in language has resulted in the use of other paradoxical definitions in the field. For example, if the gain medium of a misaligned laser is emission-seeded but non-oscillating radiation, it is said to emit amplified spontaneous emission or ASE. This ASE is regarded as unwanted or parasitic. Some researchers would add to this definition the presence of insufficient feedback or unmet lasing threshold: that is, the users wish the system to behave as a laser. The emission from astrophysical masers is, in fact, ASE but is sometimes termed superradiant emission to differentiate it from the laboratory phenomenon. This simply adds to the confusion, since both sources are superradiant. In some laboratory lasers, such as a single pass through a regeneratively amplified Ti:Sapph stage, the physics is directly analogous to an amplified ray in an astrophysical maser.
Furthermore, the practical limits of the use of the m to stand for microwave in maser are variously employed. For example, when lasers were initially developed in the visible portion of the spectrum, they were called optical masers. Charles Townes advocated that the m stand for molecule, since energy states of molecules generally provide the masing transition. Along these lines, some use the term laser to describe any system that exploits an electronic transition and the term maser to describe a system that exploits a rotational or vibrational transition, regardless of the output frequency. Some astrophysicists use the term iraser to describe a maser emitting at a wavelength of a few micrometres, even though the optics community terms similar sources lasers.
The term taser has been used to describe laboratory masers in the terahertz regime, although astronomers might call these sub-millimeter masers and laboratory physicists generally call these gas lasers or specifically alcohol lasers in reference to the gain species. The electrical engineering community typically limits the use of the word microwave to frequencies between roughly 1 GHz and 300 GHz; that is, wavelengths between 30 cm and 1 mm, respectively.

Astrophysical conditions

The simple existence of a pumped population inversion is not sufficient for the observation of a maser. For example, there must be velocity coherence along the line of sight so that Doppler shifting does not prevent inverted states in different parts of the gain medium from radiatively coupling. While polarisation in laboratory lasers and masers may be achieved by selectively oscillating the desired modes, polarisation in natural masers will arise only in the presence of a polarisation-state–dependent pump or of a magnetic field in the gain medium.
The radiation from astrophysical masers can be quite weak and may escape detection due to the limited sensitivity, and relative remoteness, of astronomical observatories and due to the sometimes overwhelming spectral absorption from unpumped molecules of the maser species in the surrounding space. This latter obstacle may be partially surmounted through the judicious use of the spatial filtering inherent in interferometric techniques, especially very long baseline interferometry.
The study of masers provides valuable information on the conditions—temperature, density, magnetic field, and velocity—in environments of stellar birth and death and the centres of galaxies containing black holes, leading to refinements in existing theoretical models.

Discovery

Historical background

In 1965 an unexpected discovery was made by Weaver et al.: emission lines in space, of unknown origin, at a frequency of 1665 MHz. At this time many researchers still thought that molecules could not exist in space, even though they had been discovered by McKellar in the 1940s, and so the emission was at first attributed to a hypothetical form of interstellar matter named "mysterium", but the emission was soon identified as line emission from hydroxide molecules in compact sources within molecular clouds. More discoveries followed, with water emission in 1969, methanol emission in 1970, and silicon monoxide emission in 1974, all emanating from within molecular clouds. These were termed masers, as from their narrow line widths and high effective temperatures it became clear that these sources were amplifying microwave radiation.
Masers were then discovered around highly evolved late-type stars, named OH/IR stars. First was hydroxide emission in 1968, then water emission in 1969 and silicon monoxide emission in 1974. Masers were discovered in external galaxies in 1973, and in the Solar System in comet halos.
Another unexpected discovery was made in 1982 with the discovery of emission from an extra-galactic source with an unrivalled luminosity about 10⁶ times larger than any previous source. This was termed a megamaser because of its great luminosity; many more megamasers have since been discovered.
A weak disk maser was discovered in 1995 emanating from the star MWC 349A, using NASA's Kuiper Airborne Observatory.
Evidence for an anti-pumped sub-thermal population in the 4830 MHz transition of formaldehyde was observed in 1969 by Palmer ''et al.''

Detection

The connections of maser activity with far infrared emission has been used to conduct searches of the sky with optical telescopes, and likely objects are then checked in the radio spectrum. Particularly targeted are molecular clouds, OH-IR stars, and FIR active galaxies.

Known interstellar species

The following species have been observed in stimulated emission from astronomical environments:

OH
CH
H₂CO
H₂O
NH₃, ¹⁵NH₃
CH₃OH
HNCNH
SiS
HC₃N
SiO, ²⁹SiO, ³⁰SiO
HCN, H¹³CN
H
CS
Characteristics of maser radiation

The amplification or gain of radiation passing through a maser cloud is exponential. This has consequences for the radiation it produces:

Beaming

Small path differences across the irregularly shaped maser cloud become greatly distorted by exponential gain. Part of the cloud that has a slightly longer path length than the rest will appear much brighter, and so maser spots are typically much smaller than their parent clouds. The majority of the radiation will emerge along this line of greatest path length in a "beam"; this is termed beaming.

Rapid variability

As the gain of a maser depends exponentially on the population inversion and the velocity-coherent path length, any variation of either will itself result in exponential change of the maser output.

Line narrowing

Exponential gain also amplifies the centre of the line shape more than the edges or wings. This results in an emission line shape that is much taller but not much wider. This makes the line appear narrower relative to the unamplified line.

Saturation

The exponential growth in intensity of radiation passing through a maser cloud continues as long as pumping processes can maintain the population inversion against the growing losses by stimulated emission. While this is so the maser is said to be unsaturated. However, after a point, the population inversion cannot be maintained any longer and the maser becomes saturated. In a saturated maser, amplification of radiation depends linearly on the size of population inversion and the path length. Saturation of one transition in a maser can affect the degree of inversion in other transitions in the same maser, an effect known as competitive gain.

High brightness

The brightness temperature of a maser is the temperature a black body would have if producing the same emission brightness at the wavelength of the maser. That is, if an object had a temperature of about 10⁹K it would produce as much 1665-MHz radiation as a strong interstellar OH maser. Of course, at 10⁹K the OH molecule would dissociate, so the brightness temperature is not indicative of the kinetic temperature of the maser gas but is nevertheless useful in describing maser emission. Masers have incredible effective temperatures, many around 10⁹K, but some of up to 10¹²K and even 10¹⁴K.

Polarisation

An important aspect of maser study is polarisation of the emission. Astronomical masers are often very highly polarised, sometimes 100% in a circular fashion, and to a lesser degree in a linear fashion. This polarisation is due to some combination of the Zeeman effect, magnetic beaming of the maser radiation, and anisotropic pumping which favours certain magnetic-state transitions.
Many of the characteristics of megamaser emission are different.