Astrophysical maser
An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum.
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.
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.
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.
This latter obstacle may be partially surmounted through the judicious use of the spatial filtering inherent in interferometric techniques, especially very long baseline interferometry (VLBI).
[citation needed] 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,[1][2] leading to refinements in existing theoretical models.
In 1965 an unexpected discovery was made by Weaver et al.:[3] emission lines in space, of unknown origin, at a frequency of 1665 MHz.
These were termed masers, as from their narrow line widths and high effective temperatures it became clear that these sources were amplifying microwave radiation.
[8] Evidence for an anti-pumped (dasar) sub-thermal population in the 4830 MHz transition of formaldehyde (H2CO) was observed in 1969 by Palmer et al.[9][10][11][12] The connections of maser activity with far infrared (FIR) emission has been used to conduct searches of the sky with optical telescopes (because optical telescopes are easier to use for searches of this kind), and likely objects are then checked in the radio spectrum.
This has consequences for the radiation it produces: Small path differences across the irregularly shaped maser cloud become greatly distorted by exponential gain.
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.
In a saturated maser, amplification of radiation depends linearly on the size of population inversion and the path length.
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.
[citation needed] The impact of comet Shoemaker-Levy 9 with Jupiter in 1994 resulted in maser emissions in the 22 GHz region from the water molecule.
[17] Despite the apparent rarity of these events, observation of the intense maser emission has been suggested as a detection scheme for extrasolar planets.
Various pumping schemes – both radiative and collisional and combinations thereof – result in the maser emission of multiple transitions of many species.
[27] Astronomical masers remain an active field of research in radio astronomy and laboratory astrophysics due, in part, to the fact that they are valuable diagnostic tools for astrophysical environments which may otherwise elude rigorous quantitative study and because they may facilitate the study of conditions which are inaccessible in terrestrial laboratories.
Intensity variations can occur on timescales from days to years indicating limits on maser size and excitation scheme.
Perhaps the most spectacular success of this technique is the dynamical determination of the distance to the galaxy NGC 4258 from the analysis of the motion of the masers in the black-hole disk.
In general, astrophysical masers are discovered empirically then studied further in order to develop plausible suggestions about possible pumping schemes.
Quantification of the transverse size, spatial and temporal variations, and polarisation state, typically requiring VLBI telemetry, are all useful in the development of a pump theory.
For example, the magnetic dipole transitions of the OH molecule near 53 MHz are expected to occur but have yet to be observed, perhaps due to a lack of sensitive equipment.
