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 from various conditions in interstellar space. In optics, stimulated emission is the process by which, when perturbed by a photon, matter may lose energy resulting in the creation of another photon. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ... This page is about the radiation; for the appliance, see microwave oven. ... Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultrahigh frequency VHF = Very high frequency HF = High frequency... A Molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecular hydrogen, H2. ... Comet Hale-Bopp A comet (denoted by ☄) is a small body in the solar system that orbits the sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both due primarily to the effects of solar radiation upon the comets nucleus, which itself is a... A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. ... Saturns atmosphere is made up of hydorgen, helium and methane ... Different stars have different atmospheres. ... The interstellar medium (or ISM) is a term used in astronomy to describe the rarefied gas and dust that exists between the stars (or their immediate circumstellar environment) within a galaxy. ... Wikiquote has a collection of quotations related to: Space Attempting to understand the nature of space has always been a prime occupation for philosophers and scientists. ...

Background

main article: maser

A Hydrogen RF discharge, the first element inside an Hydrogen Maser (see description below), courtesy NASA/JPL-Caltech. ...

Discrete Transition Energy

Like a laser, the emission from a maser is stimulated (or seeded) 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. Indeed, 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 of laboratory instruments. The range of sizes in which lasers exist is immense, extending from microscopic diode lasers (top) to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion. ... A Hydrogen RF discharge, the first element inside an Hydrogen Maser (see description below), courtesy NASA/JPL-Caltech. ... In optics, stimulated emission is the process by which, when perturbed by a photon, matter may lose energy resulting in the creation of another photon. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ... Max Planck This article is about Planck, the German physicist. ... A simple introduction to this subject is provided in Basics of quantum mechanics. ... Laser pumping is the act of energy transfer from an external source into the laser gain medium. ... Statistical mechanics is the application of statistics, which includes mathematical tools for dealing with large populations, to the field of mechanics, which is concerned with the motion of particles or objects when subjected to a force. ... In physics, specifically statistical mechanics, the concept of population inversion is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a laser. ... In physics, resonance is the tendency of a system to absorb more energy when the frequency of the oscillations matches the systems natural frequency of vibration (its resonant frequency) than it does at other frequencies. ... A cavity resonator uses resonance to amplify a wave. ... Coherence is a property of waves that measures the ability of the waves to interfere with each other. ... Normal modes in an oscillating system are special solutions where all the parts of the system are oscillating with the same frequency (called normal frequencies or allowed frequencies). ...

Nomenclature

Due to the differences between engineered and naturally occurring masers, it is often stated that astrophysical masers are not "true" masers because they lack an oscillation cavity. Actually, the reverse is true.

Laboratory lasers and masers which employ oscillation cavities should actually be called losers because they are oscillators. However, due to the unattractiveness of this acronym, the scientific laboratory community consciously choose to incorrectly call oscillator-based sources lasers.[1] Astrophysical masers are, in fact, amplifiers of stimulated emission and properly called masers. Acronyms and initialisms are abbreviations formed from the initial letter or letters of words, such as NATO and XHTML, and are pronounced in a way that is distinct from the full pronunciation of what the letters stand for. ...

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 emits 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 loser. The emission from astrophysical masers is, in fact, ASE but is sometimes termed superradiant emission to differentiate it from the laboratory phenomenon. This is more unneeded confusion since both sources are superradiant. (Note that in some laboratory lasers, such as a single pass through a regeneratively amplified Ti:Sapph stage, the physics is directly analgous to an amplified ray in an astrophysical maser.) Amplified spontaneous emission (ASE) is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in an gain medium. ... For other uses, including Audio feedback, see Feedback (disambiguation) In cybernetics and control theory, feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input. ... In optics, the lasing threshold is the lowest excitation level at which laser output is dominated by stimulated emission rather than by spontaneous emission. ... The regenerative circuit (or self-regenerative circuit) allows a signal to be amplified many times by the same vacuum tube or other active component such as a field effect transistor. ... Part of a Ti-sapphire oscillator. ...

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. Townes advocated that the m stand for molecule since energy states of molecules generally provide the masing transition. Along these lines, some will use the term laser to describe any system which exploits an electronic transition and the term maser to describe a system which 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 has similar sources which they call 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 for frequencies roughly between 1 GHz and 300 GHz. Charles Hard Townes (born July 28, 1915) is an American physicist and educator. ... An astrophysicist is a person whose profession is astrophysics. ... The wavelength is the distance between repeating units of a wave pattern. ... A micrometre (American spelling: micrometer), symbol µm, is an SI unit of length. ... See also: List of optical topics Optics (appearance or look in ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter. ... A terahertz (THz) is 1012 hertz or a thousand gigahertz, a measure of frequency. ... An astronomer or astrophysicist is a scientist whose area of research is astronomy or astrophysics. ... A physicist is a scientist trained in physics. ... Ethanol, also known as ethyl alcohol or grain alcohol, is a flammable, colorless chemical compound, one of the alcohols that is most often found in alcoholic beverages. ... Electrical engineers design power systems. ... A gigahertz is a billion hertz or a thousand megahertz, a measure of frequency. ...

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. Also, while polarization in laboratory lasers and masers may be achieved by selectively oscillating the desired modes, polarization in natural masers will only arise in the presence of a polarization-state dependent pump or of a magnetic field in the gain medium. Finally, 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 (VLBI). Current flowing through a wire produces a magnetic field (M) around the wire. ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ... Very Long Baseline Interferometry (VLBI) is a type of interferometry in which the data received at each antenna in the array is paired with timing information, usually from a local atomic clock, and then stored for later analysis on magnetic tape or hard disk. ...

The major use of maser study is that they give valuable information on the conditions in space, such as temperature, number density, magnetic field and velocity, in the most interesting of environments - including stellar birth and death, and the centre of galaxies containing black-holes. The conditions involved in these events still need more accurate measuring so that theoretical models can be refined or revised.

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 people still thought that molecules could not exist in space, so the emission was at first put down to an interstellar species named Mysterium, but the emission was soon identified as line emission from OH molecules in compact sources within molecular clouds. More discoveries followed, with H₂O emission in 1969, CH₃OH emission in 1970 and SiO emission in 1974, all coming 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; First was OH emission in 1968, then H₂O emission in 1969 and SiO emission in 1974. Masers were also discovered in external galaxies in 1973, and in our own solar system in comet halos. In astronomy, stellar evolution is the sequence of changes that a star undergoes during its lifetime, the hundreds of thousands, millions or billions of years during which it emits light and heat. ...

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, and many more megamasers have since been discovered. A new type of extra-galactic maser was discovered with the detection of OH emission from Arp220 (IC 4553) with a then unequalled luminosity of ~103LO. The most luminous galactic OH maser (in HII regions) has a luminosity of ~10-3LO, so this maser was called a megamaser. Now more...

Detection

The connections of maser activity with far Infra-red (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. Particularly targeted are molecular clouds, OH-IR stars, and FIR active galaxies.

Anti-pumped absorption

Evidence for an anti-pumped sub-thermal population in the 4830 MHz transition of formaldehyde (H₂CO) was observed in 1969 by Palmer et al.

Known Interstellar Species

The following species have been observed in stimulated emission from astronomical environments:[2]

• OH
• CH
• H₂CO
• H₂O
• NH₃, ¹⁵NH₃
• CH₃OH
• SiS
• HC₃N
• SiO, ²⁹SiO, ³⁰SiO
• HCN, H¹³CN
• H

Natural Occurance Astronomical OH masers were the first masers to be discovered in space and have been observed in more environments than any other type of maser. ...

Characteristics of Maser Radiation

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

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 (as it is the exponent of the path length that is relevant), 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 path length, any variation of either will itself result in exponential change of the maser output. These exponential changes mean the maser output changes much more rapidly the physical changes causing them.

Line narrowing

Exponential gain also amplifies the centre of the gaussian line shape more than the edges or wings. This results in an emission line shape that is approximately gaussian, but much taller and 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

Since maser emission is obviously non-thermal, maser brightness is in no way indicative of their physical temperature. However a useful comparison between masers is their brightness temperature. The brightness temperature, T_B, of a maser is the temperature at which a black-body would have to be to show the same emission brightness at the wavelength of the maser. Masers have huge 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 the case of some OH masers) in a circular fashion, and to a lesser degree in a linear fashion. This polarisation is due to the Zeeman effect - interactions between magnetic fields and moving material containing dipolar molecules, and magnetic beaming of the maser radiation.

It should be noted that many of the characteristics of megamaser emission are different. A new type of extra-galactic maser was discovered with the detection of OH emission from Arp220 (IC 4553) with a then unequalled luminosity of ~103LO. The most luminous galactic OH maser (in HII regions) has a luminosity of ~10-3LO, so this maser was called a megamaser. Now more...

Maser environments

Star-forming regions

Star-forming regions (SFR), in the form of molecular clouds and giant molecular clouds (sometimes known by the related term HII regions), support the bulk of astrophysical masers. Various pumping schemes - both radiative and collisional and combinations thereof - result in the maser emission of multiple transitions of many species. For example, the OH molecule has been observed to mase at 1612, 1665, 1667, 1720, 4660, 4750, 4765, 6031, 6035, and 13440 MHz. Water and methanol masers are also typical of these environments. Relatively rare masers such as ammonia and formaldehyde may also be found in SFR's.[3] A Molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecular hydrogen, H2. ... Arguably the most famous dark nebula, the Horsehead Nebula. ... NGC 604, a giant H II region in the Triangulum Galaxy. ... Methanol, also known as methyl alcohol or wood alcohol, is a chemical compound with chemical formula CH3OH. It is the simplest alcohol, and is a light, volatile, colourless, tasteless, flammable, poisonous liquid with a very faint odor. ... Ammonia is a compound of nitrogen and hydrogen with the formula NH3. ... The chemical compound formaldehyde (also known as methanal), is a gas with a strong pungent smell. ...

Comets

Comets are small bodies (5-15km diameter) of frozen volatiles (H₂O, CO₂, NH₃, CH₄) embedded in a crusty silicate filler. They orbit the sun in eccentric orbits and as they approach the sun the volatiles vaporise to form a halo, and later a tail, around the nucleus. Once vaprorised these molecules can form inversions and mase. Comet Hale-Bopp A comet (denoted by ☄) is a small body in the solar system that orbits the sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both due primarily to the effects of solar radiation upon the comets nucleus, which itself is a...

The impact of comet Shoemaker-Levy 9 with Jupiter in 1994 resulted in maser emission at 22 GHz from the water molecule.[4] Despite the apparent rarity of these events, observation of the intense maser emission has been suggested as a detection scheme for extrasolar planets.[5] Hubble Space Telescope image of Comet Shoemaker-Levy 9, taken on May 17, 1994. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ...

Ultraviolet light from the sun breaks down some H₂O molecules forming OH molecules that can mase. In 1997, 1667-MHz maser emission from the OH molecule was observed from comet Hale-Bopp.[6] A close-up of Hale-Bopp Comet Hale-Bopp (formally designated C/1995 O1) was probably the most widely observed comet of the 20th century, and one of the brightest seen for many decades. ...

Stellar atmospheres

The conditions in the atmospheres of late-type stars support the pumping of different maser species at different distances from the star. Hydroxyl masers occur at a distance of about 1000 to 10000 astronomical units (AU), water masers at a distance of about 100 to 400 AU, and silicon monoxide masers at a distance of about 5 to 10 AU.[7] In astronomy, stellar evolution is the sequence of changes that a star undergoes during its lifetime, the hundreds of thousands, millions or billions of years during which it emits light and heat. ... The astronomical unit (AU or au or a. ...

Supernova remnants

The 1720 MHz maser transition of the OH molecule is known to be associated with supernova remnants that interact with molecular clouds.[8] The Crab Nebula is an expanding cloud of gas created by the 1054 supernova. ... A Molecular cloud is a type of interstellar clouds whose density and size permits the formation of molecular hydrogen, H2. ...

Planetary Atmospheres

It is predicted that masers exist in the atmospheres of giant planets, e.g. [9]. Such masers would be highly variable due to planetary rotation (10-hour period for Jovian planets).

Extragalactic sources

While some of the SFR masers can achieve luminosities sufficient for detection from external galaxies (such as the nearby Magellanic Clouds), masers observed from distant galaxies generally arise in wholly different conditions. Some galaxies possess central black holes into which a disk of molecular material (about 0.5 parsec in size) is falling. Excitations of these molecules in the disk or in a jet can result in megamasers with large luminosities. Hydroxyl, water, and formaldehyde masers are known to exist in these conditions. [10] The two Magellanic Clouds are irregular dwarf galaxies orbiting our Milky Way galaxy, and thus are members of our Local Group of galaxies. ... A black hole is a concentration of mass great enough that the force of gravity prevents anything from escaping it except through quantum tunnelling behaviour (known as Hawking Radiation). ... The parsec (symbol pc) is a unit of length used in astronomy. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... Relativistic Jet. ... A new type of extra-galactic maser was discovered with the detection of OH emission from Arp220 (IC 4553) with a then unequalled luminosity of ~103LO. The most luminous galactic OH maser (in HII regions) has a luminosity of ~10-3LO, so this maser was called a megamaser. Now more...

Ongoing research

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. Microwave image of 3C353 galaxy at 8. ...

Variability

Maser variability is generally understood to mean the change in apparent brightness to the observer. However, masers change in various ways over various timescales.

Masers, especially those in star-forming regions, are known to move across the sky along with the material that is flowing out from the forming star(s). Also, since the emission is a narrow spectral line, line-of-sight velocity can be determined from the Doppler shift of the maser. This permits a three-dimensional mapping of the dynamics of the maser environment. 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.[11] The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ... The Spiral Galaxy M106 (also known as Messier Object 106, Messier 106, M106, or NGC 4258) is a spiral galaxy in the Canes Venatici constellation. ...

Intensity variations can occur on timescales from days to years indicating limits on maser size and excitation scheme.

Open questions

Unlike terrestrial lasers and masers for which the excitation mechanism is known and engineered, the reverse is true for astrophysical masers. 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 polarization state (typically requiring VLBI) are all useful in the development of a pump theory. Galactic formaldehyde emission is an example of a maser that has yet to be explained.[12] In electrodynamics, polarization (also spelled polarisation) is a property of waves, such as light and other electromagnetic radiation. ...

On the other hand, some masers have been predicted to occur theoretically but have yet to be observed in nature. 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.[13] This article is about the electromagnetic phenomenon. ...

See also

• Laser
• Maser
• Amplified spontaneous emission
• Interstellar medium