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ScienceWeek
ASTROPHYSICS: ON ASTRONOMICAL MASER EMISSIONS
The following points are made by Moshe Elitzur (Science 2005 309:71):
1) In 1963, radio emission from interstellar OH (the hydroxyl radical) was discovered. The emission patterns in the astronomical sources deviated considerably from expectations based on laboratory conditions. Two years later, researchers realized that some of the most peculiar emission properties of interstellar OH could only be explained in terms of maser amplification. (Maser is an acronym for microwave amplification by stimulated emission of radiation; masers operate on the same principles as lasers, except that they involve microwave radiation instead of visible light.)
2) Maser emission has now been detected from many different molecules in a variety of astronomical sources, from nearby comets to faraway galaxies. But the evidence for amplification is indirect in most cases. Observations from 18 pulsars, reported by Weisberg et al[1], provide direct evidence for an interstellar amplifier in the direction of one of these pulsars, B1641-45.
3) Every 0.455 seconds, B1641-45 emits a pulse of radio radiation toward Earth that passes through an OH cloud. Spectra of the four OH ground-state lines detected from the cloud display absorption features in three lines and an emission feature in the fourth. When the pulsar is on, passage of its radiation through the cloud deepens the absorption features, just like the shadow cast by an object in front of a bright light. But the emission feature in the fourth line becomes stronger after passing through the intervening screen, the equivalent of an object amplifying a background light instead of casting a shadow.
4) Almost 30 years ago, Rieu et al[2] observed a similar effect for an OH cloud in front of a distant radio galaxy when they switched the telescope between pointing directly at the source and pointing away from it. In both cases [1,2], the input signals are amplified by only a few percent, but to date, these weak masers are the only unambiguous direct evidence for amplification. The pulsar method [1] increases confidence in the results, thanks to the repeated detection of signals that arrive more than twice every second.[3-5]
References (abridged):
1. J. M. Weisberg, S. Johnston, B. Koribalski, S. Stanimirovíc, Science 309, 106 (2005)
2. N. Q. Rieu et al., Astron. Astrophys. 46, 413 (1976)
3. M. Miyoshi et al., Nature 373, 127 (1995)
4. J. Crovisier et al., Astron. Astrophys. 393, 1053 (2002)
5. N. Kanekar et al., Phys. Rev. Lett. 93, 051302 (2004)
Science http://www.sciencemag.org
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ASTRONOMY: ON ASTRONOMICAL MASERS
The following points are made by Mark Claussen (Science 2004 306:235):
1) Maser emission from molecules such as water, hydroxyl (OH), and silicon monoxide (SiO) is an important tracer of the gas kinematics and magnetic field strength in astrophysically interesting regions. Since their discovery in 1965, these emissions have provided clues about the molecular gas in and around young stellar and protostellar objects, around stars at the end of their life, at the interface of supernova remnants and molecular clouds, and near the black holes at the centers of active galaxies. Because they are bright, they can be observed with the finest angular resolution currently possible in astronomy. They can thus be used to probe much smaller physical scales than with other astronomical methods, and to infer accurate distances to objects within and outside the Milky Way.
2) The first interstellar masers were discovered from the ground state of OH (at a wavelength near 18 cm) but were not recognized as such initially (1). It was only because laboratory masers had already been invented (2) that the discoverers could understand the physical mechanism of the maser. Many early observations characterized the emission from OH masers as time-variable, polarized (both linearly and circularly), and having narrow line widths. These characteristics are typical of most astronomical masers.
3) As radio telescopes became more sensitive and able to look at a broader range of frequencies, and interferometry provided much better angular resolution, more molecular masers were discovered and their sizes were measured. Brightness temperatures (the temperature that they would have if they were emitting as thermal sources) of greater than 10^(9) K and sizes of less than 0.001 arc sec (1 arc sec = ~1/2000 of the angular diameter of the Sun as seen from Earth) were found to be typical of these natural masers. Masers have been found in transitions of OH, SiO, water, methanol, ammonia, and other molecules, and also in recombination lines of hydrogen.
4) The study of masers has gone hand-in-hand with the development of very long baseline interferometry (VLBI), which enables angular resolutions of 0.0001 arc sec at the highest radio frequencies. The Very Long Baseline Array (VLBA), built and operated since 1993 by the National Radio Astronomy Observatory, has provided the lion's share of recent maser observations.
5) Masers occur in several places in the Universe: in the vicinity of newly forming stars and regions of ionized hydrogen (H II regions) (OH, water, SiO, and methanol masers); in the circumstellar shells of stars at the end of their life -- that is, red giants and supergiants (OH, water, and SiO masers); in the shocked regions where supernova remnants are expanding into an adjacent molecular cloud (OH masers); and in the nuclei and jets of active galaxies (OH and water masers).(3-5)
References (abridged):
1. H. Weaver, D. R. W. Williams, N. H. Dieter, T. W. Lum, Nature 208, 29 (1965)
2. R. L. Walsworth, Science 306, 236 (2004)
3. R. Genzel, M. J. Reid, J. M. Moran, D. Downes, Astrophys. J. 244, 884 (1981)
4. M. J. Claussen, K. B. Marvel, A. Wootten, B. A. Wilking, Astrophys. J. 507, L79 (1998)
5. V. L. Fish, M. J. Reid, A. L. Argon, K. M. Menten, Astrophys. J. 596, 328 (2003)
Science http://www.sciencemag.org
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Notes by ScienceWeek:
optical interferometer: An interferometer combines the light from two or more widely spaced mirrors, resulting in an array with the same resolution as a single large telescope of the same diameter as the separation between the mirrors. Radio astronomers using the Very Long Baseline Array are thus able simulate a single radio telescope the size of North America. Unfortunately, most stars emit radiation at optical rather than radio wavelengths, and technical obstacles have prevented optical telescopes being built on this scale. The Palomar Testbed Interferometer (PTI), located on Palomar Mountain in California (US), has the resolving capability of a single 110-meter infrared telescope, much larger than any single telescope built so far, and good enough to resolve stars as small as a few milliarcseconds in diameter. Such interferometers are now operating or under construction in Britain, continental Europe, Australia, Hawaii, and North and South America.
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APPLIED PHYSICS: ON THE MASER
The following points are made by Ronald L. Walsworth (Science 2004 306:236):
1) In 1954, Gordon, Zeiger, and Townes (1) developed the ammonia maser, the first device to demonstrate "Microwave Amplification by Stimulated Emission of Radiation" from atoms or molecules. The maser and its younger optical cousin, the laser, remain prototypical examples of the powerful technologies inspired by quantum mechanics and 20th-century physics. Today, masers are extending the reach of quantum mechanics to revolutionary new methods of computation and communication and are probing theories that seek to unify quantum mechanics with general relativity --the other major part of 20th-century physics.
2) Masers produce coherent, monochromatic electromagnetic radiation at a characteristic frequency and wavelength. All share a few general features:
a) A "population inversion" -- that is, a larger population in the higher energy of two selected quantum states of an ensemble of atoms, molecules, or ions -- is created in the maser medium. Through stimulated emission, the population inversion amplifies electromagnetic fields that are resonant with the transition frequency between the two quantum states.
b) A surrounding electromagnetic resonator is tuned to the maser medium's transition frequency. The resonator typically has low electromagnetic loss at its resonant frequency, and thereby enhances the ability of electromagnetic fields to induce stimulated emission by the maser.
c) Some fraction of the radiated electromagnetic field is released from the resonator to provide the output signal.
d) In many masers, a steady, continuous output is desired. Such "active oscillation" has two requirements: There must be a continuous means of creating a population inversion, and the time for self-induced maser action (the radiation damping time) must be shorter than the decay time for the radiating electromagnetic moment of the maser medium (that is, the decay time for a coherent superposition of the two quantum states).
These conditions are met in a wide variety of systems. Indeed, the definition of a maser has expanded since 1954 to include the entire audio-to-microwave range of the electromagnetic spectrum, corresponding to wavelengths of millimeters to kilometers.
3) To operate at these long wavelengths, masers usually exploit magnetic dipole transitions (such as hyperfine or Zeeman transitions) in atoms, molecules, and other media. Because magnetic dipoles interact weakly with each other, with electromagnetic fields, and with environmental perturbations, masers typically provide weak but spectrally pure and temporally stable signals. An important exception to this weak signal behavior is the electron cyclotron maser, which can be used to create very high power signals -- up to hundreds of thousands of watts -- in the millimeter wavelength regime (2). When placed in a very cold environment, masers can also amplify applied resonant signals with very little added noise beyond the small effects of spontaneous emission and remnant thermal (blackbody) radiation.(3-5)
References (abridged):
1. J. P. Gordon, H. J. Zeiger, C. H. Townes, Phys. Rev. 95, 282 (1954)
2. K. R. Chu, Rev. Mod. Phys. 76, 489 (2004)
3. H. M. Goldenberg, D. Kleppner, N. F. Ramsey, Phys. Rev. Lett. 5, 361 (1960)
4. J. M. Raimond, M. Brune, S. Haroche, Rev. Mod. Phys. 73, 565 (2001)
5. B. T. H. Varcoe, S. Brattke, M. Weidinger, H. Walther, Nature 403, 743 (2000)
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