Zeeman measurements of OH masers are used to probe the magnetic field around regions of massive star formation. Previous observations suggested that OH maser field directions were aligned in a clockwise sense in the Milky Way, but recent data from a large-scale VLA survey do not support this hypothesis. However, these observations suggest that the magnetic field of the Milky Way is correlated on kiloparsec scales.

The strength and structure of the magnetic fields in circumstellar envelopes have been measured through polarization observations of OH and SiO masers. Here we present the first results obtained by observing the circular polarization of the H2O masers using VLBI. SiO masers are probes of the high temperature and density regime close to the central star. OH masers are found at much lower densities and temperatures, generally much further out in the circumstellar envelope. The detection of the Zeeman splitting of the (616-523) rotational transition of the H2O maser allows us to determine the magnetic field strength in the intermediate temperature and density regime.

Study of extragalactic H2O masers has progressed significantly in the 25 years since their discovery. Existing in star forming regions and in the accretion disks supermassive black holes, they are familiar and unfamiliar at the same time. A review of how our understanding has grown, up to the present day, is followed by comments on future prospects.

We present single-dish monitoring of the 22 GHz water maser lines from the Seyfert 2 galaxies NGC 3079, M51(NGC 5194), NGC 5793, and the radio galaxy NGC 315 with the Effelsberg 100-m radio telescope. During the monitoring period of 1995 − 2001, the H2O masers flared in M51 and NGC 5793, while maser emission from NGC 315 was not detected in 1996 and 2000. During 2000, we discovered new red-shifted velocity features in NGC 3079 and blue-shifted features in M51. These velocity components are crucial to model the distribution of maser emission in each galaxy.

Water maser emission is detected towards the nuclei of 22 galaxies, all of them showing some sign of nuclear activity. Except for a couple of cases, maser features have been found only within 150 km s−1 of the systemic galaxy recession velocity but are often offset from systemic within that window. Spectral features in maser-detected galaxies are monitored regularly and these observations show that velocity drifts like those seen in the systemic masers of NGC 4258 are rare. Hence the conclusion that either masers in other galaxies are generally located farther from the central black hole than in NGC 4258, that the black holes in those galaxies are less massive than the one in NGC 4258, or that the masers' acceleration is in the plane of the sky (as in the case of maser gas at the projected extremities of an edge-on disk). Assuming the disk model holds, the last of these options leads to the possibility that, despite the relative faintness of high velocity maser features in the case of NGC 4258, the brightest maser emission originates from the projected edges of the disk, in general. A selection effect would have caused a bias, then, in previous surveys whereby masers in larger, more slowly rotating disks are favorably detected. New wide-bandwidth capabilities at the 100-m Effelsberg telescope and the emergence of the GBT will help to overcome any such bias and provide new examples of maser sources.

Our observations of H2O masers have detected some high-velocity features and a secular velocity drift of the systemic features in the Seyfert 2 Galaxy IC 2560. The high-velocity features were blue- and red-shifted from the systemic velocity of 220-420 km s−1 and 210-350 km s−1, respectively. The velocity of the systemic features drifted at a secular rate of 2.62 km s−1 yr−1. Assuming the existence of a compact rotating disk as in NGC 4258, IC 2560 possesses a nuclear disk with inner and outer radii of 0.07 pc and 0.26 pc, respectively, and a confined mass of 2.8 × 106M⊙ at the center, making the central density > 2.1 × 109M⊙ pc−3. Such a dense object cannot be a cluster of stars, and this strongly suggests that the central mass is a super-massive black hole. Since the 2-10 keV luminosity of IC 2560 is 1 × 1041 erg s−1, the mass accretion rate of the suggested black hole must be 2 × 10−5M⊙ yr−1.

We present high resolution studies of the nuclear region in NGC 3079 with phase-referenced VLBI monitoring between its water maser and continuum emission. From results of the observations, NGC 3079 showed relative motion between core-jet-like continuum components. One of the components (A) relative to the reference maser feature moved with an apparent subluminal velocity along the SE-NW direction, almost parallel to alignment of the components. On the other hand, the position of the other component (B) did not show such a significant change. If the maser gas is associated with a rotating disk, the maser gas should be gravitationally bound to the central mass. This suggests that B and A are the nucleus and jet, respectively.

The IR-properties of all the host galaxies which exhibit H2O megamaser emission have been studied in this paper. The most striking feature is the anticorrelation of the log[S(60)/S(100)] vs. log[S(12)/S(25)] and log[S(25)/S(60)] vs. log[S(12)/S(25)], and the correlation of log[S(60)/S(100)] vs. log[S(25)/S(60)]. These anticorrelations and the correlation in the flux density ratios can be explained by the coexistence of large and very small dust particles. The relationship between the luminosity of the H2O megamaser and the infrared luminosity vs. the flux density ratio of S(60)/S(100), S(25)/S(60), and S(12)/S(25) have been studied. The correlation of the luminosity vs. S(60)/S(100) and the luminosity vs. S(25)/S(60), and the anticorrelation of the lumonosity vs. S(12)/S(25) are obtained, respectively. The pumping mechanism of the H2O megamaser is discussed according to the these results. The characteristics of the central sources are also studied, according to these relationships.

The history of OH megamaser (OHM) research is reviewed, and recent developments in the field are summarized. Particular attention is paid to results from VLBI, and the recognition of a wide range of maser properties within a single galaxy. A peculiar characteristic of compact parsec-scale OHM features is a very broad velocity width, which poses a challenge for radiative pumping models, and suggests filamentary geometries for the emitting clouds. The compact masers may be associated with shocks in a violent nuclear environment, and collisional pumping may play a role. A link between the compact masers and newly formed active galactic nuclei (AGNs) cannot be ruled out. The low-gain amplification model of the diffuse maser component is strongly supported by recent data, and detailed information about the parameters of the amplifying screen is starting to emerge. OHM can be used as powerful probes of dusty, obscured IR galaxy nuclei, and the prospects for these objects being detectable, and therefore useful as probes, at cosmological distances is discussed.

We report on high-sensitivity EVN and MERLIN observations of the OH maser emission and continuum in the luminous infrared galaxy III Zw 35. Earlier VLBI observations have shown two compact maser clumps containing 40%–50% of the single-dish flux. In addition to these compact masers, we detect more diffuse maser emission which lies in a clear ring structure of radius r ∼ 22 pc. The ring appears inclined to the line of sight and the compact masers occur at the tangent points. This structure suggests that the compact and diffuse masers appear different because of geometrical effects and not pumping or other physical conditions. Our observations reveal a velocity gradient along the western side of the disc which indicates rotation. The 18 cm continuum radiation is resolved into a few unresolved sources, plus more diffuse emission and is broadly consistent with emission originating in a starburst.

We present EVN observations of hydroxyl (OH) main-line emission in two megamaser sources Mrk 231 and IC 694. The observations indicate that the broad maser emission lines originate within the nuclear regions. A single 1667 MHz main-line feature is seen at the nucleus of IC 694. In Mrk 231 both main-lines were observed in an extended “half-circular” region of 150 pc by 50 pc surrounding the radio nucleus.

We report new observations of the H2O megamaser in the Seyfert 2 galaxy Mrk 348. Following our initial detection in 2000 March using the Effelsberg 100 m telescope, a re-analysis of previous data on this source indicates that the maser was present but only marginally detectable in late 1997. Monitoring through late 2000 shows that the maser has again decreased to its original level. The H2O line is redshifted by ∼130 km s−1 with respect to the systemic velocity, is extremely broad, with a FWHM of 130 km s−1, and has no detectable high velocity components within 1500 km s−1 on either side of the strong line. Followup VLBA observations show that the maser emission emanates entirely from a region ≤0.25 pc in extent, toward the base of the radio jet.

MERLIN images of Mrk 348 at 22 GHz show H2O maser emission at 0.02–0.11 Jy, within ∼ 0.8 pc of the nucleus. This is the first direct confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk 348 was observed one month after Falcke et al. (2000) first identified the maser in single-dish spectra. The peak maser flux density has increased about threefold. The masing region is ≲ 0.6 pc in radius. The flux density of radio continuum emission from the core has been rising for about 2 years. The maser-core separation is barely resolved but at the 3σ significance level maser and continuum peaks are not coincident along the line of sight. The masers lie in the direction of the northern radio lobes and probably emanate from material shocked by a jet with velocity close to c although direct maser amplification of the core by masers tracing a Keplerian disc is not completely ruled out.

The 18-cm lines of the OH radical are the only well-documented masers in comets. They have been observed in more than 65 comets since 1973. The good knowledge of their excitation mechanisms and their linear regime allow us to estimate the water production rate and its variation with heliocentric distance from radio observations of OH in comets. Two bright comets, C/1996 B2 (Hyakutake) and C/1995 01 (Hale-Bopp), recently passed perihelion. We give an account of their observing campaigns and of the new insights they provide in the physics of comets. We also discuss the possibility of masering emission for other cometary molecules: H2O, CH, CH3OH.

This review covers some of the developments in basic theory of astronomical masers over the past ten years. Topics included are the effects of three dimensional geometry and polarization, with special emphasis on the differences between maser and non-maser radiation.

Theoretical research on extragalactic masers (and hence this review) is almost entirely directed toward 22 GHz water masers in circumnuclear disks with the focus being the near ideal masing disk at the nucleus of the galaxy NGC4258. The discussion here is organized around (1) the excitation and conditions for the masers, (2) the spatial and spectral appearance of the maser emission, and (3) the mass accretion rate and structure of the disk. In addition, a summary is given of the basic physics (spectral linebreadths and maser polarization) that underlies certain interpretations of these and other astrophysical masers.

Hundreds of strong astrophysical masers have been detected since the first, serendipitous detection of such a maser in 1965. Surprisingly, no high-gain natural lasers in the optical domain and only one in the infrared domain have been detected during these decades. What does inhibit natural lasing? An answer is proposed in this presentation.

I introduce a new model of OH maser pumping in star-forming regions, based on exact radiation transfer models of the interface between an expanding ucHII region and undisturbed molecular material. The model incorporates the most recent set of rate-coefficients for collisions of OH with molecular hydrogen, and an improved model of continuum emission from dust. FIR line overlap is incorporated into the model in a manner which avoids the inconsistencies associated with the Sobolev approximation.

We obtain integrated gains for the OH ground-state lines from an exact radiation transfer code with parameters suitable for a large molecular cloud. Results show that the lines are usually formed in NLTE, and that the satellite lines typically show conjugate behaviour. Constraints are set on the OH abundance and kinetic temperature by the requirement that there be no strong maser emission in the main lines.

Electron cyclotron maser emission is an accepted physical mechanism for generating coherent planetary and stellar radio emissions. Observational data has indicated evidence of nonlinear and chaotic temporal variability in some cosmic masers such as solar microwave spikes. The nonlinear and chaotic characteristics of cosmic masers can be attributed to plasma turbulence, such as Alfvén chaos, embedded in the emission region. We report a chaos theory of Alfvén waves which can account for chaotic acceleration of electrons in the source region of cosmic masers. Two types of Alfvén intermittency are identified: Pomeau-Manneville intermittency and crisis-induced intermittency. Since Alfvén waves may be responsible for accelerating electrons that emit maser radiations, the chaotic dynamics of Alfvén waves may be the origin of chaotic time variations of cosmic masers. Hence, we suggest that Alfvén intermittency may cause intermittent temporal fluctuations which can be observed in cosmic masers.