Nuclear Maser Research Articles

Molecular line emission in NGC 4945, imaged with ALMA

NGC 4945 is one of the nearest (D ≈ 3.8 Mpc; 1″ ≈ 19 pc) starburst galaxies. To investigate the structure, dynamics, and composition of the dense nuclear gas of this galaxy, ALMA band 3 (λ ≈ 3−4 mm) observations were carried out with ≈2″ resolution. Three HCN and two HCO+ isotopologues, CS, C3H2, SiO, HCO, and CH3C2H were measured. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10″ × 2″ reaching out to a galactocentric radius of r ≈ 100 pc with position angle PA = 45° ± 2°, inclination i = 75° ± 2° and an unresolved bright central core of size ≲2″. The continuum source, representing mostly free-free radiation from star forming regions, is more compact than the nuclear disk by a linear factor of two but shows the same position angle and is centered 0.′′39 ± 0.′′14 northeast of the nuclear accretion disk defined by H2O maser emission. Near the systemic velocity but outside the nuclear disk, both HCN J = 1 → 0 and CS J = 2 → 1 delineate molecular arms of length ≳15″ (≳285 pc) on opposite sides of the dynamical center. These are connected by a (deprojected) ≈ 0.6 kpc sized molecular bridge, likely a dense gaseous bar seen almost ends-on, shifting gas from the front and back side into the nuclear disk. Modeling this nuclear disk located farther inside (r ≲100 pc) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojected velocity of ≈50 km s−1. All our molecular lines, with the notable exception of CH3 C2H, show significant absorption near the systemic velocity (≈571 km s−1), within the range ≈500–660 km s−1. Apparently, only molecular transitions with low critical H2 density (ncrit ≲ 104 cm−3) do not show absorption. The velocity field of the nuclear disk, derived from CH3 C2H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of Mtot = (2.1 ± 0.2) × 108 M⊙ inside a galactocentric radius of 2.′′45 (≈45 pc), with a significantly flattened rotation curve farther out. Velocity integrated line intensity maps with most pronounced absorption show molecular peak positions up to ≈1.′′5 (≈30 pc) southwest of the continuum peak, presumably due to absorption, which appears to be most severe slightly northeast of the nuclear maser disk. A nitrogen isotope ratio of 14N/15N ≈ 200–450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because 15N is less abundant than expected, the question for strong 15N enrichment by massive star ejecta in starbursts still remains to be settled.

Nicolaas Bloembergen as a scientist and a mentor

Nicolaas Bloembergen made rich contributions to nuclear magnetic resonance, masers and lasers, nonlinear optics and ultrafast laser–matter interactions. The Nobel laureate sadly passed away on 5 September 2017. Here are my memories of my Harvard mentor, a remarkable person and a wonderful scientist.

DISCOVERY OF NUCLEAR WATER MASER EMISSION IN CENTAURUS A

We report the detection of a 22GHz water maser line in the nearest (D~3.8Mpc) radio galaxy Centaurus A using the Australia Telescope Compact Array (ATCA). The line is centered at a velocity of ~960kms-1, which is redshifted by about 400kms-1 from the systemic velocity. Such an offset, as well as the width of ~120kms-1, could be consistent with either a nuclear maser arising from an accretion disk of the central supermassive black hole, or for a jet maser that is emitted from the material that is shocked near the base of the jet in Centaurus\,A. The best spatial resolution of our ATCA data constrains the origin of the maser feature within <3pc from the supermassive black hole. The maser exhibits a luminosity of ~1Lo, which classifies it as a kilomaser, and appears to be variable on timescales of months. A kilomaser can also be emitted by shocked gas in star forming regions. Given the small projected distance from the core, the large offset from systemic velocity, as well as the smoothness of the line feature, we conclude that a jet maser line emitted by shocked gas around the base of the AGN is the most likely explanation. For this scenario we can infer a minimum density of the radio jet of ~>10cm-3, which indicates substantial mass entrainment of surrounding gas into the propagating jet material.

Resolving the obscuring torus in NGC 1068 with the power of infrared interferometry: revealing the inner funnel of dust

We present new interferometric data obtained with MIDI (MID infrared Interferometric instrument) for the Seyfert II galaxy NGC 1068, with an extensive coverage of sixteen uv points. These observations resolve the nuclear mid-infrared emission from NGC 1068 in unprecedented detail with a maximum resolution of 7 mas. For the first time, sufficient uv points have been obtained, allowing us to generate an image of the source using maximum entropy image reconstruction. The features of the image are similar to those obtained by modelling. We find that the mid-infrared emission can be represented by two components, each with a Gaussian brightness distribution. The first, identified as the inner funnel of the obscuring torus, is hot (800K), 1.35 parsec long, and 0.45 parsec thick in FWHM at a PA=-42 degrees (from north to east). It has an absorption profile different than standard interstellar dust and with evidence for clumpiness. The second component is 3 by 4 pc in FWHM with T=300K, and we identify it with the cooler body of the torus. The compact component is tilted by 45 degrees with respect to the radio jet and has similar size and orientation to the observed water maser distribution. We show how the dust distribution relates to other observables within a few parsecs of the core of the galaxy such as the nuclear masers, the radio jet, and the ionization cone. We compare our findings to a similar study of the Circinus galaxy and other relevant studies. Our findings shed new light on the relation between the different parsec-scale components in NGC 1068 and the obscuring torus.

Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy

To test the dust torus model for active galactic nuclei directly, we study the extent and morphology of the nuclear dust distribution in the Circinus galaxy using high resolution interferometric observations in the mid-infrared with the MIDI instrument at the Very Large Telescope Interferometer. We find that the dust distribution in the nucleus of Circinus can be explained by two components, a dense and warm disk-like component of 0.4 pc size and a slightly cooler, geometrically thick torus component with a size of 2.0 pc. The disk component is oriented perpendicular to the ionisation cone and outflow and seems to show the silicate feature at 10 micron in emission. It coincides with a nuclear maser disk in orientation and size. From the energy needed to heat the dust, we infer a luminosity of the accretion disk corresponding to 20% of the Eddington luminosity of the nuclear black hole. We find that the interferometric data are inconsistent with a simple, smooth and axisymmetric dust emission. The irregular behaviour of the visibilities and the shallow decrease of the dust temperature with radius provide strong evidence for a clumpy or filamentary dust structure. We see no evidence for dust reprocessing, as the silicate absorption profile is consistent with that of standard galactic dust. We argue that the collimation of the ionising radiation must originate in the geometrically thick torus component. Our findings confirm the presence of a geometrically thick, torus-like dust distribution in the nucleus of Circinus, as required in unified schemes of Seyfert galaxies. Several aspects of our data require that this torus is irregular, or clumpy.

The Nature of the Nuclear H2O Masers of NGC 1068: Reverberation and Evidence for a Rotating Disk Geometry

We report new (1995) Very Large Array observations and (1984 - 1999) Effelsberg 100m monitoring observations of the 22 GHz H2O maser spectrum of the Seyfert 2 galaxy NGC 1068. The sensitive VLA observations provide a registration of the 22 GHz continuum emission and the location of the maser spots with an accuracy of ~ 5 mas. Within the monitoring data, we find evidence that the nuclear masers vary coherently on time-scales of months to years, much more rapidly than the dynamical time-scale. We argue that the nuclear masers are responding in reverberation to a central power source, presumably the central engine. Between October and November 1997, we detected a simultaneous flare of the blue-shifted and red-shifted satellite maser lines. Reverberation in a rotating disk naturally explains the simultaneous flaring. There is also evidence that near-infrared emission from dust grains associated with the maser disk also responds to the central engine. We present a model in which an X-ray flare results in both the loss of maser signal in 1990 and the peak of the near-infrared light curve in 1994. In support of a rotating disk geometry for the nuclear masers, we find no evidence for centripetal accelerations of the redshifted nuclear masers; the limits are +/- 0.006 km/s/year, implying that the masers are located within 2 degrees of the kinematic line-of-nodes. We also searched for high velocity maser emission like that observed in NGC 4258. In both VLA and Effelsberg spectra, we detect no high velocity lines between +/- 350 km/s to +/- 850 km/s relative to systemic, arguing that masers only lie outside a radius of ~ 0.6 pc (1.9 light years) from the central engine (assuming a distance of 14.4 Mpc).

BeppoSAXObservations of the Maser Seyfert 2 Galaxy ESO 103‐G35

We have made BeppoSAX observations of the Seyfert 2/1.9 galaxy ESO 103-G35, which contains a nuclear maser source and is known to be heavily absorbed in the X-rays. Analysis of the X-ray spectra observed by BeppoSAX in 1996 October and 1997 October yields a spectral index αE = 0.74 ± 0.07 (Fν ∝ ν), which is typical of Seyfert galaxies and consistent with earlier observations of this source. The strong, soft X-ray absorption has a column density NH of 1.79 ± 0.09 × 1023 cm-2, again consistent with earlier results. The best-fitting spectrum is that of a power law with a high-energy cutoff at 29 ± 10 keV, a cold (E = 6.3 ± 0.1 keV, rest frame), marginally resolved (σ = 0.35 ± 0.14 keV, FWHM ~ 31 ± 12 × 103 km s-1) Fe Kα line with EW 290 eV (1996), and a mildly ionized Fe K edge at 7.37 keV, τ = 0.24. The Kα line and cold absorption are consistent with origin in an accretion disk/torus through which our line of sight passes at a radial distance of ~50 pc. The Fe K edge is mildly ionized, suggesting the presence of ionized gas, probably in the inner accretion disk close to the central source or in a separate warm absorber. The data quality is too low to distinguish between these possibilities, but the edge-on geometry implied by the water maser emission favors the former. Comparison with earlier observations of ESO 103-G35 shows little or no change in spectral parameters while the flux changes by factors of a few on timescales of a few months. The 2-10 keV flux decreased by a factor of ~2.7 between 1996 October and 1997 October with no detectable change in the count rate greater than 20 keV (i.e., the Phoswich Detector System data). Spectral fits to the combined data sets indicate either a significant hardening of the spectrum (αE ~ 0.5) or an approximately constant or delayed response reflection component. The high-energy cutoff (29 ± 10 keV) is lower than the typical ~300 keV values seen in Seyfert galaxies. A significant subset of similar sources would affect current models of the active galactic nucleus contribution to the cosmic X-ray background which generally assume a high-energy cutoff of ~300 keV.

Water-vapour maser emission from galactic nuclei

We report here the progress of a survey of galactic nuclei for water-vapour maser emission at 22.235 GHz. We observed 29 late-type galaxies in November 1982, January 1983, and September 1983 using the 40-m radio telescope of the Owens Valley Radio Observatory (OVRO) equipped with a travelling-wave maser receiver1. We have detected maser emission from four nuclei: NGC4258 (M106), NGC1068 (M77), NGC3034 (M82), and NGC6946. The masers in NGC4258 and NGC1068 are extremely luminous; NGC1068 has the most luminous water-vapour maser yet reported—350 L⊙ assuming isotropic emission. We suggest that the extremely bright nuclear masers indicate an ongoing burst of star formation, and therefore provide a useful probe of the physical conditions of the starburst phenomenon.

Nuclear spin analogue of a molecular beam maser with cavities in series

The operation of a nuclear spin maser oscillator with two independent resonant coils through which the active maser medium flows in succession is described. It is shown experimentally that the variation of the oscillation amplitude in the second coil as a function of particular parameters of the nuclear maser system is analogous to that obtained with a molecular beam maser operated with two cavities in series. A simple univelocity theory is presented which accounts for the gross features of the experimental results obtained with the two-coil nuclear maser. The influence of the velocity distribution of the nuclei is also investigated.

Theory of a proton spin maser with two emission coils

A nuclear maser oscillator with two emission coils, coupled solely by a stream of a prepolarized liquid which is initially at negative spin temperature, is investigated theoretically. The second emission coil is assumed to be tuned by a condenser to the proton spin resonance, while the first emission coil can be detuned. If the spin-spin interaction time is sufficiently long, then the voltage amplitude available from the terminals of the second coil can go through two maxima as the first coil is detuned. For maximum detuning of the first coil, a self-modulation effect can arise in the second resonance circuit.

Method for precise tuning of a nuclear maser magnetometer

A method is described which permits exact tuning of a nuclear maser oscillator type of magnetometer using a liquid or gaseous maser medium which flows through two decoupled coils in succession.

Quantum Radio Frequency Physics

D. Stepin (translated by Scripta Technica Inc.; edited by H. H. Stroke) Cambridge, Mass: M.I.T. Press. 1965. Pp. viii + 227. Price £2 17s. This book aims to provide an introduction to electron and nuclear magnetic resonance, masers and radiofrequency spectrometry. It concludes with a chapter on ammonia-beam oscillators.

A Secondary Standard of Current Based on a Nuclear Resonance Spin Oscillator

A description is given of the use of a nuclear spin oscillator as a transducer, connecting a magnetic field (10 g) to a frequency (42 kc/s). It is shown that for this function the spin oscillator is to be preferred to the nuclear maser. The system offers the advantage of permitting work in an ordinary laboratory with a background of parasitic field of some 1 mg; this insensibility to fluctuations is very convenient for the preliminary adjustment. For an accurate determination of current by means of a standard solenoid the stray field must be compensated statically and dynamically; for this purpose a feedback arrangement using a galvanometer-photocell amplifier is suggested. Calculations and experiments on an efficient mu-metal screen are presented. But this last arrangement, for absolute measurement of I, would require too large a screen when using a solenoid that shows a high-leakage field. The screen looks promising when Helmholtz coils are used, if construction can be perfected and if a sufficient degree of accuracy and reliability can be reached.