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 (VLA) observations and (1984-1999) Effelsberg 100 m 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 timescales of months to years, much more rapidly than the dynamical timescale. We argue that the nuclear masers are responding in reverberation to a central power source, presumably the central engine. Between 1997 October and November, we detected a simultaneous flare of the blueshifted and redshifted satellite maser lines. Reverberation in a rotating disk naturally explains the simultaneous flaring. There is also evidence that near-IR 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-IR light curve in 1994. In support of 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-1 yr-1, implying that the masers are located within 2° 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 approximately ±350 and ±850 km s-1 relative to systemic, arguing that masers only lie outside a radius of ~0.6 pc (1.9 lt-yr) from the central engine (assuming a distance of 14.4 Mpc). We also consider possible models for the jet masers near radio continuum component C. We favor a shock precursor model, in which the molecular gas surrounding the jet is heated by X-ray emission from a shock front between the jet and a molecular cloud.

[1]  D. Neufeld.  Response of Circumnuclear Water Masers to Luminosity Changes in an Active Galactic Nucleus , 2000, astro-ph/0008325.

[2]  J. M. Moran,et al.  A geometric distance to the galaxy NGC4258 from orbital motions in a nuclear gas disk , 1999, Nature.

[3]  G. Neugebauer,et al.  Diffraction-limited Imaging and Photometry of NGC 1068 , 1999, astro-ph/9904076.

[4]  Makoto Kishimoto,et al.  The Location of the Nucleus of NGC 1068 and the Three-dimensional Structure of Its Nuclear Region , 1999, astro-ph/9902346.

[5]  Lunar,et al.  Workshop on Thermal Emission Spectroscopy and Analysis of Dust, Disks, and Regoliths : Houston, Texas April 28-30, 1999 , 1999 .

[6]  A. Sprague,et al.  Workshop on Thermal Emission Spectroscopy and Analysis of Dust, Disk, and Regoliths , 1999 .

[7]  Edward J. M. Colbert,et al.  The Compact Radio Sources in the Nucleus of NGC 1068 , 1998 .

[8]  R. Terlevich,et al.  HI Observations of NGC 1068 , 1997 .

[9]  Stefi A. Baum,et al.  A direct image of the obscuring disk surrounding an active galactic nucleus , 1997, Nature.

[10]  B. Peterson,et al.  Reverberation Mapping and the Physics of Active Galactic Nuclei , 1997, astro-ph/9706039.

[11]  Y. Taniguchi,et al.  Why Does NGC 1068 Have a More Powerful Active Galactic Nucleus than NGC 4258 , 1997, astro-ph/9705076.

[12]  C. McKee,et al.  Doppler Shift Asymmetry in High-Velocity Maser Emission from Shocks in Circumnuclear Disks , 1997, astro-ph/9704050.

[13]  F. Macchetto,et al.  The Nuclear Region of NGC 1068: High-Accuracy Alignment of the Optical and Radio Emission , 1997 .

[14]  W. D. Watson,et al.  Excitation for Masing by Water in X-irradiated Environments at the Nuclei of Active Galaxies , 1997 .

[15]  C. Gwinn,et al.  VLBI Imaging of Water Maser Emission from a Nuclear Disk in NGC 1068 , 1997 .

[16]  I. Glass.  Variability in the Nucleus of NGC 1068 , 1997 .

[17]  J. Bland-Hawthorn,et al.  A well-fed black hole , 1997, Nature.

[18]  Dan Maoz,et al.  Astronomical Time Series , 1997 .

[19]  W. Baan,et al.  Molecular Tori and H2O Megamaser Variability , 1996 .

[20]  R. Antonucci,et al.  VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068 , 1996, astro-ph/9609082.

[21]  S. Terebey,et al.  A Monthly Survey of Water Masers Associated with Low-Mass Stars , 1996 .

[22]  B. Nelson.  A Correlated Optical-Infrared Outburst of Markarian 744: The Strongest Evidence Yet for Thermal Dust Reverberation , 1996 .

[23]  A. Tielens,et al.  X-Ray--irradiated Molecular Gas. I. Physical Processes and General Results , 1996 .

[24]  S. Baum,et al.  The Subarcsecond Radio Structure in NGC 1068. II. Implications for the Central Engine and Unifying Schemes , 1996 .

[25]  S. Baum,et al.  H 2O and OH Masers as Probes of the Obscuring Torus in NGC 1068 , 1996 .

[26]  S. Baum,et al.  The Subarcsecond Radio Structure in NGC 1068. I. Observations and Results , 1996 .

[27]  R. Antonucci,et al.  The compact radio nucleus of the Seyfert galaxy NGC 1068 , 1996 .

[28]  M. Dopita,et al.  Spectral Signatures of Fast Shocks. II. Optical Diagnostic Diagrams , 1995 .

[29]  W. Sparks,et al.  Hubble Space Telescope Imaging Polarimetry of the Inner Nuclear Region of NGC 1068 , 1995 .

[30]  D. Neufeld,et al.  The Mass Accretion Rate through the Masing Molecular Disk in the Active Galaxy NGC 4258 , 1995 .

[31]  Naomasa Nakai,et al.  Evidence for a black hole from high rotation velocities in a sub-parsec region of NGC4258 , 1995, Nature.

[32]  D. Neufeld,et al.  Water maser emission from X-ray-heated circumnuclear gas in active galaxies , 1994 .

[33]  A. Königl,et al.  DISK-DRIVEN HYDROMAGNETIC WINDS AS A KEY INGREDIENT OF ACTIVE GALACTIC NUCLEI UNIFICATION SCHEMES , 1994 .

[34]  A. Tielens,et al.  Grain destruction in shocks in the interstellar medium , 1994 .

[35]  W. D. Watson,et al.  Evidence from masers for a rapidly rotating disk at the nucleus of NGC 4258 , 1994 .

[36]  E. Pier,et al.  The intrinsic nuclear spectrum of NGC 1068 , 1994 .

[37]  Julian H. Krolik,et al.  Infrared Spectra of Obscuring Dust Tori around Active Galactic Nuclei. II. Comparison with Observations , 1993 .

[38]  M. Inoue,et al.  Extremely-high-velocity H20 maser emission in the galaxy NGC4258 , 1993, Nature.

[39]  R. Barvainis,et al.  Dust reverberation : a model for the infrared variations of Fairall 9 , 1992 .

[40]  G. Voit.  X-ray irradiation of interstellar grains in active galaxies : evaporation and infrared spectra , 1991 .

[41]  R. Goodrich,et al.  Multidirectional views of the active nucleus of NGC 1068 , 1991 .

[42]  Julian H. Krolik,et al.  The physical state of the obscuring torus in Seyfert galaxies , 1989 .

[43]  D. Hollenbach,et al.  H2O masers in star-forming regions , 1989 .

[44]  Willem Wamsteker,et al.  Hot dust on the outskirts of the broad-line region in Fairall 9 , 1989 .

[45]  R. Tully.  Nearby Galaxies Catalog , 1988 .

[46]  Carl Heiles,et al.  Galactic and Extragalactic Radio Astronomy , 1988 .

[47]  M. Claussen,et al.  Circumnuclear water vapor masers in active galaxies , 1986 .

[48]  J C Tarter,et al.  A cloud collision model for water maser excitation. , 1986, The Astrophysical journal.

[49]  P. I. Nelson,et al.  Statistical methods for astronomical data with upper limits. II - Correlation and regression , 1986 .

[50]  J. Miller,et al.  Spectropolarimetry and the nature of NGC 1068 , 1985 .

[51]  H. M. Lee,et al.  Optical properties of interstellar graphite and silicate grains , 1984 .

[52]  M. Claussen,et al.  Water-vapour maser emission from galactic nuclei , 1984, Nature.

[53]  E. Ney,et al.  The isothermal dust condensation of Nova Vulpeculae 1976 , 1978 .

[54]  K. Nordsieck,et al.  The Size distribution of interstellar grains , 1977 .

[55]  E. Salpeter.  FORMATION AND DESTRUCTION OF DUST GRAINS , 1977 .

[56]  D. Schramm,et al.  Dust in the Universe , 1977 .

[57]  D. Weedman,et al.  An atlas of Seyfert galaxies , 1974 .

[58]  Kenneth I. Kellermann,et al.  Galactic and Extragalactic Radio Astronomy , 1974 .

[59]  P. Goldreich,et al.  Astrophysical Masers. V. Pump Mechanism for h, O Masers , 1974 .

[60]  C. K. Seyfert.  Nuclear Emission in Spiral Nebulae. , 1943 .