The Geometry of and Mass Accretion Rate through the Maser Accretion Disk in NGC 4258

A maximum likelihood analysis of the NGC 4258 maser positions and velocities reveals a ~2 σ deviation from Keplerian motion in the projected rotation curve of the high-velocity features, corresponding to a ~9 km s-1, or 0.8%, flattening of the LOS velocities with respect to Keplerian motion over the range of the high-velocity masers. While there are a number of potential explanations for this flattening, we argue for pure Keplerian rotation in an inclination-warped disk on the basis of the ability of this model to explain a number of otherwise puzzling features of the system. A radial gradient in the disk inclination of 0.034 mas-1 is not only consistent with the observed rotation curve, but it generates a bowl along the near edge of the disk that naturally explains the otherwise puzzling narrow spread in the declinations of the systemic masers. It also explains the existence and location of an apparently recurring flare among the systemic masers. There is no significant evidence for non-Keplerian rotation in the inclination-warped disk. An additional implication of the inclination warp is that the disk rises in front of and obscures the central engine at a disk radius of about 8.3 mas, or 0.29 pc. By comparing the observed X-ray column to conditions in the disk at this radius, we argue that the disk must be atomic at 0.29 pc. Hence, we conclude that the molecular-to-atomic transition occurs just beyond the outermost maser at 0.28 pc, and from this we infer an accretion rate of ~10-4α M☉ yr-1, where α (1) is the standard dimensionless parameterization of the kinematic viscosity. Our model suggests that most of the observed X-ray column arises in the warped accretion disk at 0.29 pc and that the maser emission is truncated at large radii predominantly as a result of the molecular-to-atomic phase transition originally proposed by Neufeld & Maloney. The inferred accretion rate is consistent with the jet-dominated accretion models of Yuan et al.

[1]  R. Chary,et al.  NGC 4258: A Compact Central Infrared Source Revealed , 1996 .

[2]  L. Ho,et al.  Low-Luminosity Active Galactic Nuclei at the Highest Resolution: Jets or Accretion Flows? , 2003, astro-ph/0311303.

[3]  H. Falcke,et al.  NGC4258: a jet-dominated low-luminosity AGN? , 2002, astro-ph/0205531.

[4]  R. Blandford,et al.  What Is the Accretion Rate in NGC 4258? , 1998, astro-ph/9808036.

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

[6]  J. Moran,et al.  Discovery of a Subparsec Jet 4000 Radii Away from the Central Schwarzschild Engine of NGC 4258 , 1997 .

[7]  K. Lo,et al.  The optical jet of the galaxy NGC 4258: interaction with the interstellar medium , 1989 .

[8]  L. A. Antonelli,et al.  The BeppoSAX View of the X-Ray Active Nucleus of NGC 4258 , 2001, astro-ph/0102438.

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

[10]  James M. Moran,et al.  The Warp in the Subparsec Molecular Disk in NGC 4258 as an Explanation for Persistent Asymmetries in the Maser Spectrum , 1996 .

[11]  G. Cecil,et al.  Hot Shocked Gas along the Jets of NGC 4258 (M106) , 1995 .

[12]  E. Peng,et al.  The Masering Torus in NGC 4258 , 1994 .

[13]  Roger D. Blandford,et al.  Relativistic jets as compact radio sources , 1979 .

[14]  Eyal Maoz Dynamical Constraints on Alternatives to Supermassive Black Holes in Galactic Nuclei , 1997, astro-ph/9710309.

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

[16]  J. Moran,et al.  Probing active galactic nuclei with H2O megamasers. , 1995, Proceedings of the National Academy of Sciences of the United States of America.

[17]  Paul S. Smith,et al.  Optical Detection of the Hidden Nuclear Engine in NGC 4258 , 1995, astro-ph/9510006.

[18]  E. Quataert Radiatively Inefficient Accretion Flow Models of Sgr A , 2003, astro-ph/0304099.

[19]  H. Ford,et al.  Bubbles and braided jets in galaxies with compact radio nuclei , 1986 .

[20]  A. Evans,et al.  High-Resolution Infrared Imaging of the Compact Nuclear Source in NGC 4258 , 1999, astro-ph/9910557.

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

[22]  Eyal Maoz A Stringent Constraint on Alternatives to a Massive Black Hole at the Center of NGC 4258 , 1995, astro-ph/9503113.

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

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

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

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

[27]  G. Cecil,et al.  The braided jets in the spiral galaxy NGC 4258 , 1992 .

[28]  J. M. Moran,et al.  Accelerations of Water Masers in NGC 4258 , 2000, astro-ph/0001543.

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

[30]  Probing the magnetic FIELD at subparsec radii in the accretion disk of NGC 4258 , 2005, astro-ph/0502240.

[31]  M. Elvis,et al.  Ubiquitous Variability of X-Ray-absorbing Column Densities in Seyfert 2 Galaxies , 2001, astro-ph/0107510.

[32]  Andrew King,et al.  Accretion Power in Astrophysics: Contents , 2002 .

[33]  L. Ho,et al.  Double-peaked Broad Emission Lines in NGC 4450 and Other LINERs , 2000, astro-ph/0004401.

[34]  M. Inoue,et al.  VLBA Continuum Observations of NGC 4258: Constraints onan Advection-dominated Accretion Flow , 1998, astro-ph/9802264.

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

[36]  A. Filippenko,et al.  X-Ray Luminosity and Absorption Column Fluctuations in the H2O Maser Galaxy NGC 4258 from Weeks to Years , 2005, astro-ph/0501105.

[37]  James M. Moran,et al.  Detection of a Subparsec Diameter Disk in the Nucleus of NGC 4258 , 1995 .