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

Abstract

A maximum likelihood analysis of the NGC 4258 maser positions and velocities reveals a $2 deviation from Keplerian motion in the projected rotation curve of the high-velocity features, corresponding to a $9 km s À1 , 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 inclinationwarped 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 À1 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 À4 M yr À1 , where (P1) 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.

Magnetic fields of AGNs and standard accretion disk model: testing by optical polarimetry

“…This picture coincides with one by Herrnstein et al (2005) pointed out earlier. According to them the observing intrinsic absorption in the X-ray spectrum can arise in the outer layers of the warped geometrically-thin accretion disk at the distance ∼29 pc from the black hole, where the molecular-to-atomic transition occurs.…”

The structure of the accretion disk in NGC 4258

“…where r is the radius in mas, by Herrnstein et al (2005) based on pre-1997 data. Hence, the disk is defined by y = r cos α and y = r sin i in the plane of the sky and along the line of sight, respectively.…”

PDRs and XDRs: Theory and Observations

“…This object is famous for the prominent 22 GHz H 2 O water masers that have been resolved via VLBI in a rotating circumnuclear disk (e.g., Herrnstein et al 2005;slide 29). The Keplerian orbital motions imply that the central supermassive black hole mass is 3 × 10 7 M .…”