THE MASS ACCRETION RATE THROUGH THE MASING MOLECULAR DISK IN THE ACTIVE GALAXY NGC 4258

摘要

Recent observations (Miyoshi et al. 1995; Greenhill et al. 1995b) of water maser emission from the galaxy NGC 4258 have revealed the presence of a thin molecular disk of outer radius 0.25 pc which orbits a central object of mass 3.6 X 10~7 solar mass. This molecular disk, which we view nearly edge on, shows a substantial warp. NGC 4258 is also observed to contain a central X-ray source of inferred intrinsic luminosity ～4 X 10~(40) ergs s~(-1) over the 2-10 keV energy range (Makishima et al. 1994), suggesting that the warped circumnuclear disk is illuminated obliquely by X-rays. Here we show how the physical conditions within the orbiting disk may be inferred from these observations. Modeling this system as a viscous accretion disk that is illuminated obliquely by a central X-ray source, we infer that material accretes through the disk at a rate of 7 X 10~(-5) α solar masses per year, where α is the dimensionless parameter (α approx< 1) that conventionally characterizes the disk viscosity. This value suggests that the active nucleus must convert rest-mass energy into 2-10 keV X-rays with an efficiency of 0.01α~(-1); the total energy output over the entire electromagnetic spectrum must be generated with an overall efficiency which is ～10 times larger. We have also investigated how the properties of molecular circumnuclear disks are expected to depend on the mass and luminosity of an active galactic nucleus: our results suggest the possible existence of extragalactic water masers that are several orders of magnitude more luminous than any observed to date.