The Mass Accretion Rate through the Masing Molecular Disk in the Active Galaxy NGC 4258

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 × 10⁷ M_☉. 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 × 10⁴⁰ 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 7×10^-5α solar masses per year, where α is the dimensionless parameter (α ≲ 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.