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 w ater maser emission from the galaxy NGC 4258 have revealed the presenc e of a thin molecular disk of outer radius 0.25 pc which orbits a cent ral object of mass 3.6 x 10(7) 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 simil ar to 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 illumin ated obliquely by X-rays. Here we show how the physical conditions wit hin the orbiting disk may be inferred from these observations. Modelin g this system as a viscous accretion disk that is illuminated obliquel y by a central X-ray source, we infer that material accretes through t he disk at a rate of 7 x 10(-5) alpha solar masses per year, where alp ha is the dimensionless parameter (alpha less than or similar to 1) th at conventionally characterizes the disk viscosity. This value suggest s that the active nucleus must convert rest-mass energy into 2-10 keV X-rays with an efficiency of 0.01 alpha(-1); the total energy output o ver the entire electromagnetic spectrum must be generated with an over all efficiency which is similar to 10 times larger. We have also inves tigated how the properties of molecular circumnuclear disks are expect ed to depend on the mass and luminosity of an active galactic nucleus: our results suggest the possible existence of extragalactic water mas ers that are several orders of magnitude more luminous than any observ ed to date.