Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...447l..17n&link_type=abstract
Astrophysical Journal Letters v.447, p.L17
Astronomy and Astrophysics
Astronomy
112
Accretion, Accretion Disks, Galaxies: Nuclei, Galaxies: Seyfert, Masers
Scientific paper
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 3.6x107 Msolar . 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 ~4x1040 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 7x10^{-5} alpha solar masses per year, where alpha is the dimensionless parameter (alpha <~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 0.01 alpha -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.
Maloney Philip R.
Neufeld David A.
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