IS THE ACCRETION FLOW IN NGC-4258 ADVECTION DOMINATED

The mass of the central black hole in the active galaxy NGC 4258 (M106 ) has been measured to be M = 3.6 x 10(7) M(.) by Miyoshi et al. The E ddington luminosity corresponding to this mass is L(E) = 4.5 x 10(45) ergs s(-1). By contrast, the X-ray luminosity of the nucleus of NGC 42 58 between 2 and 10 keV is (4 +/- 1) x 10(40) ergs s(-1) while the opt ical/UV luminosity is less than 1.5 x 10(42) ergs s(-1). The luminosit y of NGC 4258 is therefore extremely sub-Eddington: L similar to 10(-5 )L(E) in X-rays and L similar to 3 x 10(-4)L(E) even if we take the ma ximum optical/UV luminosity. Assuming the usual accretion efficiency o f 0.1 would imply accretion rates orders of magnitude lower than in Se yfert galaxies and quasars. We show that the properties of the active galactic nuclei (AGNs) in NGC 4258 can be explained by an accretion fl ow in the form of a very hot, optically thin plasma that advects most of the viscously generated thermal energy into the central black hole and radiates only a small fraction of the energy. In this case, the ac cretion rate in Eddington units could be as high as similar to 0.16 al pha, where alpha is the standard viscosity parameter; and the size of the hot disk should be larger than similar to 10 times the Schwarzschi ld radius. We compare the predictions of this model with observations and discuss its application to other low-luminosity AGNs.