Abstract

The possible relation between observed variability behaviour and slim disk stability properties is examined. It is argued that processes which give rise to QPOs in galactic sources are operative in AGN as well. Thus, it may be that unstable acoustic modes in the inner part of the accretion disk give rise to both the quasi-periodic short-term X-ray variability in NGC 6814 and the horizontal branch oscillations (HBOs) in X-ray binaries. The estimated central mass in NGC 6814 is ∼ 106M⊗.

Introduction

The majority of compact galactic and extragalactic sources seem to accrete at a rate ṁ ∼ 1, where ṁ = L/LE, LE = 1038m erg s−1 being the Eddington accretion rate and where m = M/M⊗ is the central mass in solar units. Some examples corresponding to AGN are shown in Figure 1. It follows that the standard Shakura-Sunyaev model (Shakura & Sunyaev 1973, 1976) is simply inadequate when it comes to a relevant description of, especially, the inner accretion disk in these sources, where the bulk of the luminosity is generated. One may also note that Shakura-Sunyaev disks contain an artificial singularity at the inner edge, due to an improper neglect of some inertial terms in the radial structure equations. Thus, any model which attempts to combine the effects of magnetic fields, electron/positron pairs, winds or whatever, with a Shakura-Sunyaev Keplerian disk, is bound to yield questionable results.