Showing papers by "Luigi Stella published in 1984"

Magnetic field instabilities in accretion disks

Abstract: The stability of magnetic fields within the accretion disks is examined, including the effects of magnetic buoyancy. Rayleigh-Taylor, and Kelvin-Helmholtz instabilities. The purpose of the study is to examine the efficacy of turbulent accretion (..cap alpha..-) disk models which depend upon ''magnetic viscosity'' to provide the dissipation necessary to account for radial infall. We provide estimates for the time scales of the dominant instabilities in both the gas pressure-dominated and radiation pressure-dominated regions of a thin accretion disk; provide constraints on dynamo processes such that ''magnetic viscosity'' operates to permit stationary accretion; show that field amplification may take place even in vertically stably stratified regions of a thin disk by the action of ''magnetic buoyancy-driven'' convection; and demonstrate that the turbulent Reynolds stress responsible for accretion is proportional to the gas pressure even in the radiation-dominated portion of the disk (so that the disk is stable to viscous and thermal instabilities). Furthermore, we show that if a ''seed'' magnetic field is injected into a thin accretion disk, then the formation of buoyancy-generated hot corona above the disk appears inevitable.

Short-term X-ray variability of the globular cluster source 4U 1820 - 30 (NGC 6624)

Abstract: Analytical techniques for improved identification of the temporal and spectral variability properties of globular cluster and galactic bulge X-ray sources are described in terms of their application to a large set of observations of the source 4U 1820 - 30 in the globular cluster NGC 6624. The autocorrelation function, cross-correlations, time skewness function, erratic periodicities, and pulse trains are examined. The results are discussed in terms of current models with particular emphasis on recent accretion disk models. It is concluded that the analyzed observations provide the first evidence for shot-noise variability in a globular cluster X-ray source.