SUPPRESSION OF MASS ACCRETION RATES BY ORDERED MAGNETIC FIELDS

摘要

It has gradually become evident that many nearby low-luminosity galactic nuclei, including that at our own Galactic center, Sgr A~*, are less luminous by several orders of magnitude than would be expected based on their Bondi accretion rates as derived from recent X-ray observations. Although this may be attributed partly to inefficiency of the radiative processes taking place in the central accretion flows of such objects, the accretion rate itself seems also to be suppressed strongly from the Bondi rate by some mechanism. Some high-resolution observations in the optical and X-ray bands nevertheless suggest that the radii of such accretion flows (or disks) extend up to scales consistent with the Bondi theory. In this paper, a plausible explanation of these facts is derived from a theoretical model previously proposed for geometrically thin, inefficiently radiating accretion disks in an ordered magnetic field. When the outer edge of such an accretion disk is identified physically with a rotational discontinuity (or an Alfven shock) between the disklike accretion flow and its surrounding hot diffuse gas, an expression similar to Bondi's critical radius appears for its location. The suppression of the accretion rate from the Bondi value results from reductions in the infall velocity at the outer boundary and also in the solid angle subtended by the inlet of a nonspherical (i.e., disklike) accretion flow, both of which are caused by the presence of an ordered magnetic field.