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Astron. Astrophys. 362, L41-L44 (2000)

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1. Introduction

Recent X-ray observations reveal that optically thick disks in AGN and LMXBs truncate within (10-100) [FORMULA] (Gilfanov et al. 1999, see also Frontera et al. 2000 and the references therein). The theoretical studies on the other hand predict a truncation radius [FORMULA] between [FORMULA] (Narayan & Yi 1995, see also Abramowicz et al. 1998 and the references therein). Although conduction plays a key role in the disk-corona interaction, and specifically in evaporating the innermost part of the disk, and hence determining the transition radius, it has been considered only within the context of 1D or 1+1D (Meyer et al. 2000; Rozanska & Czerny 2000). The transition radii in the latter studies did not differ significantly from those in the formers.

Very recently, Hujeirat &Camenzind (2000a, b, henceforth HCa, HCb) presented new numerical solutions based on solving the 2D problem self-consistently. They showed that, in the absence of conduction, the disk truncates close to the last stable orbit, to form a hot sub-keplerian and advective ion torus. The disk here is multi-layered and outflows are powered by ion-pressure and centrifugal barriers.

Another problem related to the validity of the 2T description of magnetized accretion flows is whether other mechanisms that are faster than Coulomb interaction may exist, forcing the ions and electrons to be in thermal equilibrium. Collective plasma waves (Begelman &Chiueh 1988) and magnetic reconnection (Bisnovatyi-Kogan &Lovelace 2000) were suggested, but their applicabilities to disk-torus accretion around BHs are still not verified. On the other hand, Esin et al. (1996) found that their 1T self-similar one-dimensional solutions are not only inconsistent with the low-luminosities observed in LMXBs and AGN, they even produce "unusual" disk structure.

In this letter, the effects of the ion- and electron-conduction, as well as the sensitivity of the obtained configuration to coupling enhancement are investigated. These results are obtained using the 2D robust implicit radiative hydrodynamical solver IRMHD2 in spherical coordinates (Hujeirat 1998).

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000