Towards a New Standard Model for Black Hole Accretion

We briefly review recent developments in black hole accretion disk theory, emphasizing the vital role played by magnetohydrodynamic (MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong indicator that large-scale MHD torques facilitate vertical transport of angular momentum. This leads to an enhanced overall rate of angular momentum transport and allows accretion of matter to proceed at an interesting rate. Furthermore, we argue that when vertical transport is important, the radial structure of the accretion disk is modified at small radii and this affects the disk emission spectrum. We present a simple model demonstrating how energetic, magnetically-driven outflows modify the emergent disk emission spectrum with respect to that predicted by standard accretion disk theory. A comparison of the predicted spectra against observations of quasar spectral energy distributions suggests that mass accretion rates inferred using the standard disk model may severely underestimate their true values.Comment: To appear in the Fifth Stromlo Symposium Proceedings special issue of ApS

MÖSSBAUER-BORRMANN SUPERRADIANCE

Considérons le comportement cinétique d'un laser pour produire des rayons gamma, construit de noyaux isomériques aux sites de réseau d'un cristal parfait et simple, dont les dimensions et la structure sont choisies pour favoriser l'émission anormale dans le mode de Borrmann qui contient le nombre maximum de reflexions de Bragg. Ceci réduit fortement l'excitation requise pour le lasing. Notre analyse de plusieurs systèmes hypothétiques démontre que la superradiance, plutôt que l'amplification d'émission spontanée, sera le mode d'opération, pour autant que les noyaux isomériques soient pompés très rapidement à un niveau Mossbauer de durée courte et que l'intégrité du cristal soit préservée. Ce résultat justifie la recherche de solution aux problèmes principaux : noyaux candidats, préparation de l'isomère, et transfert du niveau isomérique au niveau lasant.We consider the kinetic behavior of a gamma-ray laser comprising an array of isomeric nuclei located at regular lattice sites in a perfect single crystal of dimensions and structure so chosen as to favor anomalous emission into that Borrmann mode having the maximum possible number of component Bragg-reflected beams, which greatly reduces the excitation requirements. Our analysis of several hypothetical systems shows that superradiance, rather than amplified spontaneous emission, will then be the preferred mode of deexcitation, provided the nuclei can be pumped rapidly to a short-lived Mossbauer level while preserving crystal integrity. This warrants a search for solutions to the major problems : candidate nuclides. preparation of a storage isomer, and interlevel transfer from storage to lasing state