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Thermal and Non-Thermal Emission from Accretion Disks

from IV - X-rays and Accretion Disks

Published online by Cambridge University Press:  04 August 2010

A.C. Fabian
Affiliation:
Institute of Astronomy, Madingley Road, Cambridge CB3 0HA
R.R. Ross
Affiliation:
Institute of Astronomy, Madingley Road, Cambridge CB3 0HA and Physics Department, College of the Holy Cross, Worcester, MA, USA
Andrew Robinson
Affiliation:
University of Cambridge
Roberto Juan Terlevich
Affiliation:
Royal Greenwich Observatory, Cambridge
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Summary

Abstract

Luminous accretion discs around black holes are expected to be optically thick and radiate much of their emission in the EUV and soft X-ray bands. Quasiblackbody emission consistent with such discs is observed in many Seyfert 1 galaxies and from Galactic black hole candidates such as Cygnus X-1. The harder, rapidly variable, X-rays from such objects must originate above the disc, probably from non-thermal processes involving magnetic fields. The disc is therefore irradiated by a hard X-ray continuum. Backscattering and fluorescence from the disc produce a reflection spectrum, which is now observed in X-rays. Features in the reflection spectrum act as a diagnostic of the geometry and conditions of the inner disc, offering the strong possibility that it can be mapped in the near future.

Introduction

We begin by reviewing the case for the presence of accretion discs in many Active Galactic Nuclei (AGN), such as the Seyfert 1 galaxies. Here we are concentrating on the inner disc within radii R ≲ 100Rs, where Rs is the Schwarzschild radius of the central object (assumed here to be a black hole). Such discs were first detected from the UV excess and in particular by the variable soft X-ray emission that they produce. Further rapid progress has been hindered by the unfortunate coincidence that most of the direct thermal radiation produced by accretion discs around massive objects is emitted in the EUV, where photoelectric absorption by the interstellar medium of our Galaxy is strong.

Type
Chapter
Information
The Nature of Compact Objects in Active Galactic Nuclei
Proceedings of the 33rd Herstmonceux Conference, held in Cambridge, July 6-22, 1992
, pp. 280 - 290
Publisher: Cambridge University Press
Print publication year: 1994

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