Accretion disc coronae as magnetic reservoirs |
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| Authors: | A Merloni A C Fabian |
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| Institution: | Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA |
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| Abstract: | Most astrophysical sources powered by accretion on to a black hole, either of stellar mass or supermassive, when observed with hard X-rays show signs of a hot Comptonizing component in the flow, the so-called corona , with observed temperatures and optical depths lying in a narrow range (0.1? τ ?1 and 1×109 K? T ?3×109 K). Here we argue that these facts constitute strong supporting evidence for a magnetically dominated corona. We show that the inferred thermal energy content of the corona, in all black hole systems, is far too low to explain their observed hard X-ray luminosities, unless either the size of the corona is at least of the order of 103 Schwarzschild radii, or the corona itself is in fact a reservoir , where the energy is mainly stored in the form of a magnetic field generated by a sheared rotator (probably the accretion disc). We briefly outline the main reasons why the former possibility is to be discarded, and the latter preferred. |
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| Keywords: | accretion accretion discs magnetic fields |
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