Low-Luminosity Accretion in Black Hole X-Ray Binaries and Active Galactic Nuclei |
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| Authors: | Ramesh Narayan |
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| Institution: | (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, U.S.A. |
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| Abstract: | At luminosities below a few percent of Eddington, accreting black holes switch to a hard spectral state which is very different
from the soft blackbody-like spectral state that is found at higher luminosities. The hard state is well-described by a two-temperature,
optically thin, geometrically thick, advection-dominated accretion flow (ADAF) in which the ions are extremely hot (up to
1012 K near the black hole), the electrons are also hot (∼109−10.5 K), and thermal Comptonization dominates the X-ray emission. The radiative efficiency of an ADAF decreases rapidly with decreasing
mass accretion rate, becoming extremely low when a source reaches quiescence. ADAFs are expected to have strong outflows,
which may explain why relativistic jets are often inferred from the radio emission of these sources. It has been suggested
that most of the X-ray emission also comes from a jet, but this is less well established. |
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| Keywords: | accretion accretion disks active galactic nuclei black hole physics radiation mechanisms X-rays: binaries |
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