Iron line profiles including emission from within the innermost stable orbit of a black hole accretion disc |
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Authors: | A J Young R R Ross & A C Fabian |
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Institution: | Institute of Astronomy, Madingley Road, Cambridge CB3 0HA,;Instituto de Física de Cantabria (Consejo Superior de Investigaciones Científicas —Universidad de Cantabria), 39005 Santander, Spain |
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Abstract: | We propose a model for the source of the X-ray background (XRB) in which low-luminosity active nuclei ( L ~ 1043 erg s?1) are obscured ( N ~ 1023 cm?2) by nuclear starbursts within the inner ~ 100 pc. The obscuring material covers most of the sky as seen from the central source, rather than being distributed in a toroidal structure, and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is turbulent with velocity dispersion ~ few × 100 km s?1 and cloud–cloud collisions lead to copious star formation. Although supernovae tend to produce outflows, most of the gas is trapped in the gravity field of the star-forming cluster itself and the central black hole. A hot ( T ~ 106 ? 107 K) virialized phase of this gas, comprising a few per cent of the total obscuring material, feeds the central engine of ~ 107 M⊙ through Bondi accretion, at a sub-Eddington rate appropriate for the luminosity of these objects. If starburst-obscured objects give rise to the residual XRB, then only 10 per cent of the accretion in active galaxies occurs close to the Eddington limit in unabsorbed objects. |
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Keywords: | galaxies: active X-rays: galaxies |
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