期刊
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
卷 491, 期 3, 页码 3553-3561出版社
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stz3294
关键词
accretion, accretion discs; galaxies: active; galaxies: Seyfert; X-rays: galaxies
A warm corona at the surface of an accretion disc has been proposed as a potential location for producing the soft excess commonly observed in the X-ray spectra of active galactic nuclei (AGNs). In order to fit the observed data, the gas must be at temperatures of similar to 1 keV and have an optical depth of tau(T) approximate to 10-20. We present one-dimensional calculations of the physical conditions and emitted spectra of a tau(T) = 10 or 20 gas layer subject to illumination from an X-ray power law (from above), a blackbody (from below), and a variable amount of internal heating. The models show that a warm corona with kT similar to 1 keV can develop, producing a strong Comptonized soft excess, but only if the internal heating flux is within a relatively narrow range. Similarly, if the gas density of the layer is too large then efficient cooling will stop a warm corona from forming. The radiation from the hard X-ray power law is crucial in producing a warm corona, indicating that a warm and hot corona may coexist in AGN accretion discs, and their combined effect leads to the observed soft excess. Intense heating of a warm corona leads to steep X-ray spectra with ionized Fe K alpha lines, similar to those seen in some narrow-line Seyfert 1 galaxies.
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