Examining the physical conditions of a warm corona in active galactic nuclei accretion discs

Abstract

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 ∼ 1 keV and have an optical depth of τ T ≈ 10-20. We present one-dimensional calculations of the physical conditions and emitted spectra of a τ 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 ∼ 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 co-exist 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 ionised Fe Kα lines, similar to those seen in some narrow-line Seyfert 1 galaxies.An alternative model for the soft excess returns to the idea of direct emission from the AGN accretion disc, but now proposes

Radiation spectra of warm and optically thick coronae in AGNs

“…On the other hand, only a few attempts of realistic modelings of the warm corona emission have been done so far (e.g. Ballantyne 2020). At such low temperature (around 1 keV) large atomic opacities could dominate over the Thomson opacities.…”

Towards Precision Measurements of Accreting Black Holes Using X-Ray Reflection Spectroscopy

“…If the soft excess emission is not dominated by disk reflection, the spin measurements inferred from the full band spectra (Walton et al 2013b, Jiang et al 2019c) may indeed disagree with those obtained from the iron line alone (Kara et al 2017, Parker et al 2018, Frederick et al 2018. For example, Ballantyne (2020) has suggested a hybrid model combining a warm corona with disk reflection to explain the soft excess and Jiang et al (2019c) have shown that the warm corona and the high density disk reflection solutions provide equally good fits for the data of Ton S180, so a spectral analysis alone does not seem to be able to select the correct model. The combined analysis of other data, like rms (e.g.…”

“…The degree of reflection Compton-scattering in the corona will scale, of course, with the coronal emission strength, but the precise relationship will depend on disk-coronal geometry. Centrally compact coronae are likely to be less efficient at intercepting disk and disk-reflection emission compared to other disk-hugging planar geometries such as the surface ("warm") corona described by Ballantyne (2020).…”

“…The coronal geometry can evolve with time, and this is seen either in BH binaries (e.g., Malzac et al 2006, Kara et al 2019 and AGN (e.g., Wilkins & Gallo 2015b). More than one corona may coexist at the same time (see, e.g., Różańska et al 2015, Fürst et al 2015, Petrucci et al 2018, Ballantyne 2020.…”

“…Two coronae may coexist at the same time, which inevitably increases the complexity of the problem. Models with a hot and a warm corona have been proposed (see, e.g., Petrucci et al 2018, Ballantyne 2020) and the possibility of the presence of two coronae, with two different photon indices, has been advocated even as a solution of the well known problem of inferred high iron abundance common to several sources (Fürst et al 2015).…”