Intrinsic disc emission and the soft X‐ray excess in active galactic nuclei

By C. Done, S. Davis, C. Jin, O. Blaes, M. Ward. 2011

390 citations recorded by the source corpus.

Narrow-line Seyfert 1 (NLS1) galaxies have low-mass black holes and mass accretion rates close to (or exceeding) Eddington, so a standard blackbody accretion disc should peak in the extreme ultraviolet. However, the lack of true absorption opacity in the disc means that the emission is better approximated by a colour temperature corrected blackbody, and this colour temperature correction is large enough (∼2.4) that the bare disc emission from a zero spin black hole can extend into the soft X-ray bandpass. Part of the soft X-ray excess seen in these objects must be intrinsic emission from the disc unless the vertical structure is very different to that predicted. None the less, this is not the whole story even for the extreme NLS1 as the shape of the soft excess is much broader than predicted by a bare disc spectrum, indicating some Compton upscattering by warm, optically thick material. We associate this with the disc itself, so it must ultimately be powered by mass accretion. We build an energetically self-consistent model assuming that the emission thermalizes to a (colour temperature corrected) blackbody only at large radii. At smaller radii the gravitational energy is split between powering optically thick Comptonized disc emission (forming the soft X-ray excess) and an optically thin corona above the disc (forming the tail to higher energies). We show examples of this model fit to the extreme NLS1 RE J1034+396, and to the much lower Eddington fraction broad-line Seyfert 1 PG 1048+231. We use these to guide our fits and interpretations of three template spectra made from co-adding multiple sources to track out a sequence of active galactic nucleus (AGN) spectra as a function of L/LEdd. Both the individual objects and template spectra show the surprising result that the Compton upscattered soft X-ray excess decreases in importance with increasing L/LEdd. The strongest soft excesses are associated with low mass accretion rate AGN rather than being tied to some change in disc structure around Eddington. We argue that this suggests a true break in accretion flow properties between stellar and supermassive black holes. The new model is publicly available within the xspec spectral fitting package.

Subfields: Astronomy and Astrophysics, Biomedical Engineering

View Paper: https://www.semanticscholar.org/paper/5ea3764f180659f6429ba0046631f4721eaa77b3

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Citations in the corpus

The Suzaku View of Highly Ionized Outflows in AGN. 1; Statistical Detection and Global Absorber Properties (2012, background): “The thermal temperature of this ‘soft-excess’ suggests that it is unlikely to be the direct emission from a standard accretion disc without additional reprocessing (Sobolewska & Done 2007; Done et al. 2012).”
A Combined Optical and X-ray Study of Unobscured Type 1 AGN. I. Optical Spectra and SED Modeling (2011, methodology): “In order to account for this, and to estimate the bolometric luminosity, we fit the X-ray and UV/optical continua all together using a new broadband SED model (Done et al. 2011, XSPEC model: optxagn).”
Suzaku observations of 'bare' active galactic nuclei (2012, background): “…the data, in which the soft excess arises due to Comptonization by thermal electrons within the inner disc, possibly existing as a skin-like layer on the surface of the disc, while the higher energy continuum arises in a separate, more traditional corona (Done et al. 2012; Jin, Ward & Done 2012).”
Superradiance (2015, methodology): “Both the (mass-independent) shape of the iron Kα line seen in reflection [628] and the thermal emission from the inner edge of the disc [629] – assumed to be the ISCO [630, 631] – are commonly employed for both stellar mass BHs in binaries and supermassive BHs in active galactic nuclei (although the”
Titans of the early Universe: The Prato statement on the origin of the first supermassive black holes (2018, methodology): “Radiative feedback from the accretion disk can be modelled in a straightforward manner, by determining the appropriate SED for a given black hole mass and accretion rate (e.g., Done et al. 2012).”
The first ultraviolet quasar-stacked spectrum at z ≃ 2.4 from WFC3 (2015, methodology): “…corrected) blackbody, the inner disc is where a fraction of the disc emission is Compton up-scattered, while the X-ray corona is where also a fraction of the emission is Compton up-scattered producing the power-law tail at high energies (see Section 4 in Done et al. 2012 for further details).”
A physical model of the broad-band continuum of AGN and its implications for the UV/X relation and optical variability (2018, background): “…emission, which can extend into the soft X-ray bandpass for the lowest mass, Narrow Line Seyfert-1 (NLS1) galaxies, but these still have a small fraction of their bolometric power emitted in a soft X-ray excess component (Jin et al. 2012a,b; Done et al. 2012; Jin et al. 2013; Matzeu et al. 2017).”