X-Ray Weak AGNs from Super-Eddington Accretion onto Infant Black Holes

TL;DR

The paper proposes that JWST-detected, X-ray weak high-redshift AGNs are powered by mildly super-Eddington accretion onto infant black holes, forming thick, funnel-shaped disks in which the inner hot corona is embedded and irradiated by a largely isotropic soft-photon field. This configuration cools the coronal electrons via Comptonization, yielding extremely soft X-ray spectra with $\Gamma \approx 2.8$–$4.0$ and large $2$–$10$ keV bolometric corrections, potentially explaining the lack of Chandra detections. The authors develop a semi-analytic thick-disk/ funnel model with representative parameters (Models A and B) and show that modest external photon flux ($m\sim2$–3) can produce $kT_e\sim40$ keV and $\Gamma\sim2.8$, while more extreme funnel narrowing can drive even softer spectra. While offering a plausible intrinsic X-ray weakness mechanism, the work also emphasizes the need for more comprehensive radiative transfer, including Comptonization and reflection, and considers caveats from winds and geometry.

Abstract

A simple model for the X-ray weakness of JWST-selected broad-line AGNs is proposed under the assumption that the majority of these sources are fed at super-Eddington accretion rates. In these conditions, the hot inner corona above the geometrically thin disk that is responsible for the emission of X-rays in "normal" AGNs will be embedded instead in a funnel-like reflection geometry. The coronal plasma will Compton upscatter optical/UV photons from the underlying thick disk as well as the surrounding funnel walls, and the high soft-photon energy density will cool down the plasma to temperatures in the range 30-40 keV. The resulting X-ray spectra are predicted to be extremely soft, with power-law photon indices Gamma=2.8-4.0, making high-z super-Eddington AGNs largely undetectable by Chandra.

Figures (4)

• Figure 1: Meridional cross-sections (over one quadrant) for the supercritical thick disks described by Models A (magenta line) and B (green line). As $r_{\rm in}$ decreases, $r_{\rm out}$ increases as does the ratio $L/L_{\rm Edd}$, while the efficiency of mass to energy conversion drops. Smaller values of $r_{\rm in}$ imply steeper and deeper funnels, where pressure gradients are balanced by centrifugal forces rather than by gravity and luminosities exceed the Eddington limit. The square points mark the location on the surface inside which 90% of the disk luminosity $L_{\rm rad}$ is actually emitted. The inset shows the Keplerian specific angular momentum distribution for the adopted pseudo-Newtonian potential (black line), and the angular momentum distributions corresponding to our supercritical disks (Models A and B).
• Figure 2: The funnel regions of supercritical disks around black holes. The two configurations (Models A and B) have funnel opening half-angles of $\Theta={\rm arccot}\,(h/r)_{\rm max}= 35^\circ$ and $44^\circ$, respectively. The dot on each funnel indicates the point where $h/r$ reaches a maximum, and the percentage next to it denotes the fraction of total disk luminosity emitted interior to that point.
• Figure 3: Equilibrium electron temperature ($\theta=kT_e/m_ec^2$) of the hot coronal phase (lower curves) and the resulting photon index $\Gamma$ of the X-ray Comptonized component (upper curves). The corona has an assumed electron scattering opacity of $\tau=0.5$, comparable to the value inferred in Seyfert galaxies. Two values, $f_c=1/2$ and $f_c=1/3$, have been assumed for the fraction of the locally-generated radiation power that is dissipated in the corona. The parameter $m$ on the horizontal axis measures the strength of the incoming external radiation field in units of the net flux. In our two supercritical thick disk models, we estimate $m\simeq 3$ (Model A) and $m\simeq 2$ (Model B) at the bottom of the funnel.
• Figure 4: Sketch illustrating the super-Eddington corona-funnel model discussed in the main text. Embedded in a funnel-like reflection geometry, the inner hot corona will cool down by Comptonizing a largely isotropic soft radiation field. The resulting X-ray spectra are predicted to be extremely soft, independently of the detailed geometry of the corona.