[Paper Review] Weakness of X-rays and Variability in High-redshift AGNs with Super-Eddington Accretion

Kohei Inayoshi, Shigeo KIMURA+1 more|arXiv (Cornell University)|Dec 4, 2024

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper proposes that super-Eddington accretion in high-redshift AGNs with low-mass black holes ($\lesssim 10^{7-8}~M_\odot$) drives radiation-driven outflows forming warm, moderately thick coronae, leading to softer X-ray spectra and suppressed UV/optical variability due to photon trapping. The model explains the observed X-ray weakness and low variability in JWST-identified 'Little Red Dots' and aligns with bolometric corrections and X-ray faintness seen in local super-Eddington AGNs.

ABSTRACT

The James Webb Space Telescope (JWST) observations enable the exploration of active galactic nuclei (AGNs) with broad-line emission in the early universe. Despite their clear radiative and morphological signatures of AGNs in rest-frame optical bands, complementary evidence of AGN activity - such as X-ray emission and UV/optical variability - remains rarely detected. The weakness of X-rays and variability in these broad-line emitters challenges the conventional AGN paradigm, indicating that the accretion processes or environments around the central black holes (BHs) differ from those of low-redshift counterparts. In this work, we study the radiation spectra of super-Eddington accretion disks enveloped by high-density coronae. Radiation-driven outflows from the disk transport mass to the poles, resulting in moderately optically-thick, warm coronae formed through effective inverse Comptonization. This mechanism leads to softer X-ray spectra and larger bolometric correction factors for X-rays compared to typical AGNs, while being consistent with those of JWST AGNs and low-redshift super-Eddington accreting AGNs. In this scenario, UV/optical variability is suppressed due to photon trapping within super-Eddington disks, while X-ray emissions remain weak yet exhibit significant relative variability. These characteristics are particularly evident in high-redshift AGNs powered by lower-mass BHs with $\lesssim 10^{7-8}~M_\odot$, which undergo rapid mass accretion following overmassive evolutionary tracks relative to the BH-to-stellar mass correlation in the local universe.

Motivation & Objective

• To explain the observed X-ray weakness and lack of UV/optical variability in high-redshift AGNs identified by JWST, particularly 'Little Red Dots' (LRDs) at $z \gtrsim 4$.
• To address the discrepancy between strong optical-line and morphological AGN signatures and the absence of X-ray and variability signatures in these sources.
• To reconcile the low X-ray luminosity and high bolometric correction with known properties of local super-Eddington AGNs and NLSy1 galaxies.
• To investigate how super-Eddington accretion in low-mass black holes ($M_{\rm BH} \lesssim 10^{7-8}~M_\odot$) can produce the observed SEDs and variability patterns.
• To test whether radiation-driven outflows and inverse Comptonization in dense coronae can naturally explain the anti-correlation between X-ray faintness and relative X-ray variability.

Proposed method

• Modeling radiation spectra of super-Eddington accretion disks surrounded by high-density, warm coronae formed via radiation-driven outflows from the disk surface.
• Applying effective inverse Comptonization in moderately optically-thick coronae to compute X-ray emission and spectral softening.
• Using photon trapping in super-Eddington disks to suppress UV/optical variability, while allowing for significant relative X-ray variability.
• Calculating bolometric correction factors for X-rays and comparing them with observed $\alpha_{\rm ox}$ values in high-redshift AGNs.
• Simulating broadband SEDs for low-mass black holes ($M_{\rm BH} \lesssim 10^{7-8}~M_\odot$) undergoing rapid super-Eddington accretion to match JWST and Chandra observations.
• Assessing the consistency of the model with observed $\alpha_{\rm ox} < -1.8$ and non-detections in deep X-ray stacking, independent of obscuration.

Figure 1: X-ray spectral index ( $\Gamma$ ; $F_{\nu}\propto\nu^{1-\Gamma}$ ) as a function of Compton $y$ -parameter, based on three different studies; Titarchuk & Lyubarskij ( 1995 ) (black, our fiducial model), Pozdniakov et al. ( 1979 ) (blue), and Beloborodov ( 1999 ) (red). Three values of elec

Experimental results

Research questions

• RQ1 Why are high-redshift AGNs with broad-line emission (especially LRDs) X-ray weak despite strong optical signatures?
• RQ2 How can UV/optical variability be suppressed in AGNs that show strong broad emission lines and compact morphology?
• RQ3 What physical mechanism in super-Eddington accretion disks leads to softer X-ray spectra and larger X-ray bolometric corrections than in typical AGNs?
• RQ4 Why is the X-ray weakness observed in both unobscured and obscured AGNs, suggesting an intrinsic origin?
• RQ5 Can radiation-driven outflows and coronal structure formation explain the observed anti-correlation between X-ray faintness and relative X-ray variability?

Key findings

• Radiation-driven outflows from super-Eddington disks transport mass to the poles, forming moderately optically-thick, warm coronae via effective inverse Comptonization.
• This mechanism produces softer X-ray spectra and larger X-ray bolometric correction factors than in typical AGNs, consistent with observed $\alpha_{\rm ox} < -1.8$ in high-redshift AGNs.
• Photon trapping within super-Eddington disks suppresses UV/optical variability, explaining the lack of significant flux variations in JWST-identified LRDs.
• X-ray emissions remain weak but exhibit significant relative variability, which could be detectable through long-term monitoring or transient events like tidal disruption events.
• The model successfully explains the X-ray faintness and low variability in high-redshift AGNs powered by low-mass black holes ($M_{\rm BH} \lesssim 10^{7-8}~M_\odot$) undergoing rapid super-Eddington growth.
• The scenario is consistent with the observed overmassive black hole growth relative to the local $M_{\rm BH}/M_\star$ relation, especially if seed black holes formed with low $M_{\rm BH}/M_\star$ ratios at $z > 10$.

Figure 2: Broadband SEDs of a $z=4$ AGN with a bolometric luminosity of $L_{\rm bol}=10^{46}~{\rm erg~s}^{-1}$ for different values of the optical depths ( $0.1\leq\tau_{\rm es}\leq 10$ ). The Compton parameter is set to $y=2/3$ . The gray curve shows the SED of disk seed photons. For reference, we