You can't see me: super-Eddington growth hindering X-ray detection in high-z broad-line AGNs

Abstract

We revisit black hole mass estimates for high-redshift broad-line AGNs discovered with JWST by jointly analysing their broad emission lines and their systematic non-detections in deep Chandra imaging. Building upon the self-shadowed, super-Eddington accretion flow framework and the coronal over-cooling prescription of Madau & Haardt (2024), we couple funnel-dependent Comptonization physics with slim-disc spectra from Kubota & Done (2019) and explore the resulting parameter space through a full MCMC inference. Using the sample analysed by Lupi et al. (2024) and Maiolino et al. (2025), we show that X-ray weakness - manifested as extreme bolometric corrections, suppressed 2-10 keV luminosities, and non-detections in the 0.5-5 keV Chandra band - naturally arises when the corona is confined and radiatively over-cooled inside a narrow super-Eddington funnel. The combined broad line+X-ray analysis yields strongly bimodal posteriors: either very massive, very low-Eddington black holes (physically disfavoured), or a population of low-mass ($\sim 10^{6}-10^{7} ~M_{\odot}$) black holes accreting at $f_{\rm Edd} \gg 1$. The latter solution is strongly preferred for nearly all objects and returns masses consistent with, or lower than, local $M_{\rm BH}-M_{\rm star}$ relations, mitigating the extreme mass ratios implied by single-epoch virial estimators. The predicted intrinsic spectra are redder and exhibit reduced hard-X-ray output but higher bolometric luminosities, implying bolometric corrections larger than those typical of the local AGN population, yet consistent with low-redshift highly accreting counterparts. These results support a picture in which many JWST broad-line AGNs are powered by rapidly growing, super-Eddington black holes whose suppressed coronal emission and self-shadowed BLR geometry combine to mimic overmassive black holes at $z \gtrsim 6$.

Figures (11)

• Figure 1: Effective electron temperature (red solid line) and power-law index $\Gamma$ (blue dashed line) as a function of the half-opening angle of the disc funnel.
• Figure 2: Comparison between the expected $K_{\rm X}$ for our sample of high-redshift AGN (assuming the $M_{\rm BH}$ and $f_{\rm Edd}$ estimates from Maiolino2025) for spins ranging between $a_\bullet=0$ and $a_\bullet=0.99$, and the observed lower limit from the X-ray non-detection, shown as triangles. Our models are shown with solid vertical lines spanning values expected for different spins, with lower spin values implying higher bolometric corrections. The local scaling relation is shown as a red line, with the shaded area corresponding to a 1$\sigma$ uncertainty ($\pm 0.26 ~\rm dex$), and the local observations by duras2020 as grey points.
• Figure 3: Same as Fig. \ref{['fig:Kx_M25']} but assuming BH masses and accretion rates from Lupi2024. We selected here the combination of $M_{BH}, f_{Edd}, a$ showing the highest $K_{X}$ correction among the 10 solutions with highest probability over the 32k samples. Bolometric corrections are corrected accordingly to the predicted bolometric luminosity of each source.
• Figure 4: Example of a corner plot from the MCMC X-ray analysis for the GS8083 source. Blue lines and squares show the reference values of $M_{\rm BH}$ and $f_{\rm Edd}$ from Maiolino2025. The values reported above the marginalized distributions correspond to the medians of the respective posteriors. A bimodal structure is clearly visible in the posterior distributions of BH mass and accretion rates.
• Figure 5: Comparison between the best-fit values of $M_{\rm BH}$ (left panel) and $\dot{M}/\dot{M}_{\rm Edd}$ (right panel) derived from the MCMC analysis (which includes constraints on the X-ray emission) and the estimates from Maiolino2025 obtained using the single-epoch method. Given the bimodal posterior distributions found for all objects, the Monte Carlo samples are divided into two groups: low-mass, highly accreting systems (filled data points) and high-mass, almost inactive black holes (empty data points). Each pair of solutions for a given object is represented by a different marker. Data points for the low mass solution are colour-coded according to the ratio between the mean likelihoods obtained for the low-mass and high-mass solutions, where high values (red colours) suggest a preference for the former. The grey shaded regions indicate a deviation of 0.5 dex from the 1:1 relation.