The X-ray weakness of Little Red Dots and JWST-selected AGN: comparison with local AGN in different accretion regimes

Abstract

We investigate the origin of the observed X-ray weakness in high z LRDs and other JWST-selected broad line AGN by comparing their X-ray and optical properties with those of a diverse sample of low z AGN, including super-Eddington accreting massive black holes (SEAMBHs), NLS1s, and type I AGN from large surveys. We examine the relations between X-ray luminosity, broad Hα line luminosity, Eddington ratio, bolometric luminosity and X-ray-to-bolometric luminosity correction, and explore whether high z sources may represent analogues of local highly accreting systems. While a few LRDs and JWST-selected AGN are consistent with the SEAMBHs population in the $L_x/L_{Hα}$ versus $λ_{Edd}$ plane, most lie below it, suggesting either more extreme accretion conditions, suppressed coronal emission or heavy obscuration. We identify an anti-correlation between $L_x/L_{Hα}$ and $λ_{Edd}$ in the low z, high accreting subsample, consistent with theoretical expectations of slim-disc accretion. We further show that, for SEAMBHs, $Hα$-based bolometric luminosities underestimate SED-based values even after dust correction. We find that SEAMBHs, LRDs, and JWST-selected AGN occupy a similar high-$κ_{bol,x}$ regime, indicating that the relative deficit of X-ray emission compared to the bolometric output could potentially support the view that suppression of the hot corona emission is a common feature of highly accreting systems across cosmic time. Our results are consistent with the idea that the observed X-ray weakness of LRDs and JWST-selected AGN may be linked to the physics of highly accreting SMBHs. Moreover, observational limitations at high z, including instrumental sensitivity and the steep X-ray spectra expected for highly accreting systems, likely further suppress the detected X-ray signal.

Figures (5)

• Figure 1: Histogram of the distribution of the Eddington ratio values ($\lambda_{\rm Edd}$) of the samples considered in this work. We report: low-redshift type I AGN from BASS Gupta2024, low-redshift type I AGN from SDSSDR16 Wu_2022, NLS1 Jin2012a, LRDs Yue_2024, JWST-selected AGN Maiolino2025 and SEAMBHs Du2014Du2015.
• Figure 2: Left: The $L_{2-10\,\rm keV}$-$L_{\rm H\alpha}$ relation; Right: The $L_{2-10\,\rm keV}$/$L_{\rm H\alpha}$-$\lambda_{\rm Edd}$ relation. We report: the BASS sample Gupta2024, the SDSSDR16--4XMM sample Wu_2022 and NLS1 sample Jin2012a. Red circles are the upper limits of LRDs from Yue_2024 derived assuming $N_{\rm H}=10^{21}\,\rm cm^{-2}$. Orange circles are the upper limits of JWST-selected AGN from Maiolino2025. Yellow squares are the SEAMBHs from Tortosa2023. The cyan dashed line marks the best fit linear relation from Jin2012a respectively. White circles with red edges are the low-z LRDs analogues from Lin2026. We report also the $1-2-3\sigma$ contour levels (relative to the peak) for the low-redshift type I AGN.
• Figure 3: The $L_{2-10\,\rm keV}$/$L_{\rm H\alpha}$-$\lambda_{\rm Edd}$ relation. We used the same colour code as Fig. \ref{['fig:lx_lha']}. Solid black line is the best fitting linear relation considering the complete sample. We reported also the best fitting linear relation for the sub- (orange) and super- (green) Eddington sources. The stars represent the mean value in each $\lambda_{\rm Edd}$ bin and are shown for illustrative purposes only. Given the large intrinsic scatter (spanning $\geq 2$ orders of magnitude in $L_{2-10\,\rm keV}$/H$\alpha$), these averages are not used for $\chi^2$ minimization or quantitative inference. A quadratic fit (dotted-dashed curve) is shown to illustrate curvature in the trend. This figure includes only low-redshift AGN with joint detections in both X-rays and H$\alpha$; no upper limits are included in this analysis.
• Figure 4: Bolometric luminosity estimated from SED-fitting ($L_{\rm bol}^{\rm SED}$) vs bolometric luminosity derived from the broad H$\alpha$ luminosity following Stern_2012 ($L_{\rm bol}^{H\alpha}$). Left panel: we report $L_{\rm bol}^{H\alpha}$ using the observed $L_{\rm H\alpha}$ (green squares) and the extinction corrected one (yellow squares) for SEAMBHs; Central panel: the BASS AGN (blue crosses); Right panel: NLS1s galaxies from Jin2012a (cyan diamonds). The black dashed line indicates the one-to-one relation in each panel.
• Figure 5: X-ray bolometric correction, $\kappa_{\mathrm{bol,X}} = L_{\rm bol}/L_{2-10\,\rm keV}$, as a function of $L_{\rm bol}$. Shown are LRDs Yue_2024, JWST-selected AGN Maiolino2025, BASS AGN Gupta2024, SEAMBHs Tortosa2023 and NLS1s Jin2012a. The orange star marks the stacked X-ray measurement of the non-detected sources from Maiolino2025. $L_{\rm bol}$ is derived from the broad H$\alpha$ luminosity following Stern_2012, as in Maiolino2025. $L_{\rm bol}$ for BASS, SEAMBHs and NLS1 is from SED-fitting while for LRDs and JWST-selected AGN $L_{\rm bol}$ is derived from the broad H$\alpha$ luminosity following Stern_2012, as in Maiolino2025. Also shown are low $z$ LRDs analogues Lin2026, hyper-luminous QSOs at $z\sim 2-4$ from the WISSH sample Zappacosta2020, SDSS DR7 QSOs at $z\simeq 3.0-3.3$Trefoloni2023 and $0.4<z<3.3$ radio-quiet highly accreting AGN Laurenti20222024AA...689A.337L. The black line indicates the relation from Duras2020, with the shaded region showing its $1\sigma$ uncertainty and the hatched region indicates the unphysical regime where $\kappa_{\rm bol,X}<1$.