Mildly Super-Eddington Accretion Onto Slowly-Spinning Black Holes Explains the X-Ray Weakness of the Little Red Dots

TL;DR

This study uses simulations to show that mildly super-Eddington accretion onto slowly-spinning black holes explains the X-ray weakness observed in low-luminosity AGN at high redshift, matching recent JWST findings.

Contribution

It introduces a model of mildly super-Eddington accretion onto slowly-spinning SMBHs that accounts for the X-ray faintness of Little Red Dots at z>4, supported by detailed simulations.

Findings

X-ray bolometric corrections reach ~10^4 at z=6.

Most SEDs are steep and soft in X-rays with median photon index 3.1.

Viewing angle affects X-ray detectability, with a 1.5% probability of pole-on orientation.

Abstract

JWST has revealed a population of low-luminosity AGN at $z>4$ in compact, red hosts (the "Little Red Dots", or LRDs), which are largely undetected in X-rays. We investigate this phenomenon using GRRMHD simulations of super-Eddington accretion onto a SMBH with $M_\bullet=10^7 \,\rm M_\odot$ at $z\sim6$, representing the median population; the SEDs that we obtain are intrinsically X-ray weak. The highest levels of X-ray weakness occur in SMBHs accreting at mildly super-Eddington rates ($1.4<f_{\rm Edd}<4$) with zero spin, viewed at angles $>30^\circ$ from the pole. X-ray bolometric corrections in the observed $2-10$ keV band reach $\sim10^4$ at $z=6$, $\sim5$ times higher than the highest constraint from X-ray stacking. Most SEDs are extraordinarily steep and soft in the X-rays (median photon index $\Gamma=3.1$, mode of $\Gamma=4.4$). SEDs strong in the X-rays have harder spectra with a high-energy bump when viewed near the hot ($>10^8$ K) and highly-relativistic jet, whereas X-ray weak SEDs lack this feature. Viewing a SMBH within $10^\circ$ of its pole, where beaming enhances the X-ray emission, has a $\sim1.5\%$ probability, matching the LRD X-ray detection rate. Next-generation observatories like AXIS will detect X-ray weak LRDs at $z\sim6$ from any viewing angle. Although many SMBHs in the LRDs are already estimated to accrete at super-Eddington rates, our model explains $50\%$ of their population by requiring that their masses are overestimated by a mere factor of $\sim3$. In summary, we suggest that LRDs host slowly spinning SMBHs accreting at mildly super-Eddington rates, with large covering factors and broad emission lines enhanced by strong winds, providing a self-consistent explanation for their X-ray weakness and complementing other models.