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

TL;DR

This work addresses the puzzling X-ray weakness of JWST-detected Little Red Dots by simulating mildly super-Eddington accretion onto a $10^7\,M_0$ black hole at $z\sim6$ with GRRMHD and post-processing to produce angle-dependent SEDs. It demonstrates that intrinsic X-ray weakness arises for $1.4\lesssim f_{ m Edd}\lesssim4$, low or zero spin, and off-pole viewing, yielding bolometric corrections up to $k_X\sim10^4$ and extremely soft X-ray spectra ($\Gamma\gtrsim3$). The results show that a significant fraction of LRDs can be explained by mildly super-Eddington, slowly spinning SMBHs, with some sources potentially arising from modest BH-mass overestimates while beaming remains a minor contributor to detections. The study offers a self-consistent framework that complements obscuration and beaming scenarios and predicts that future missions like AXIS will detect these X-ray-weak LRDs across viewing angles, enabling robust tests of SMBH growth at high redshift.

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 $Γ=3.1$, mode of $Γ=4.4$). 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.

Figures (7)

• Figure 1: Comparison of the $2-10$ keV bolometric correction as a function of the logarithm of the Eddington ratio in the restframe (left panel) and in the observed$2-10$ keV band when the source placed at the median redshift of $z=6$ (right panel). Different colors indicate varying inclination angles from the pole (degrees), and different markers represent different spin values: triangles for $a=0.9$, circles for $a=0.68$, and stars for $a=0$. Mildly super-Eddington accretion rates ($1.4<\,{f_{\rm Edd}}<4$) onto slowly spinning black holes, if observed at large inclination angles from the pole, lead to extremely large values of the bolometric correction, especially if the SMBH is highly redshifted.
• Figure 2: The relationship between $\,{\alpha_{\rm ox}}$ and $\log_{10} L_{2500}$ for our super-Eddington SEDs. The dashed red line (with its intrinsic scatter) represents the standard $\,{\alpha_{\rm ox}}(L_{2500 \, \mathrm{\AA}})$ relation from Lusso_2010. Many of our super-Eddington SEDs are characterized by values of the $\,{\alpha_{\rm ox}}$ that are significantly higher, indicating a severe lack of X-ray emission. Our SEDs become X-ray stronger, instead of weaker, with increasing optical-UV luminosity.
• Figure 3: The photon index distribution for our $96$ super-Eddington SEDs. These values of $\Gamma$ indicate extremely steep, X-ray soft SEDs, declining rapidly with increasing energy.
• Figure 4: Top Row: Comparison (at $z=0$) between typical X-ray weak (left, mildly super-Eddington with zero spin) and X-ray strong (right, mildly super-Eddington with high spin) SEDs. Bottom Row: Same as in the top row, but at $z=6$. The 8 gray lines represent the spectrum for lines of sight ranging from $80^\circ$ (bottom) to $10^\circ$ (top) from the pole. The red line indicates the spectrum averaged over $4\pi$ steradians. In the bottom panel, the arrows indicate typical flux limits for deep-field X-ray surveys with Chandra ($2-10$ keV) and AXIS ($0.2-10$ keV). Some of these SEDs extend up to $\sim 1$ MeV.
• Figure 5: Effect on the spectrum of a changing line of sight from the pole. The strongest X-ray emission is achieved by looking from a direction close to the pole (i.e., from small inclination angles). The red line indicates the spectrum averaged over $4\pi$.