Mildly Super-Eddington Accretion Onto Slowly-Spinning Black Holes Explains the X-Ray Weakness of the Little Red Dots
arxiv(2024)
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_∙=10^7 M_⊙ at z∼6,
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 (1.430^∘ from the pole. X-ray bolometric corrections in the observed
2-10 keV band reach ∼10^4 at z=6, ∼5 times higher than the
highest constraint from X-ray stacking. Most SEDs exhibit α_ ox
values outside standard ranges, with X-ray weakness increasing with optical-UV
luminosity; they are also 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^∘ of its pole, where beaming enhances the X-ray
emission, has a ∼1.5% probability, matching the LRD X-ray detection rate.
Next-generation observatories like AXIS will detect X-ray weak LRDs at z∼6
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
∼3. 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.
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