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

• Published in: The Astrophysical journal 2024-11, Vol.976 (1), p.96
• Main Authors: Pacucci, Fabio, Narayan, Ramesh
• Format: Article
• Language: English
• Subjects: Accretion; Active galactic nuclei; Active galaxies; Black holes; Deposition; Emission; Emission lines; Emissions; Luminosity; Magnetohydrodynamic simulation; Magnetohydrodynamics; Observatories; Relativistic effects; Spectral energy distribution; Strong winds; Supermassive black holes; Viewing; X-ray astronomy; X-ray emissions; X-rays
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad84f7

JWST has revealed a population of low-luminosity active galactic nuclei 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 General Relativistic Radiation Magnetohydrodynamics simulations of super-Eddington accretion onto a supermassive black hole (SMBH) with M • = 10 7 M ⊙ at z ∼ 6, representing the median population; the spectral energy distributions (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 Edd < 4) with zero spin, viewed at angles >30° 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 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 an 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.