Date: 7/24/2024
Harvard ADS: Mildly Super-Eddington Accretion Onto Slowly-Spinning Black Holes Explains the X-Ray Weakness of the Little Red Dots
Paper 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~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^\circ 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 \alpha_{\rm 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 \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 ~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.
