Weakness of X-rays and variability in high-redshift active galactic nuclei with super-Eddington accretion
Kohei Inayoshi, Shigeo S. Kimura, Hirofumi Noda
Abstract
Abstract JWST observations have enabled the exploration of active galactic nuclei (AGNs) with broad-line emission in the early Universe. Despite the clear radiative and morphological signatures of AGNs in rest-frame optical bands, complementary evidence of AGN activity—such as X-ray emission and ultraviolet/optical variability—remains rarely detected. The weakness of X-rays and variability in these broad-line emitters challenges the conventional AGN paradigm, indicating that the accretion processes or environments around the central black holes (BHs) differ from those of their low-redshift counterparts. In this work, we study the radiation spectra of super-Eddington accretion disks enveloped by high-density coronae. Radiation-driven outflows from the disk transport mass to the poles, resulting in moderately optically thick, warm coronae formed through effective inverse Comptonization. This mechanism leads to softer X-ray spectra and larger bolometric correction factors for X-rays compared with typical AGNs, while being consistent with those of JWST AGNs and low-redshift super-Eddington accreting AGNs. In this scenario, ultraviolet/optical variability is suppressed due to photon trapping within super-Eddington disks, while X-ray emissions remain weak yet exhibit significant relative variability. These characteristics are particularly evident in high-redshift AGNs powered by lower-mass BHs with $\lesssim 10^{7-8}$ M$_\odot$, which undergo rapid mass accretion following overmassive evolutionary tracks relative to the BH-to-stellar mass correlation in the local Universe.