2025ApJ...995..171C

Super-Eddington accretion is a crucial phase in the growth of supermassive black holes. However, identifying super-Eddington accreting quasars observationally is challenging due to uncertain black-hole mass estimates and other complications. The Eddington ratio parameter does not accurately represent the accretion rate in the super-Eddington regime. On the other hand, super-Eddington accreting quasars appear to show large hard X-ray (rest-frame > 2 keV) power-law photon indices, and they also exhibit distinct optical spectral features including weak [O III] λ5007 emission and strong Fe II emission. We test steep X-ray photon-index selection of super-Eddington accreting quasars by obtaining Palomar 200-inch Hale Telescope near-infrared (NIR) spectra for a pilot sample of nine Γ = 2.0-2.6 quasars at z ≈ 1.4-2.5. We derive Hβ-based single-epoch virial black-hole masses (median value 4.3 × 10⁸M_⊙) and Eddington ratios (median value 0.6). The Eddington ratio distribution is consistent with that of the comparison sample, which is a flux-limited sample of quasars at z ≈ 1.5-3.5 with NIR spectroscopy. However, our super-Eddington candidates do show statistically weaker [O III] emission (P_null = 0.0075) and marginally stronger Fe II emission (P_null = 0.06). We also find one candidate with broad (width of 1960 km s^–1) and blueshifted (690 km s^–1) [O III] λ4959 and [O III] λ5007 lines, which probably originate from a strong [O III] outflow driven by super-Eddington accretion. Overall, the steep X-ray photon-index selection of super-Eddington accreting quasars appears promising. However, a larger sample is needed to further assess the reliability of the selection.