2020ApJ...896...23W

Luminous z >= 7 quasars provide direct probes of the evolution of supermassive black holes (SMBHs) and the intergalactic medium (IGM) during the epoch of reionization (EoR). The Lyα damping wing absorption imprinted by neutral hydrogen in the IGM can be detected in a single EoR quasar spectrum, allowing the measurement of the IGM neutral fraction toward that line of sight. However, damping wing features have only been detected in two z > 7 quasars in previous studies. In this paper, we present new high-quality optical and near-infrared spectroscopy of the z = 7.00 quasar DES J025216.64-050331.8 obtained with Keck/Near-Infrared Echellette Spectrometer and Gemini/GMOS. By using the Mg II single-epoch virial method, we find that it hosts a (1.39±0.16)×10⁹M_☉ SMBH accreting at an Eddington ratio of λ_Edd = 0.7 ± 0.1, consistent with the values seen in other luminous z ∼ 7 quasars. Furthermore, the Lyα region of the spectrum exhibits a strong damping wing absorption feature. The lack of associated metal absorption in the quasar spectrum indicates that this absorption is imprinted by a neutral IGM. Using a state-of-the-art model developed by Davies et al., we measure a volume-averaged neutral hydrogen fraction at z = 7 of <x_HI>=0.70_–0.23^+0.20(_–0.48^+0.28) within 68% (95%) confidence intervals when marginalizing over quasar lifetimes of 10³≤ t_Q≤ 10⁸yr. This is the highest IGM neutral fraction yet measured using reionization-era quasar spectra.