Luminous z \geq 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 Ly \alpha 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 towards 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/NIRES and Gemini/GMOS . \edit 1By using the Mg ii single-epoch virial method , we find that it hosts a ( 1.39 \pm 0.16 ) \times 10 ^ { 9 } ~ { } M _ { \odot } SMBH accreting at an Eddington ratio of \lambda _ { Edd } = 0.7 \pm 0.1 , consistent with the values seen in other luminous z \sim 7 quasars .
Furthermore , the Ly \alpha 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 \langle x _ { HI } \rangle = 0.70 ^ { +0.20 } _ { -0.23 } ( ^ { +0.28 } _ { -0.48 } ) within 68 % ( 95 % ) confidence intervals when marginalizing over quasar lifetimes of 10 ^ { 3 } \leq t _ { Q } \leq 10 ^ { 8 } yr .
This is the highest IGM neutral fraction yet measured using reionization-era quasar spectra .