2018ApJ...864..143D

Measuring the proximity effect and the damping wing of intergalactic neutral hydrogen in quasar spectra during the epoch of reionization requires an estimate of the intrinsic continuum at rest-frame wavelengths of λ_rest ∼ 1200-1260 Å. In contrast to previous works which used composite spectra with matched spectral properties or explored correlations between parameters of broad emission lines, we opted for a nonparametric predictive approach based on principal component analysis (PCA) to predict the intrinsic spectrum from the spectral properties at redder (i.e., unabsorbed) wavelengths. We decomposed a sample of 12764 spectra of z ∼ 2-2.5 quasars from the Sloan Digital Sky Survey (SDSS)/Baryon Oscillation Spectroscopic Survey (BOSS) into 10 red-side (1280 Å < λ_rest < 2900 Å) and 6 blue-side (1180 Å < λ_rest < 1280 Å) PCA basis spectra, and constructed a projection matrix to predict the blue-side coefficients from a fit to the red-side spectrum. We found that our method predicts the blue-side continuum with ∼6%-12% precision and <=1% bias by testing on the full training set sample. We then computed predictions for the blue-side continua of the two quasars currently known at z > 7: ULAS J1120+0641 (z = 7.09) and ULAS J1342+0928 (z = 7.54). Both of these quasars are known to exhibit extreme emission line properties, so we individually calibrated the uncertainty in the continuum predictions from similar quasars in the training set, finding comparable precision but moderately higher bias than the predictions for the training set as a whole, although they may face additional systematic uncertainties due to calibration artifacts present in near-infrared echelle spectra. We find that both z > 7 quasars, and in particular ULAS J1342+0928, show signs of damping wing-like absorption at wavelengths redward of Lyα.