2021MNRAS.503.2077B

Reconstruction techniques for intrinsic quasar continua are crucial for the precision study of Lyman α (Ly α) and Lyman β (Ly β) transmission at z > 5.5, where the λ < 1215 Å emission of quasars is nearly completely absorbed. While the number and quality of spectroscopic observations have become theoretically sufficient to quantify Ly α transmission at 5.0 < z < 6.0 to better than 1 per cent, the biases and uncertainties arising from predicting the unabsorbed continuum are not known to the same level. In this paper, we systematically evaluate eight reconstruction techniques on a unified testing sample of 2.7 < z < 3.5 quasars drawn from the Extended Baryon Oscillation Spectroscopic Survey. The methods include power-law extrapolation, stacking of neighbours, and six variants of principal component analysis (PCA) using direct projection, fitting of components, or neural networks to perform weight mapping. We find that power-law reconstructions and the PCA with fewest components and smallest training sample display the largest biases in the Ly α forest (-9.58 per cent/+8.22 per cent, respectively). Power-law extrapolations have larger scatters than previously assumed of +13.1 per cent/-13.2 per cent over Ly α and +19.9 per cent/-20.1 per cent over Ly β. We present two new PCAs that achieve the best current accuracies of 9 per cent for Ly α and 17 per cent for Ly β. We apply the eight techniques after accounting for wavelength-dependent biases and scatter to a sample of 19 quasars at z > 5.7 with IR X-Shooter spectroscopy, obtaining well-characterized measurements for the mean flux transmission at 4.7 < z < 6.3. Our results demonstrate the importance of testing and, when relevant, training, continuum reconstruction techniques in a systematic way.