A comparison of quasar emission reconstruction techniques for z ≥ 5.0 Lyman α and Lyman β transmission

Reconstruction techniques for intrinsic quasar continua are crucial for the precision study of Lyman alpha (Ly alpha) and Lyman beta (Ly beta) transmission at z > 5.5, where the lambda < 1215 angstrom emission of quasars is nearly completely absorbed. While the number and quality of spectroscopic observations have become theoretically sufficient to quantify Ly alpha transmission at 5.0 < z < 6.0 to better than , 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 alpha 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 alpha and +19.9 per cent/-20.1 per cent over Ly beta. We present two new PCAs that achieve the best current accuracies of for Ly alpha and for Ly beta. 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.