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

ABSTRACT 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.