A New Measurement of the Mean Transmitted Flux in the Lyman-alpha and Lyman-beta Forest

We present new measurements of the mean transmitted flux in the hydrogen Lyman-alpha and Lyman-beta forest using 27,008 quasar spectra from the Fourteenth Data Release (DR14) of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS). Individual spectra are first combined into 16 composites with mean redshifts in the range of 2.8 < z < 4.9. We then apply Markov Chain Monte Carlo (MCMC) inference to produce a piecewise fit of the effective {\tau_{Ly\alpha}} (corrected for metal lines and optically thick absorptions) and relative {\Delta}{\tau_{Ly\beta}} optical depths. A detailed evaluation of our large data set shows a systematic offset towards lower values of {\tau_{Ly\alpha}} compared to recent results by Kamble et al. (2020) based on SDSS DR12.