RELIKE: Reionization Effective Likelihood from Planck 2018 Data
  We release RELIKE (Reionization Effective Likelihood), a fast and accurate
effective likelihood code based on the latest Planck 2018 data that allows one
constrain any model for reionization between $6 < z < 30$ using five
constraints from the CMB reionization principal components (PC). We tested the
code on two example models which showed excellent agreement with sampling the
exact Planck likelihoods using either a simple Gaussian PC likelihood or its
full kernel density estimate. This code enables a fast and consistent means for
combining Planck constraints with other reionization data sets, such as kinetic
Sunyaev-Zeldovich effects, line-intensity mapping, luminosity function, star
formation history, quasar spectra, etc, where the redshift dependence of the
ionization history is important. Since the PC technique tests any reionization
history in the given range, we also derive model-independent constraints for
the total Thomson optical depth $\tau_{\rm PC} = 0.0619^{+0.0056}_{-0.0068}$
and its $15\le z \le 30$ high redshift component $\tau_{\rm PC}(15, 30) < 0.020
$ (95\% C.L.). The upper limits on the high-redshift optical depth is a factor
of $\sim3$ larger than those reported in the Planck 2018 cosmological parameter
paper using the FlexKnot method and we validate our results with a direct
analysis of a two-step model which permits this small high-$z$ component.
