Reionization optical depth determination from Planck HFI data with ten
  percent accuracy
  We present an estimation of the reionization optical depth $\tau$ from an
improved analysis of the High Frequency Instrument (HFI) data of Planck
satellite. By using an improved version of the HFI map-making code, we greatly
reduce the residual large scale contamination affecting the data,
characterized, but not fully removed, in the Planck 2018 legacy release. This
brings the dipole distortion systematic effect, contaminating the very low
multipoles, below the noise level. On large scale polarization only data, we
measure $\tau=0.0566_{-0.0062}^{+0.0053}$ at $68\%$ C.L., reducing the Planck
2018 legacy release uncertainty by $\sim40\%$. Within the $\Lambda$CDM model,
in combination with the Planck large scale temperature likelihood, and the
high-$\ell$ temperature and polarization likelihood, we measure
$\tau=0.059\pm0.006$ at $68\%$ C.L. which corresponds to a mid-point
reionization redshift of $z_{\rm re}=8.14\pm0.61$ at $68\%$ C.L.. This
estimation of the reionization optical depth with $10\%$ accuracy is the
strongest constraint to date.
