Peer review of: Evolving parsec-scale radio structure in the most distant blazar known (2020)

RR2

Reviewers' comments:
Reviewer #1 (Remarks to the Author):
The authors have carefully considered the comments of the various referees and have made amendments that materially improve the paper. I would say that this work certainly deserves to be published somewhere and the revision work that the authors have done will therefore be useful. I am, however, not confident that this is the right venue. In the end re-reading the paper it seemed that the 'discovery of polarization' is not in and of itself surprising enough to merit a Nature communications publication. A detailed measurement of the polarization properties and exploitation of these to constrain the source physics would be very valuable. But that is for the future.
It seems that the various arguments about the jet dynamics are the real heart of the paper. I actually found these more interesting, although I did not agree with all conclusions. However these were too weakly justified at this point to make this paper very influential. Nor is it clear whether the authors' conclusions teach us much about high redshift jets, as a class. In sum, I think this paper is evolutionary rather than revolutionary. If the editors and other referees endorse publication, fine, but -- after this second reading -- I would advocate submission to a different Journal.
Reviewer #2 (Remarks to the Author):
The revised draft and the authors' response address the issues raised by the referees well. The revision is written clearly, and significant findings of this work, measurements of the radio polarization and the proper motion, are justified well.
However, there seem to be a few things to be further clarified (see below). So I recommend that this article is accepted for publication after clarifying the issues.
(1) The estimation of the BH mass is now lowered to ~10^7 M_sun from ~10^9 M_sun. Implications of this on the SMBH growth in the early Universe need to be discussed.

(2) It is said that in J0906+6930 a nascent jet is embedded in a "dense medium". However, the density of the ambient medium is estimated to be >1.3, which doesn't seem to be dense. This should be clarified.
(3) In appendix C and the author's response (page 4), it appears that the authors took a quadratic "mean" of the statistical and the fitting error. It is not clear why "mean" not "sum"?
(4) In the author's response (page 5), the error formula for T_b is strange; shouldn't it be σ<2><Tb>=(dTb/dS)^2 dS^2 + (dTb/dDmaj)^2 dDmaj^2 + (dTb/dDmin)^2 dDmin^2 ?
If so, I am afraid that the error reported in the article is wrong.
(5) For the jet power estimation based on the radio luminosity, they used an empirical formula. In general, the data used for deriving the empirical formula (e.g., those in Foschini 2014) have large scatter (an order of magnitude) which is not explained with the formula. So simply accounting for the parameter errors in the empirical formula may be an underestimation of the uncertainty of the jet power. Although it is hard to quantify the scatter and doing so may not change the conclusion of the paper, the scatter should be discussed qualitatively at least.
(6) The authors say "A prominent X-ray emission indicates a high accretion rate, characteristic of a luminous optically-thick geometrically-thin disk". It is still not clear how X-rays are related to accretion or a disk in these SMBHs (not stellar-mass BHs). So this should be explained. If the authors would like to argue for a strong disk (or accretion) in the source, it may be better to say strong disk emission in the optical band seen in high-z blazars; see Romani (2016) for J0906+6930, and An & Romani (2018) for four high-z blazars.
Reviewer #3 (Remarks to the Author):
The revised manuscript is fine for me. I warrant the publication.