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

Reviewers' comments: 

 

Reviewer #1 (Remarks to the Author): 

 

Referee's Report for Tao An et al MS 213259 

 

This paper discusses some new VLBI measurements of the most 

distant blazar, J0906+6930. The VLBI maps are nice and make 

an improvement over the previous publication in Zhang et al 2017. 

The new detection of significant (10\%) linear polarization 

is what is featured here. The paper proceeds to some interpretation 

of component motion in the multi-epoch VLBI maps. 

 

On the whole the paper is fine, with a useful addendum of information 

on an interesting source. I found it well worth reading. A few aspects 

did bother me though. 

 

1) It is rather disturbing to see in Figure 2 the relatively strong 

linear polarization flux with no indication of EVPA. I see that 

in Appendix D the authors note that they did not attempt to get the EVPA; 

why not?? You made the Q and U maps; if you trust these we should have a 

EVPA value, if not, why do you trust your Pi? The leakage terms 

should be known, albeit perhaps imperfectly. 

The EVPA is the most important datum for physical 

interpretation; this is the aspect that is likely to influence scientists 

in this field. Thus, I believe that you should plot an EVPA estimate on 

Figure 2 (with all appropriate caveats), and discuss at least briefly. 

If the data do not allow any useful estimate, please state clearly why 

and why the Pi measurement is then still robust. 

 

2) The interpretation focuses on the idea that J1 is a termination shock 

with the deflected jet represented by J2, motivated by the apparent 

stable core-J1 distance. I think this is plausible, but surely 

there are other interpretations -- perhaps J1 and the core are two 

standing shocks in the outflow. So while the idea that this is a nascent 

jet tunneling through the host ISM is interesting, some alternatives 

should be mentioned. One motivation for doing this is that the present 

apparently substantially sub-Eddington jet luminosity may not be 

a characteristic state for such a large BH mass at high z. One might 

expect an earlier, more typical, high luminosity phase to have cleared 

the local ISM. Perhaps the BH has recently entered the present dense, 

uncleared ISM, but this seems odd. 

 

3) Related to 2): looking through the appendices it appears that the 

authors have been somewhat cavalier in their model estimates. Given that 

these are intrinsically imprecise the general conclusions may be OK, 

but the logic seems a bit forced. For example, they use the beta_perp~2.2 

of J2 in a number of equations when evaluating the properties of the 

shock at J1. For example the theta~7deg is applied to J1 to get a true 

core separation. These numbers are also used in the penultimate paragraph 

of the main article, which further obscures that this beta_perp is 

being applied to the pre-J1 flow. (Also the argument around Eq 13 

is circular -- beta_h was just inferred for a strong shock, which assumes 

the high density contrast.) These examples suggest that, at 

a minimum the authors need to be clearer about what they have assumed 

in making their estimates (e.g. small J1/J2 angle deflection, no 

deceleration). 

 

The authors did argue that the 2017 J1 flux was responding to the 2011 

core flare. That could give an effective beta_perp estimate for pre-J1 

flow of 17.3lt-y/<6y >2.9. That is crude but it does apply to J1. 

 

4) Still in Appendix H, I was somewhat puzzled why the authors used 

the monochromatic expression from Birzan et al. (Eq 16) in Eq 10. Since 

one sees an SED peak in the radio we have a ~bolometric luminosity 

so the bolometric expression from Birzan (Eq 13) would seem more 

appropriate; that gives L_j~3.4e45 erg/s and M_edd~0.03 for 10^9Msun, 

with the numbers quoted. A more careful integration of the rest frame 

radio luminosity should be used, though. On the other hand An et al 

(ref 25) give M as 4x10^9 Msun, which re-lowers the efficiency -- 

you might wish to use that mass in your estimates 

or at least mention it and give the scaling. On the subject of 

the radio SED peak -- this was published in Romani 2006 AJ 132, 1959. 

the reference used (14 = Zhang et al 2017) largely drew the data from 

that paper and I don't think it materially improved the spectrum measurement 

on that earlier result. 


The Coppejans reference in App. H should be 2016, not 2017. 


Overall I think that this is a good paper, but it could be better. If, 

as the authors repeatedly note, measuring the polarization is a big deal, 
they should give an EVPA estimate. If the nascent jet/ISM shock argument is the 

most important result, the arguments about its energetics should clearer. 

Perhaps with a bit of attention to the points above these aspects could be improved. 

Publication would then be appropriate. 

 


Reviewer #2 (Remarks to the Author): 

 

The paper reports on multi-epoch VLBI observations of the highest-redshift (z=5.47) blazar 
J0906+6930 to measure proper motion and polarization of its jet. Using the astrometry and 
polarimetry observations, the authors found that the bulk Lorentz factor of the jet is >3.3 with a 
viewing angle of <7.1 degree, and the jet luminosity is 8.2e44 erg/s (6e-3 Eddington luminosity), 
suggesting that the blazar's jet is relatively less powerful (compared to those in nearby Universe). In 
addition, the authors infer an ISM density of 65 cm^-3, >11 times denser than the jet. This together 
with the jet bending implies that the low-density jet is interacting with dense ambient medium. 
These results from the first proper-motion and polarization measurement of the record-holding 
blazar is certainly very interesting and can help researchers in the field to further understanding of 
high-z blazars and early Universe. 

 

The paper is well organized, but I do not recommend that this paper is accepted for publication in 
Nature Communications for reasons shown below. 

 

While observations of record-holding sources often provide new insights into physics in similar 
sources, it is not clearly presented in the manuscript that their results do so. The radio polarization 
measurement for such a high-z blazar is new, but the measurement seems not to add significantly 
new information to the results (other than strengthen the argument). It seems that the other 
measurements (e.g., proper motion) also mainly strengthen previous suggestions/conclusions for 
the same source and similar high-z blazars, and I do not find their interpretations and conclusions 
are novel. One can find similar interpretations/conclusions in (or directly infer from) previous works 
(e.g., a less powerful jet of J0906 in Zhang et al. 2017, An and Romani 2018, and jet bending due to 
interaction with ambient medium for J0906 or other high-z blazars in Frey et al. 2014, Frey et al. 
2018). Hence, the paper seems not to significantly influence current thinking of high-z blazars or 
early Universe. 


For the analyses, the authors used standard techniques (e.g., using the brightness temperature 
calculation to derive the bulk Lorentz factor, and an empirical relation for jet-power calculation) and 
assumptions. These are often accepted in the community and used in literatures, and I do not find 
that the techniques/assumptions are problematic. However, they make the authors’ conclusions 
suggestive rather than definitive. That is, the results seem not to support strongly the conclusions 
due to assumptions used, uncertainties in the measurements and/or lack of justification for 
assumptions/calculations (see below for more details). 


Below are my suggestions to improve the paper. Addressing the issues may help to improve the 
paper but may not be sufficient to make the paper acceptable. 

 

* Comments: 

These are mainly about handling uncertainties when the authors calculate derived quantities from 
measured ones. 

 

(1) Confidence intervals for the errors need to be given: 68% or 90% or something else? If 68%, the 
proper motion measurement may not be significant enough; they are only at the ~2-sigma level 
(95%). In this case, reporting upper limits may be more appropriate. 

 

(2) In Appendix C, it is said that the positional errors are 10% and 5% of FWHM of the restoring beam 
for J1 and J2, respectively. As the errors may have significant impacts on the (proper motion and 
other) results, these numbers need further justifications. 

 

(3) The authors do not propagate measurement errors adequately when reporting other derived 
quantities. Some of their measurements have relatively large uncertainties (e.g., proper motion), 
and so their conclusions may change if considering the errors. It may be very difficult to quantify all 
the errors, but they need to be considered seriously when drawing conclusions. 

 

(4) The bulk Lorentz factor is reported to be >3.3, only an upper limit. While this is consistent with 
previous estimations (e.g., Zhang et al. 2017, An & Romani 2018), it alone does not rule out a 
possibility of a powerful jet (as opposed to the claimed ‘less powerful jet’). 

 

(5) For inferring the jet power and ISM density (Appendix H), the authors assume that the spectral 
index is 0 between 1.4 GHz and 15 GHz for J1 and/or J2. Although previous measurements of the 
radio spectrum seem to support this albeit a spectral break at ~10 GHz, the measurements are for 
C+J1+J2 (or C as it dominates) as compared to J1+J2. Is it reasonable to assume the same spectrum 
for J1+J2 when the variability of C+J1+J2 is different from that of J1 (and/or J2) in Table 3 and Figure 
A1? This may be a concern when extrapolating the 15-GHz flux density to 1.4-GHz. In particular, the 
authors mention a propagating shock. The propagating shock may change the radio spectral shape in 
time (e.g., Valtaoja et al. 1992), hence assuming a radio spectral index of 0 in 2017-2018 based on 
earlier data needs a justification. In this regard, (near) contemporaneous multi-frequency 
observations may be helpful. 

 

(6) The empirical relation (between L_jet and P_1.4GHz) they used in Appendix H may have a 
relatively large uncertainty (a few orders of magnitude scatter in the data used for deriving the 
relation; Birzan et al. 2008); this (and other) uncertainty needs to be considered when deriving the 
jet power and claiming that it is low. 

 

(7) The reported number n~65 cm^-3 (in the text and Appendix H) for the ISM density is too precise 
compared to the arguments given in Appendix H (see also comments 3-5). Shouldn’t it be a limit as is 
for the density contrast rho_ISM/rho_jet>11 because beta_h is only a limit? 

 

(8) It is said “In the highest-resolution images, a sharp (>90 degree) jet bending is seen...” in the text. 
As the jet bending is seen in a previous study of the same source (e.g., Zhang et al. 2017), the 
previous work needs to be recognized. 

 

(9) It is said “a prominent X-ray emission (with a strong contribution from the disk)..” in the text. It is 
believed that X-rays in these high-z blazars are produced mainly by the synchrotron-self-Compton 
process and that disks of supermassive blackholes mostly radiate in the optical band. Hence, the 
statement needs to be clarified. 

 

(10) Contours in Figure 1-e and Figure 2 are slightly different although it appears that they are 
generated from the same observational data. 

 

(11) In Appendix H, it appears that the authors consider J1+J2. This should be mentioned explicitly. 
Assuming J1+J2, I do not reproduce S_15GHz=21 mJy from the table. In 2017-2018, the average flux 
of J1 and J2 is ~20.4 mJy. The difference is only tiny, but I wonder if there is something else I am 
missing here. 


(12) In Appendix G above equation (8) it is said “Employing equation (7) which reduces to...”. 
Equation (7) does not seem to reduce to (8), so this may be a mistake. 




Reviewer #3 (Remarks to the Author): 

The essay deals with the changes in the jet structure from high-resolutions VLBI radio observations 
of the jetted AGN J0906+6930 (z=5.47). Authors find changes in the jet direction and interpret them 
as the interaction with a dense interstellar medium, suggesting a young age for this AGN. This topic 
is important and could be of interest for a larger public, as it addresses the presence of supermassive 
black holes (SMBH) in a relatively young universe. Since there is no time enough to form a SMBH via 
a sequence of mergers of smaller black holes, the detection of SMBH in the early universe sets the 
question of how they are formed. The essay could be published, but after some points that must be 
addressed by the authors. Specific comments follows: 

 

- Authors indicated J0906+6930 as the most distant known blazar, having z=5.47. However, there is a 
recent detection by Saxena et al. (2018, MNRAS, 480, 2733) of a jetted AGN at z=5.72. The radio 
spectral index suggests it could be a radio galaxy, thus with the jet viewing angle greater than ≈10°. 
Therefore, the authors statements are formally correct. However, I would say a few words about this 
other jetted AGN. 

 

- page 1, row 19: Authors wrote: "As jetted quasars seem to harbour heavier black holes than non 
jetted ones, high-redshift blazars are probes...". The line of reasoning is inverted. High-z quasars are 
useful probes for the early universe because they are the brightest sources, and, therefore, it is 
easier to find them. But, the extreme brightness is due to: (1) the presence of a relativistic jet viewed 
by a small angle, which means that the electromagnetic emission is Doppler boosted (with a typical 
Doppler factor of 10, the luminosity of the jet is boosted to 10000 times its isotropic value); (2) as 
the jet power scales with the mass of the central black hole (P proportional to M^1.4, cf Heinz & 
Sunyaev 2003), a larger mass implies a larger observed jet power. 


- page 1, row 28-29: the radio loudness is a meaningless parameter. No need to cite it. 

 

- Section F: Doppler boosting parameter of the jet. Authors estimated the Doppler factor δ by using 
the brightness temperature as measured from the VLBI core size. The apparent jet speed 
β<sub>app</sub> is measured from VLBI observations. From δ and β<sub>app</sub>, it is then 
possible to calculate the bulk Lorentz factor Γ and the viewing angle θ. As the authors reported also 
the OVRO 15 GHz radio light curve, I suggest to calculate δ also from radio variability. An easier way 
is to refer to Liodakis et al. (2018, ApJ, 866, 137), who already did it and reported δ=8.91. Together 
with β<sub>app</sub>=2.2c measured by the authors, it results Γ=4.8 and θ=3°. Please note that 
Liodakis calculated the equipartition brightness temperature is a way different from usual. So, when 
comparing your results with him, please take into account this difference. Please note that also the 
main text -- not only Section F -- must be changed accordingly. 

 

- Section H: authors calculated the jet power by using a relationship by Birzan et al. (2008). These 
relationships links the extended radio emission measured in the MHz frequency range with the jet 
kinetic power. Therefore, the application to one component detected at 15 GHz is rather risky, as it 
is known that extrapolating from high to low frequency could give serious errors in the estimation of 
the extended emission. In addition, the jet power by Birzan refers only to the kinetic component, 
and does not include the radiative part. The kinetic part calculated in this way is based on the 
cavities excavated by the jet activity during million of years and are not comparable with a year time 
scale as from the authors of the present essay. 

I suggest to adopt the relationships by Foschini (2014, Int. J. Mod. Phys. Conf. Series, 28, 1460188), 
which links the radiative and kinetic power to the 15 GHz core luminosity. You should apply them to 
the core flux, but you can also try to apply them to the component J1. Obviously, you have to 
address the proper caveats. A quick calculation by using the core emission, resulted in a log 
P<sub>rad</sub>=45.4 [erg/s] and log P<sub>kin</sub>=46.1 [erg/s], greater than the estimates 
from the Birzan relationship, as expected. 


- the last (but not least) point refers to the estimation of the mass of the central black hole and the 
accretion rate. Authors refer to previous works, which gave a mass of the order of 
10<sup>9</sup>M<sub>Sun</sub> and accretion luminosity 0.4 times the Eddington limit. However, 
all the previous estimates adopted relationships made for non-jetted AGN. The presence of the 
Doppler boosting alters significantly the continuum emission, which implies an overestimation of the 
mass. Also the fit of the accretion disk suffers the same problem. As the optical spectra all show a 
strong Lyα emission line, I suggest to use it together the relationships by Pian et al. (2005, MNRAS, 
361, 919), which take into account the Doppler boosting. Indeed, a quick calculation of the 
λL(1350A) by using the Pian's relationships results in a value of ≈7.4×10<sup>43</sup> erg/s to be 
compared with the value of 5×10<sup>47</sup> erg/s given by Romani (2004). The radius of the 
broad-line region (BLR) from Bentz et al. (2013), results to be 18.6 light days, which implies a 
luminosity of the disk of ≈3×10<sup>43</sup> erg/s. I've not found a measurement of the FWHM of 
Lyα, but if we assume the value of 5000 km/s from other lines, the mass of the central black hole 
results to be ≈ 7×10<sup>7</sup>M<sub>Sun</sub>. 

Therefore, I suggest to recalculate the mass and accretion rate of the central black hole by using the 
Lyα line parameters and Pian's relationships. Then, revise the discussion in the text according to the 
new values. A smaller mass of the central black hole would be more suitable for a young AGN, and 
also more consistent with a high-frequency peaker (HFP). Compare also with Berton et al. (2016, 
A&A, 591, A98).