Ultra-High-Energy Cosmic Rays from Active Galactic Nuclei Jets: The Role of Supermassive Black Hole Growth and Accretion States

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General comments:  the methodology here is excellent, and the computer simulations are very good and certainly need to be presented to a wide audience.  I love the idea of using cosmic rays as an indicator or measure of AGN activity and AGN jets and
SMBH growth.

                The task of the authors is to give the reader confidence in both the computer simulations that you are using (you did that), and in the input that you are using as well.  Of course, the authors have no recourse to do other than what they did, but I am not convinced that the input data are good or accurate, which of course means that no matter how good the computer code is, the output could be poor in quality.  In particular, I don’t know how you can know much about the number or luminosity distribution of high z AGN, and even if you could know, they would be of statistically small numbers and untrustworthy.  (How many z = 20 AGN do we know anyway?)  In other words, they would implicitly be making assumptions of galaxy evolution which may or may not be correct.

                The only remedy here that I could suggest is that the authors put caveats or warnings in the text about the output being only as good as this input.  This was actually done in lines 277-279.  More statements like this would be useful.

                Another general comment I would have is to refer more to the very high energy (TeV) astronomy that is typically done with Cherenkov telescopes.  In particular, the results of VERITAS, MAGIC, and HESS are most illuminating.  I mention this because the same sources and mechanisms (Compton boosting from jets due to high energy particles) are involved here.

Specific comments: 

1.      Lines 31-32, it would be nice to include a figure here as you refer to the knee of the spectrum

2.      Line 173, please define λ.

3.      For Fig. 5, you should either explain what φ represents, or explain what the y-axis means in this figure.

 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this work, the authors estimated the contribution to UHECRs flux from AGN jets based on the JET framework. They modeled the growth of black holes through the processes such as gas accretion and merger, with considering  the feedback of launched jets to the accretion process. In particular, they compared the UHECR flux expected in the two black hole growth models, the sEDD scenario and the EDDlim scenario. While the work addresses an interesting and important topic in astroparticle physics, I think a clarification of (and possibly corresponding revision to) the treatment of some key physical processes such as UHECR interactions is needed. I would recommend the paper for publication if the authors can address my following comments. 

1. It is not clear to me how the authors estimate the UHECR flux. In fact, the attenuation of UHECRs during their propagation is quite severe. Even for UHECRs of energy as low as 1 EeV, they will lose a large fraction of their energy via the Bethe-Heitler pair production process if they are injected at high redshift. The authors argue that the sEDD model and the EDDlim model predict quite distinct UHECR flux at z>4 and hence may be used to differentiate two models. This argument cannot be justified if the attenuation is not correctly dealt with. 

 

2. A related issue is the diffuse gamma-ray and the diffuse neutrino flux generated in the interactions of UHECRs. As demonstrated in previous studies (Liu et al. 2016, PRD, 94, 043008; Heinze et al. 2016, ApJ, 825, 122), a pure proton composition of UHECRs would result in too high flux of gamma rays and neutrinos which violate observations, unless a negative cosmological evolution of UHECR source is present.  It seems not the case for the AGN jets generated in the model. This fact need be considered and discussed. 

 

3. Eq.9 need be referenced. Also, the maximum energy should be related to the Lorentz factor of the jet (AGN jets are indeed higly relativistic), which is missing in the equation. In addition, as reflected in the equation, the nuclear charge number of UHECR particles are important. Based on observations of Pierre Auger Observatory (e.g., Aab et al. 2017, JCAP, 4, 38), the composition deviates from pure protons at the highest energies, which does not support the assumption of the pure proton composition by the authors.

 

4. In the modeling, the authors turn on the jet mode only if the Eddington ratio is less than 0.01 or larger than unity. The influence of the uncertainties of the critical values of the Eddington ratio need be discussed. 

 

a few typos:

Figure 3 caption : "for different BH ans stellar mass cuts", ans -> and

Line 281: an extra "by"

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed my concerns and I recommend the paper for publication.