Modeling Gamma-Ray SEDs and Angular Extensions of Extreme TeV Blazars from Intergalactic Proton-Initiated Cascades in Contemporary Astrophysical EGMF Models
Round 1
Reviewer 1 Report
This work uses simulations and gamma-ray observations to model SEDs and angular distributions in order to understand cascade models and ALPs. The author selects HBLs 1ES 0229+200 (hard TeV spectra) and 1ES 0414+009 for this study which are well suited for studying the IGMF and EBL.
The paper in general lacks certain details at important junctures.
- State-of-the-art constraints on ALPs and indeed more details on Monte-Carlos on the Cascades need to be provided.
- A key finding claimed here (from what I understand) is on extension of the sources. Do I have this correct ? TeVCAT reports 1ES 0229+200 as a point source (http://tevcat.uchicago.edu/?mode=1&showsrc=130) whereas 1ES0414 has constraints on extension (https://inspirehep.net/literature/1768862). Predictions / findings in this paper have to be confronted with (these) detailed observational results and would be a separate work on its own needing far more detailed study than presented here. Alternately if the goal is to use these observations to eliminate certain models which predict extension but are not observed then (eg with 1ES 0229+200) then that should be stated clearly.
- The observations used must not merely be cited (HESS and VERITAS), but also explained briefly. For example, there is no mention of how long the observations are, energy threshold (critical for this study) and zenith angle, and also on whether there were any spectral hardening during these observations (these sources can have harder and softer states).
Therefore, significant improvements are needed for this work especially providing a clarification on the claims of extension, to be accepted.
Line 23 - ”….blazars are also used as standard candles….” – The term Standard Candles (or even Standardizable Candles) is most popularly for sources (like SNe) where the behaviour is highly calibrated – blazars are highly variable and in a variety of different ways so not easiest to calibrate in any absolute sense. So perhaps a phrase like “standard probe” would be more suitable
Line 35 – Spectral hardening of these blazars is important – a cosmological effect would impact every observation whereas a source effect might impact specific states ; in general it depends upon the spectral / variability state which should be acknowledged. And ideally testing for new physics (for eg. ALPs) should be done within a single, relatively steady (or non-variable) state. Hadronic cascade models would have low variability, but then that’s the context within which the subsequent results must be interpreted and not more generally.
Similarly if this is what motivates choice of sources 1ES 0229+200 and 1ES 0414+009, then this must be explicitly explained
Line 40 – Perhaps a sentence or two introducing to the non-expert (in ALPs) reader on how conversion to ALPs can allow gammas escape from attenuation (due to ALP interactions or lack thereof) will be useful.
Line 59-61 – “….stronger magnetic fields mean higher sensitivities to ALPs…” – explain briefly why.
Also, pointing out current best constraints on g_Ɣ-ALP would be useful either here or in section 2
Line 78 – Define the LSS acronym once again within the text
Line 93-95 – Please provide details of the estimates from the different models (if need be in an appendix) for the uninitiated reader. This will aide in building an intuition for them without running detailed simulations.
Line 132 – “…combining them with Monte Carlo simulations of precomputed cascade spectra….” No details are provided to explain how these precomputations are done, what are the specific model parameters and how sensitive are the results to these.
Line 140 – At this point, it would be useful to split into two sections or subsections – one describing the observations and analyses (even if this is short) and another explaining the results.
Line 144 – The PSF depends upon the energy at which observations are taken. Can you comment on the effect of this on figure 2
Line 152-154 – In figure 2, would be nice to have the 1ES 0229 and 1ES 0414 comparison on the same scale (eg (a) and (b) on same scale, (c) and (d) on same, etc.) ?
Line 169 – 173 – To the best of my knowledge, 1ES 0229 and 1ES 0414 have not reported any extensions with HESS ? Can the author point to any TeV observations (even limits) on the extension ? If this is the case at for observations at higher TeV energies, then the author must compare model predictions with those observations.
For the figures – please provide legends atleast for the 1 and 3 figures in case figure 2 becomes too busy looking.
Author Response
I am grateful to the anonymous reviewer for the timely review and the comments. I address all the suggestions made by the reviewer. The corrected portions of the text are shown in boldface in the revised manuscript. In the following text the comments of the reviewer are marked as R, in quotes, and our replies are marked as A. All the corrections made are highlighted in bold in the manuscript.
1. R: “The observations used must not merely be cited (HESS and VERITAS), but also explained briefly. For example, there is no mention of how long the observations are, energy threshold (critical for this study) and zenith angle, and also on whether there were any spectral hardening during these observations (these sources can have harder and softer states).”
A: The necessary details have been provided on the observations in the new version of the manuscript (except for the zenith angle data for the VERITAS measurements since it is not presented in the original paper).
2. R: “Line 23 - ”….blazars are also used as standard candles….” – The term Standard Candles (or even Standardizable Candles) is most popularly for sources (like SNe) where the behaviour is highly calibrated – blazars are highly variable and in a variety of different ways so not easiest to calibrate in any absolute sense. So perhaps a phrase like “standard probe” would be more suitable”
A: The text has been revised in accordance with this comment (to “standard probe”).
3. R: “Line 35 – Spectral hardening of these blazars is important – a cosmological effect would impact every observation whereas a source effect might impact specific states ; in general it depends upon the spectral / variability state which should be acknowledged. And ideally testing for new physics (for eg. ALPs) should be done within a single, relatively steady (or non-variable) state. Hadronic cascade models would have low variability, but then that’s the context within which the subsequent results must be interpreted and not more generally.
Similarly if this is what motivates choice of sources 1ES 0229+200 and 1ES 0414+009, then this must be explicitly explained”
A: The following sentences have been added to the text of the manuscript:
“Extreme TeV blazars, as compared to such blazars as Mkn 501 and Mkn 421, typically show weak and slow variability for the high energy and very high energy bands, which makes them suitable for studying the effects of intergalactic magnetic and photon fields.”
“Two extreme TeV blazars were chosen as sources for this work because of their low variability.”
4. R: “Perhaps a sentence or two introducing to the non-expert (in ALPs) reader on how conversion to ALPs can allow gammas escape from attenuation (due to ALP interactions or lack thereof) will be useful.”
A: The following sentence of the manuscript has been expanded:
“Indeed, if gamma-rays convert to ALPs in the presence of Extragalactic Magnetic Field (EGMF) and propagate an appreciable distance as ALPs (which are not absorbed on EBL photons) before converting back into gamma-rays near the observer, these gamma-rays experience a lesser degree of absorption on EBL photons, which can explain the hardening of the intrinsic blazar spectra.”
5. R: “Line 59-61 – “….stronger magnetic fields mean higher sensitivities to ALPs…” – explain briefly why.
Also, pointing out current best constraints on g_Ɣ-ALP would be useful either here or in section 2”
A: An additional reference to the H.E.S.S. constraints on ALP parameters (Abramowski et al. (2013)) has been provided and the following sentence has been rephrased:
“For ALP models stronger magnetic fields mean smaller values of the mixing constant $g_{\gamma-ALP}$ and, thus, higher sensitivities to ALPs.”
6. R: “Define the LSS acronym once again within the text”
A: The LSS acronym has been eliminated from the text since it is used only two times apart from the abstract.
7. R: “Line 93-95 – Please provide details of the estimates from the different models (if need be in an appendix) for the uninitiated reader. This will aide in building an intuition for them without running detailed simulations.”
A: It is not exactly clear what estimates are being mentioned here. If the referee could elaborate on this point, it would be much appreciated.
8. R: “Line 132 – “…combining them with Monte Carlo simulations of precomputed cascade spectra….” No details are provided to explain how these precomputations are done, what are the specific model parameters and how sensitive are the results to these.”
A: The details concerning the precomputed cascade spectra are briefly described in the previous paragraph. The following sentences have been added to this paragraph:
“The EBL model Gilmore, et al. (2012) was used for these calculations. This code was used in the previous works of this author, and its applicability and sensitivity of the results to the choice of EBL models are discussed in Dzhatdoev, et al. (2017).”
9. R: “Line 140 – At this point, it would be useful to split into two sections or subsections – one describing the observations and analyses (even if this is short) and another explaining the results.”
A: New graphs have been added in accordance to the requests various reviewers. It is now not quite convenient to split the section in two since discussions of each result follow the results themselves, and to separate them would present problems with the overall coherence of the text.
10. R: “Line 144 – The PSF depends upon the energy at which observations are taken. Can you comment on the effect of this on figure 2”
A: A new figure (4) has been added. It shows angular resolutions of various gamma-ray telescopes and IHCM angular extensions vs. observable energy.
11. R: “Line 152-154 – In figure 2, would be nice to have the 1ES 0229 and 1ES 0414 comparison on the same scale (eg (a) and (b) on same scale, (c) and (d) on same, etc.) ?”
A: The plots in figure 2 have been rescaled.
12. R: “Line 169 – 173 – To the best of my knowledge, 1ES 0229 and 1ES 0414 have not reported any extensions with HESS ? Can the author point to any TeV observations (even limits) on the extension ? If this is the case at for observations at higher TeV energies, then the author must compare model predictions with those observations.”
A: The following sentences have been added to the Results and Conclusions sections:
“Given that the source 1ES 0229+200 has been reported as a point source based on IACT observations Aharonian et al. (2007), the angular extension predicted by the IHCM diminishes this model's plausibility. Further statistical analysis is required to constrain or possibly disfavor the intergalactic hadronic cascade model.”
“Since this kind of angular extension has not been supported by the IACT observational data, the author of this paper concludes that a thorough statistical analysis of angular extension is crucial for constraining or disfavoring the IHCM.”
13. R: “For the figures – please provide legends atleast for the 1 and 3 figures in case figure 2 becomes too busy looking.”
A: The legends have been added to all figures.
To conclude, hopefully I was able to somewhat improve the manuscript following the suggestions of the reviewer.
Best regards,
Emil Khalikov
Reviewer 2 Report
Please see attachment.
Comments for author File: 
Comments.pdf
Author Response
I am grateful to the anonymous reviewer for the timely review and the comments. I address all the suggestions made by the reviewer. The corrected portions of the text are shown in boldface in the revised manuscript. In the following text the comments of the reviewer are marked as R, in quotes, and our replies are marked as A. All the corrections made are highlighted in bold in the manuscript.
1. R: “1. page 1 Line 6 Before "this work": This work focuses on investigating the ... IHCM. For
IHCM, EGMF determines the deection of primary CRs and electrons of intergalactic
cascades and thus is of vital importance. Their models differs by the seed field used in
the numerical simulations. We consider ...”
A: The following sentences have been rephrased:
“For IHCM, EGMF largely determines the deflection of primary cosmic rays and electrons of intergalactic cascades and, thus, is of vital importance.”
“The models assumed are based on simulations of the local part of Large-Scale Structure of the Universe and differ in the assumptions for the seed field.”
2. R: “2. page 1 line 12 It is demonstrated that observable SEDs of IHCM inside a typical ....
makes it possible to discriminate between it and the ALP model, which predicts a
longer high energy tail shape. [ please add ALP to figure 3].
11. page 8 figure 3. Please show difference between ALP and cascade models. It is in your
abstract, and readers will try to look for it.”
A: A new figure (2) has been added. It shows a comparison between IHCM model SEDs for the source 1ES 0229+200 and all considered EGMF models and ALP model SEDs obtained in SanchezConde et al. (2009). In addition, the following sentences have been added to the Abstract, Results and Conclusions sections:
“At the same time, the spectra for IHCM models would have longer high energy tails than some available spectra for the ALP models and the universal spectra for the Electromagnetic Cascade Model (ECM).”
“Fig. 2 demonstrates a comparison between IHCM model SEDs for the source 1ES 0229+200 (z = 0.14) and all considered EGMF models and ALP model SEDs obtained in SanchezConde et al. (2009) for a similar z = 0.116, ALP masses m_{ALP} = 10^{-10} eV and two EBL options: Primack et al. (2005) (P05) and Kneiske et al. (2004) (K04). All IHCM models were normalized at 1 TeV. It can be seen that IHCM SEDs in this case have longer high energy tails than ALP SEDs. However, this result is strongly dependent on the assumed parameters of the ALPs. A more thorough study examining different ALP models with varied parameters is underway and will be published elsewhere.}”
“In comparison with the ALP model SEDs from SanchezConde et al. (2009) the IHCM SEDs showed longer high energy tails. Further comparative analysis of these models including ALP models with varied parameter values is necessary for determining the discrimination criteria between the SEDs in IHCM and ALP models.”
3. R: “3. page 1 line 17. Not clear what is "extended sources" and why it is important to identify
"extended sources".”
A: The following sentence has been rephrased:
“The analysis of the IHCM observable angular extensions shows that the sources would likely be identified by most IACTs not as point sources but rather as extended ones.”
4. R: “12. page 6. Experimentalists are interested in answers to a few important questions: What
is the exposure and detector size / resolution you need to discriminate among models
you discussed in figure 1, and to discriminate between ALP and cascade models? What
is the expected rejection significance of LHASSO and CTA in say three years under
certain conditions? If you can provide such numbers, I believe some experimentalists
would be happy to test your ideas. It is not so easy for them to reproduce your work
and calculate the quantities they want. Thus I urge you to perform these calculations.
4. page 1 line 17-18. "We also evaluated the expected sensitivity of CTA and LHASSO."
If the author has no attention to do this work, it is not worth mentioning this sentence
in the abstract.”
A: To calculate the expected sensitivities of CTA and LHAASO to the models discussed in this work is a very important yet quite demanding task. Unfortunately, I do not have enough necessary data on these two instruments, and the time limits provided to revise the manuscript are rather strict, so I am not able to perform these calculations for this paper. However, such calculations provide an interesting prospect for my future publications.
5. R: “5. page 2 line 55. For cascade models, the account .... So readers understood relationship
between this paragraph and the previous one.”
A: The text has been corrected according to this comment.
6. R: “6. page 2 line 76. Add a few more sentences here about how EGMF models impacts
the predictions of observables of IHCM to let reader understand why it is important.
For example: without EGMF, the primary CRs and electrons has longer attenuation
length and thus the observed gamma-rays would have longer high energy tail.”
A: The following sentences have been added:
“Without the account of EGMF, the observable SEDs in the IHCM (in this case called "basic hadronic cascade model") inside a typical IACT PSF would have longer high energy tails because in this case primary protons and cascade electrons would not be deflected by the EGMF. An extensive research of basic hadronic cascade model has been done by the author of this paper and his co-authors in Dzhatdoev et al. (2017).”
7. R: “7. page 2. Add a table comparing different models you have studied.”
A: It is not exactly clear what should be compared here and for which models (propagation ones or EGMF ones). If the reviewer could elaborate on this point, it would be much appreciated.
8. R: “8. page 4 what is PSF?”
A: The definition of this acronym (point spread function) has been added.
9. R: “9. page 5 figure 1. It reads as if the black is the integral of blue. Modify it to: blue: SEDs
of IHCM with PSF = 0.1; black: integral of blue; red: universal component of SEDs
of different ECM due to xxx processes. It might be better to put six models togther
to emphasize the difference.”
A: In this case integral SEDs mean SEDs for which no cuts on PSF have been applied (these correspond to the basic hadronic model, which assumes a zero EGMF). The following sentences have been added or rephrased to the results section and the caption of fig. 1 to make the discussion more clear:
“Fig. 1 shows these observable SEDs (denoted as blue curves) in comparison with the integral observable SEDs (black curves), for which no cuts on the observable angles have been applied (these SEDs correspond to the basic hadronic cascade model, which assumes a zero EGMF), and the universal SEDs of ECM, where gamma-rays were the primary particles (red curves)”
“Black curves denote the integral SEDs or SEDs for the basic hadronic cascade model (i.e. SEDs, for which no cuts on the observable angles have been applied).”
10. R: “10. page 7 figure 2. It is confusing what is in this figure. Quantiles are defined as the cut
points corresponding to a percentage position of an ordered observable. If Y-axis is
the quantile, then x-axis should be the percentage.”
A: Quantiles in this case represent containment angle vs. observable energy dependences for different containment values. The following sentences have been added or rephrased to the results section and the caption of fig. 3 to make the discussion more clear:
“Different curves denote different quantiles or containment angle vs. observable energy dependences for different containment values (e.g. black solid curve denotes an angle, inside which 5\% of the observable spectrum is contained).”
“Containment angle vs. observable energy dependences for different containment values in the framework of the IHCM.”
To conclude, hopefully I was able to somewhat improve the manuscript following the suggestions of the reviewer.
Best regards,
Emil Khalikov
Reviewer 3 Report
The author of the manuscript
"Modeling gamma-ray energy and angular distributions of extreme TeV blazars from intergalactic proton-initiated cascades in contemporary astrophysical EGMF models"
discusses potentially observable consequences of intergalactic hadronic cascades in the propagation of very high energy gamma-rays from distant active galactic nuclei. This idea is in principle a physically sound potential addition to the widely accepted paradigm that for energies above ca. 30 GeV the Universe will not be transparent out to cosmological distances because of pair production of the O(TeV) gamma-rays in the extragalactic background light mainly from stars and dust. If this absorption is calculated, and the results compared with measured spectra of a sub-sample of TeV-peaked blazars, there may just be an indication of this absorption being less than expected (e.g. the spectra staying excessively "hard").
It is important to stress that this excess effect is statistically far from being proven, but the author of the manuscript at hand fairly comments on that by citing the formal significance (~2 sigma).
If the effect is real, there is a number of potential causes that are discussed in the literature. One of them, and the focus of the manuscript at hand, may be that part of the observed gamma-rays are actually produced in intergalactic cascades. In the manuscript, the author presents a discussion and calculations on a concrete such model (IHCM). He uses both previously available simulations and his own calculations to investigate the consequences for the spectra and angular extension of two relatively prolific sources, 1ES 0229+200 and 1ES 0414+009. Although he presents no new observational data, his calculations may indeed stimulate renewed observational interest in those sources in the VHE community. This especially as it is argued that such observations may indeed allow for a discrimination between scenarios compared in the manuscript.
After a careful review, I thus consider the manuscript publishable in "Universe".
I do however have some comments that I think should be addressed before acceptance:
As a major comment I would argue that the manuscript would much profit from a more detailed discussion of the question of the sources inheriting non-point-like characteristics from the processes described. Given the limited spectral resolution of typical IACT observations (remember, detections of such distant sources will mostly be statistically limited), this may well turn out to be a main constraint/discriminator. A more complete survey of published (e.g. from performance presentations and papers of H.E.S.S., VERITAS and MAGIC, and from projections presented in "Science with the Cherenkov Telescope Array" / Reference 34 in the manuscript) angular resolutions and thus abilities of present and future experiments to detect this angular extension would be very beneficial, and could be used especially to extend the short comment on the capabilities of CTA in the discussion section.
The article is in general well written and understandable. There are some minor points where the language could be improved, especially some missing articles, but this is probably a point for language editing. The most striking issue in that regard is actually already in the title:
"...angular distributions of extreme TeV blazar..." --> I think the author rather means 'angular extension' of a single source (due to the photons inheriting a non-point-like distribution), not the angular distribution of the sources over the sky.
This continues in the abstract and throughout the text.
In addition, the abstract language could also be improved:
Line 1 "showed" --> shown.
Line 2 "excessive" --> excess.
Line 4: "most promising" --> I honestly think the author here may be unintentionally "overselling" the ALP-based models (and by that underselling the models he studied!). As a matter of fact, the existence of ALPs is not proven. I would suggest to rather phrase it in a way to communicate that the discussion is open, and there are competing ideas. Observations will have to decide in the end.
Author Response
I am grateful to the anonymous reviewer for the timely review and the comments. I address all the suggestions made by the reviewer. The corrected portions of the text are shown in boldface in the revised manuscript. In the following text the comments of the reviewer are marked as R, in quotes, and our replies are marked as A. All the corrections made are highlighted in bold in the manuscript.
1. R: “As a major comment I would argue that the manuscript would much profit from a more detailed discussion of the question of the sources inheriting non-point-like characteristics from the processes described. Given the limited spectral resolution of typical IACT observations (remember, detections of such distant sources will mostly be statistically limited), this may well turn out to be a main constraint/discriminator. A more complete survey of published (e.g. from performance presentations and papers of H.E.S.S., VERITAS and MAGIC, and from projections presented in "Science with the Cherenkov Telescope Array" / Reference 34 in the manuscript) angular resolutions and thus abilities of present and future experiments to detect this angular extension would be very beneficial, and could be used especially to extend the short comment on the capabilities of CTA in the discussion section.”
A: A new figure (4) has been added. It shows angular resolutions of various gamma-ray telescopes and IHCM angular extensions vs. observable energy.
2. R: “"...angular distributions of extreme TeV blazar..." --> I think the author rather means 'angular extension' of a single source (due to the photons inheriting a non-point-like distribution), not the angular distribution of the sources over the sky.
This continues in the abstract and throughout the text.”
A: The text has been revised in accordance with this comment. The term “angular distribution” has been replaced with “angular extension” throughout the text.
3. R: “Line 1 "showed" --> shown.
Line 2 "excessive" --> excess.”
A: The text has been revised in accordance with this comment.
4. R: “Line 4: "most promising" --> I honestly think the author here may be unintentionally "overselling" the ALP-based models (and by that underselling the models he studied!). As a matter of fact, the existence of ALPs is not proven. I would suggest to rather phrase it in a way to communicate that the discussion is open, and there are competing ideas. Observations will have to decide in the end.”
A: The following sentence has been rephrased in the Abstract:
“Several extragalactic propagation models have been proposed to explain this possible excess transparency of the Universe to gamma-rays starting from a model which assumes the existence of so-called axion-like particles (ALPs) and the new process of gamma-ALP oscillations.”
To conclude, hopefully I was able to somewhat improve the manuscript following the suggestions of the reviewer.
Best regards,
Emil Khalikov
Round 2
Reviewer 1 Report
I thank the author for diligently addressing all the concerns raised and has improved the paper significantly. The additional figures and text have added crucial details. And indeed the conclusion on the source extension and implication for the models now (more conservative) is the more appropriate one in my opinion.
To just answer the author's question on which models referring to my comment "estimates from the different models" ; I meant the models astrophysical EGMF models: D05, and astrophysical and astrophysical R - in case there are quick guesstimates of deflections, that would be great.....else it may be skipped.
