The Galactic Population of Pulsar Wind Nebulae and the Contribution of Its Unresolved Component to the Diffuse High-Energy Gamma-ray Emission

Round 1

Reviewer 1 Report

The paper discusses the scientific cases of Pulsar Wind Nebulae and their possible contribution to the diffuse high energy gamma-ray emission.

Their possible contribution is counterintuitive to the most and hence their result is relevant, therefore I strongly support the publication of this study.

 

The paper is well written, however according to me there is not an adequate quantity of discussion about the assumptions of their work. Hence a sensation about the introduction of possible biases in their assumptions still persists and the authors should instead want to shed light on this.

 

Once solved this point, the article seems to flow smoothly until its conclusions.

 

Therefore I suggest the paper for publication, but before there are some minor points and a couple of major points that should be addressed by the authors to improve the quality of the paper. 

 

 

 

Minor comments:

 

- Their work is mainly a statistical analysis and this should be highlighted in the abstract as well, otherwise the reader is expecting a totally different work.

 

- The dominance of PWNe in the sub-PeV sky is not only an hypothesis, but an explicit fact.

In addition to [49], the dominance of PWNe in the HAWC sky, especially above 100 TeV has been recently claimed as well (e.g. COSPAR 2022, Gamma 2022 or CRIS 2022) and published by HAWC collaboration (e.g. https://ui.adsabs.harvard.edu/abs/2023JPhCS2429a2017T/abstract ).

This should be considered, since it strengthens the thesis of the authors.

 

- "CRAB nebula"? Why all letters are capital? 

Usually it is written "Crab nebula".

 

- When introducing PWNe (page 2), according to me, it would be more appropriate to give credits to the ones that first introduced these objects, e.g. Kennel, C. F., & Coroniti, F. V. 1984, ApJ, 283, 710.

 

In general, other references important to introduce the problem seem to be missing as well: I would suggest the authors to check a recent PWN review (MPDI recently published a quite extensive review) to add some crucial references for PWNe and their open issues.

 

- The large-scale diffuse signal of Milagro is surely relevant, however, either it should be substituted here by the more updated multi-TeV results by LHAASO and HAWC (e.g. www.hawc-observatory.org/details/diffuse.php#sec:diffuse_hawc ), or it should be explained why this is not done.

 

- The main assumptions of the previous works of the authors [16-18] should be reported here as well in order to have a self consistent article.

 

- The debatable relevance of PWN contribution to diffuse emission might be strengthened as well by quoting previous works which suggested as not negligible the PWN contribution to explain the TeV detections of starburst galaxies NGC 253 and M81 (Mannheim, Elsaesser & Tibolla 2012, Astrop. Phys., 35, 797-800), especially because GeV and TeV consideration for the Milky Way were introduced as well in there.

 

 

Major comments: 

 

1- The assumptions (section 2.1) of this work are not fully understandable and in some parts they seems even counterproductive to the authors' thesis itself.

These assumptions are crucial since they constitute the base of their work, hence they have to be adequately described.

 

- Why are considered only HGPS sources with TeV gamma-ray fluxes larger than 10% of the Crab?

Could the authors comment on this?

 

I see no reason for this cut in energy (cut which should anyway be explained in the text): H.E.S.S. firmly identified sources at ~1% C.U. level.

Moreover this assumption seems indeed introduce a bias towards brighter (and hence younger?) PWNe.

Finally it looks to me counterproductive in supporting the thesis of the authors; in fact, in this way, several unidentified H.E.S.S. sources which have been proposed to have a PWN nature (mainly relic PWNe) and even firmly established PWNe are excluded from the discussion: such as the so called "dark sources" (HESS J1507-622, HESS J1427-608 and HESS J1708-410), HESS J1702-420, HESS J1837-069,  IGR J1849-0000, HESS J1616-508 and HESS J1813-126, etc. 

 

- Are unidentified sources (which might be PWNe) included or not in their sample?

As implicitly (probably the authors can be even more explicit on that) claimed by the authors, >50% of HGPS sources are unidentified/not firmly identified.

However the link between this dominant population of sources and PWN has been suggested and partially established since long time (e.g. H.E.S.S. Collaboration 2011, A&A, 525, id.A45 ) and moreover it strengthen the thesis of the authors.

 

- On the other hand, it is not clear how the authors consider the bright sources affected by "source confusion" (i.e. the sources with many plausible low energy counterparts) where the PWNe contributions seem unavoidable, such as HESS J1841-055 or HESS J1843-033

Could the authors comment on this?

 

- Concluding, the authors reduce the entire HGPS sample to 29 sources (which ones?).

The assumptions used to reduce the sample does not seem clearly described and therefore it is not possible for the reader to establish if these assumptions introduce biases or not.

 

2- (section 2.2) Given its assumptions, [9] is biased towards bright and young PWN, while it seems to exclude older and fainter PWNe systems.

Moreover older and fainter PWNe systems, as claimed by the authors, seems to be more relevant here to establish a connection between PWNe and diffuse VHE gamma-ray emission.

Hence I strongly suggest the authors to compare their results with more comprehensive PWNe time evolution models and their results, such as Gaensler & Slane 2006, ARA&A, 44, 17 (for X-rays) and Vorster, Tibolla, Ferreira and Kaufmann 2013, ApJ, 773, id. 139 (for the gamma-rays).

 

3- The importance of unresolved PWNe is highlighted all over the article, however their link with identified PWNe seems to me already represented by unidentified TeV sources, which represent >50% of the TeV Galactic sources and which indeed are often described as PWN, even in absence of a detected PSR nearby.

In fact, inn order to give a couple of examples, several unidentified sources have been later identified as PWNe (e.g. HESS J1303-631, the first unidentified source of the HGPS in 2006), there should be "Vela-like PWN" where the PSRs are not detected and even the above mentioned "dark sources" (TeV sources without lower energy counterparts) can be described only as relic PWNe.

The description of this link is missing in the paper and moreover it can further strengthen the authors' results.

The paper is well written. I would anyway suggest to ask a mother tongue speaker to help in improving the readability of the article.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript studied the contribution of pulsar wind nebulae to the galactic diffusion gamma-ray emission from GeV to sub-PeV domain. The topic is interesting to be studied based on the recent results from the gamma-ray observatories. However, the results are not well organized and, especially, some figures and a table seem the same as the past studies of the authors. The manuscript should be significantly modified to clarify the similarity and the difference from the previous studies of the authors.

Author Response

We thank the Referee for the comments. As already communicated to the Editor we realized that the article type of our contribution was not the correct one and we changed it now to "review". Indeed, following the invitation of the Guest Editors (Prof. Aharonian and Amato) of this special issue on PWNe,  our principal goal was to review and to present in a complete and consistent framework our previous results on galactic PWNe, mainly obtained in Refs. [19-20-21]. 

We explicitly stated our intentions in the introduction to make them clear to the readers.

Reviewer 3 Report

Understanding the diffuse gamma ray flux from the Galaxy has recently emerged as a subject of renewed interest with several novel theoretical studies and new data, most recently from the LHAASO experiment. Soon IceCube will release a first view of the Galaxy in neutrinos. Although mostly reviewing published papers by the same authors, this review is timely and provides a useful summary of the status of one of the important challenges in interpreting the data, especially at lower energies: the contribution of unresolved sources which requires an understanding of the population of pulsar wind nebulae. The review demonstrates how theoretical insights, for instance the claim that the data reveal a decrease of the spectral index towards the Galactic center, are held hostage to the identification of the contribution of unresolved sources. I recommend publication in its present form. 

Author Response

We thank the Referee for reviewing the manuscript.