Scientific Highlights of the AGILE Gamma-ray Mission

Round 1

Reviewer 1 Report

I had a real pleasure reading the “Scientific highlights of the AGILE gamma-ray mission” manuscript. In my opinion, this text could be quite interesting and valuable even for readers who are not connected with gamma-ray astrophysics or astro particle physics. 

Nevertheless, I have some minor comments, suggestions, and questions.

1.     Authors should improve the quality of some figures (especially description labels are not all are readable) e.g.: Fig. 1, Fig. 2, Fig. 13 (especially the left panel). 

2.     Authors use to used statements “we know”, and “we observed” without details (see below). 

a)     pp. 5, 140 – 143, “We know …as shown in Figure 2, left panel.”

-       Please add information about the date of gamma-ray flares detected by the AGILE-GRID and Fermi-LAT (“rate one every few years”).

-       Please detailed information about the frequency of waves (“more frequent”)

b)    pp.6, 173 - 17, “This model … in the TeV energy range”

-       “spectral features” – what kind of features are explained?

c)     pp. 19, 405 – 407, “An interesting .. the MeV emission”

“with noticeable delay after MeV emission” – please add information about the delay, details of the set of data (time of delay)

3.     pp. 5, 148 – 150, “Gamma – ray … theoretical models”

-       What kind of models, please add citations.

4.     Chapter 7 “High redshift sources” 

-       Fig. 9, the F1 is not visible in the Panel (n). Authors should change the scale/base of visualisation

-       Why did authors compare 4C +71.07 (F2) with PKS 1830 – 211 (F1)?

Based on the results of observation from Fig. 9 and Fig. 10 it is visible that 4C +71.07 F2 (gamma-ray) is associated with flare of X-ray (Fig. 9, Panel (m)) and the same was observed for PKS 1830 – 211 F3 (Fig. 10, Panel (c)). 

-       Did Authors observe/find any correlation that could be proven by simple “correlation factors” not only description?

5.     The list of abbreviations (pp. 24) should be completed e.g. EGRET, Fermi-LAT

Author Response

Dear referee,

we thank you for your comments, which we took into account in full. In the following you can find the detailed responses.  The changes are marked in red in the revised text.

Stefano Vercellone, for the authors

I had a real pleasure reading the “Scientific highlights of the AGILE gamma-ray mission” manuscript. In my opinion, this text could be quite interesting and valuable even for readers who are not connected with gamma-ray astrophysics or astro particle physics.

Nevertheless, I have some minor comments, suggestions, and questions.

1. Authors should improve the quality of some figures (especially description labels are not all are readable) e.g.: Fig. 1, Fig. 2, Fig. 13 (especially the left panel).

A: The figures are drawn from published, open access papers, so they cannot be modified by us, unfortunately. In particular, Figure 1 is drawn from a scanned copy of a 1962 paper, so we re-scanned it with some improvement. However, we added the information in the caption. The original of Figure 2 does not have the axis marked, so we put the information in the caption and made the figure a little bigger. We enlarged the two parts of Figure 13 and added the information in the caption.

2. Authors use to used statements “we know”, and “we observed” without details (see below).   

a) pp. 5, 140 – 143, “We know …as shown in Figure 2, left panel.”

- Please add information about the date of gamma-ray flares detected by the AGILE-GRID and Fermi-LAT (“rate one every few years”).

A: 12 different gamma-ray flare alerts (published as ATels) from the Crab Nebula were identified by AGILE-GRID and/or Fermi-LAT above 100 MeV between 2007 and 2019, i.e. a rate of one every few years. We added a table and this new sentence in the text.

- Please detailed information about the frequency of waves (“more frequent”)

A: we added the sentence: "and as can be seen in [Striani et al., 2013], Figure 7."   In fact, motivated by the waving behaviour of the flux observed in the AGILE data above 100 MeV during the Crab Nebula flaring period in September/October 2007, a systematic search for a similar type of enhanced gamma-ray emission in the publicly available Fermi-LAT data during ~ 4 years (2008 September/2012 July) was presented in Striani et al. 2013. As can be seen in their Fig. 7, in addition to the three major flaring episodes, several episodes of enhanced gamma-ray emission (at least five waves above a 3.6-sigma post-trial significance) have been identified, thus justifying the sentence ("more frequent") about the frequency of the waves. Of course the Crab Nebula behaviour and its flux enhancements are still under study, and an even broader variety (in flux and timescales) of enhanced gamma-ray emission cannot be excluded.

b) pp.6, 173 - 17, “This model … in the TeV energy range”

- “spectral features” – what kind of features are explained?

A: We added a sentence at the end of this paragraph to better explain this findings.

c) pp. 19, 405 – 407, “An interesting .. the MeV emission”

“with noticeable delay after MeV emission” – please add information about the delay, details of the set of data (time of delay)

A: We rephrased part of Section 9.1 and added quantitative details.

3. pp. 5, 148 – 150, “Gamma – ray … theoretical models”

- What kind of models, please add citations.

A: We added a reference to a recent review on this subject.

4. Chapter 7 “High redshift sources”

- Fig. 9, the F1 is not visible in the Panel (n). Authors should change the scale/base of visualisation  

A: Flare (F1) is more an enhanced gamma-ray state. 4C+71.07 started to be detected by AGILE after a long period of inactivity, and therefore we started our MWL campaign, defining that a flare. Since all the panels are in linear scale, we prefer to keep the same scale/base also for panel (n) for consistency.

- Why did authors compare 4C +71.07 (F2) with PKS 1830 – 211 (F1)?

Based on the results of observation from Fig. 9 and Fig. 10 it is visible that 4C +71.07 F2 (gamma-ray) is associated with flare of X-ray (Fig. 9, Panel (m)) and the same was observed for PKS 1830 – 211 F3 (Fig. 10, Panel (c)).

A: We thank the referee for noting this possible association. We modelled the flare F1 of PKS 1830-211 because we have simultaneous data in radio, IR, optical, X-ray and γ -ray bands (IR data are relevant to constrain the synchrotron component). On the other hand, we modelled flare (F2) of 4C +71.07 since it is the most prominent one in the gamma-ray energy band and also because we could use J,H,K and all radio data in the SED fitting. Therefore, we report here the SEDs that have been already published and that maximise the numbers of observational data at different epochs.

- Did Authors observe/find any correlation that could be proven by simple “correlation factors” not only description?

A: The number of data points would not allow a precise determination of correlations between different energy bands. Moreover, we would also need a much longer temporal baseline, as the one we presented for 3C 454.3, Figure 7, left panel, when we observed this source for 18 months and we have been able to investigate possible time-delay between the R-band and the gamma-ray one, finding that the gamma-ray emission is delayed by about half a day with respect to the optical one (see Vercellone et al., ApJ 2010, 712, 405).

5. The list of abbreviations (pp. 24) should be completed e.g. EGRET, Fermi-LAT  

A: Thank you, we updated the abbreviation list.    

Reviewer 2 Report

Congratulations on preparing such a succinct and complete review on AGILE. The review covers galactic and extragalactic findings done in the MeV range and the major theoretical implications. I find it to be a well-balanced and well-structured review. The paper is very timely, given the probe's return to Earth. Below, I provide some observations on the prepared manuscript; I have ordered them in sections and indicated some typos. Generally, it would be good to have a pass-through to check hyphens, upper/lower case usage, etc. I did not encounter any major flaws and mostly have indicated changes to the phrasing.

I somehow missed a small indication of future probes to "replace" AGILE, either in the conclusions or as an additional section towards the end of the review.

I commented through the text on some instances where you use the wording “we”, as I presumed this was a review paper of a small set of AGILE members but later realised that the paper is written on behalf of the AGILE collaboration as stated in the Acknowledgments. My indications are still valid, but not crucial, as in some places you use, for example, “The AGILE collaboration provided…” and in others, you use “we”. I suggest you uniformise the style. The use of past tense in some parts of the text should be used as AGILE recently stopped operations.

ABSTRACT

1 of "the" most

4 range of energies

8 allow -> allows

Introduction.

The authors should consider adding a last paragraph after L61 to “motivate” why AGILE was needed to continue these explorations.

Split into two sentences from l44 to 46

57 Galactic ->galactic

Sec 2: The AGILE Mission (probably change to past tense some of the sentences, since it just stopped operating). for example:

69. (2007-2023)

70. was a mission

71. it combined

72. had the possibility

73. was in a low-Earth … 85 was down-linked

86 to 89. Since you mention that ASi Malandi is in Kenya, you should also mention the location of Fucino and the ADC locations as a country, country and city or just city but all the same format.

103 very-high

103 to 106. Either this sentence is incorrect or prone to misunderstandings. The Crab Nebula has been detected up to tens of TeVs with MAGIC and H.E.S.S. (see arxiv-1905.12518) 103-111. I suggest shortening this paragraph as I find it miss-placed to mention CTA prototypes and leaving behind other IACTs; it is only needed to know that the Crab has been detected at VHEs by Chrenkov Telescopes and past 100 TeV by HAWC/LHAASO.

L110. This belongs to a new paragraph, as this is starting to talk about models of its emission.

L119. Missing a “.” after the citations.

L125 was awarded. PI abbreviation not introduced before in the text.

1. Was also confirmed the day following AGILE’s discovery (otherwise it reads as if it was discovered by Fermi the day after the award)

I suggest restructuring this paragraph 123 to 134. You start to talk again about the same flare in L129 and becomes temporarly confusing since you already stated the discovery by Fermi before etc. A solution is to move L129 at the begining. and remove the repeated mention of Fermi.

L132 observatory → satellite (?)

Figure 2. Could you indicate in the caption if the red/black lines are fitted functions. Also the plots are missing axis labels. In the right panel, could you expand the x-axis so that the green bow-tie is fully shown. I encourage to add labels in the plot.

L148 to 150 Sentence a bit hard to understand.

1. Probes of cosmic-ray acceleration.

L158 use SNR

L160 add citation to W28 detection by H.E.S.S?

L165. I understand that the authors are part of the AGILE collaboration but the use of “we” in this text should be avoided as this is a review paper from a short set of authors. Same L183, L201 we demonstrate → it was demonstrated

Remove from L187 until the end of the paragraph. It is just an explanation of the colors in the figure. Replace with some discussion on the figure, implications etc. UL_TeV is not explained, the caption says TeV data only. Could you mention where the radio data comes from? Same for TeV data?. Maybe one can rephrase “Figure 2 shows the multi-wavelenght SED of W44 with observations in Radio up to TeV. A model described by a synchrotron, brems. and IC contribution is shown and constitutes the first evidence of hadronic cosmic-ray acceleration in the 50-100 MeVs”

L197 of exposure towards this source

5. The Cygnus Region

227 one-day timescales

L234 There is no explanation in the text about the “spinning” mode of AGILE.

6 . The Crazy Diamond 3C 454.3

248 reknown → renown

L260 we

The sentence starting at L267 could be rewritten to improve readability.

7. High-redshift sources (attention to the hyphen here)

L273 the sentence lacks something to make it readable.

L283 we

L286 we

L314 one-component

1. From MeV to TeV

L336 two regions near the Galactic Plane”:”

L341 we

Figure 13 and related to Line 360. You mention that the dominance of SSC + external Compton is the favoured model, but this is not shown/supported by the figure. I think either a) add the figure with the modelling or b) at the end of this sentence L362 cite and refer to a specific figure of the bibliography

L373, This scenario is quite “accepted”, but could you add a reference supporting this statement?

1. GRBs and MM astrophysics

L382 executed and “were available” in the same sentence

L384 reacted

The paragraph of L391 is hard to understand. I am unsure if you want to say that the variation is only temporal between the two bands (delay onset of the emission) or if there is also a flux-ratio evolution between the distinct temporal phases. Tried to follow the paper cited but realised that your text is an almost exact reproduction of the abstract. Since this is another “first” of AGILE, I suggest the authors rewrite and improve the readability.

L408 It is important to note that the duration here refers to the duration of the prompt emission.

L410 are spectrally hard or have a harder spectrum compared to long GRBs.

I suggest restructuring this section a bit. The features and classification of Long and Short GRBs should open this section. The part of “Today, the AGILE and Fermi…” might be better suited for the end of this section.

L438 Add citation to the LHAASO paper

1. The AGILE legacy: the Catalogs

The section is missing a reference to table 4

11. Conclusion.

I suggest adding a sentence here and in the abstract about the end of the operations of AGILE.

See my comments in the "main" review. 

Author Response

Dear referee,

we thank you for your comments, which we took into account in full.
In the following you can find the detailed responses. 
The changes are marked in red in the revised text. 

Stefano Vercellone, for the authors

Congratulations on preparing such a succinct and complete review on AGILE. The review covers galactic and extragalactic findings done in the MeV range and the major theoretical implications. I find it to be a well-balanced and well-structured review. The paper is very timely, given the probe's return to Earth. Below, I provide some observations on the prepared manuscript; I have ordered them in sections and indicated some typos. Generally, it would be good to have a pass-through to check hyphens, upper/lower case usage, etc. I did not encounter any major flaws and mostly have indicated changes to the phrasing.

I somehow missed a small indication of future probes to "replace" AGILE, either in the conclusions or as an additional section towards the end of the review.

A: we thank the referee for having brought this to our attention, we added some text. 

I commented through the text on some instances where you use the wording “we”, as I presumed this was a review paper of a small set of AGILE members but later realised that the paper is written on behalf of the AGILE collaboration as stated in the Acknowledgments. My indications are still valid, but not crucial, as in some places you use, for example, “The AGILE collaboration provided…” and in others, you use “we”. I suggest you uniformise the style. The use of past tense in some parts of the text should be used as AGILE recently stopped operations.

A: Thank you for pointing this out. We have uniformed the text. 

ABSTRACT

1 of "the" most

DONE

4 range of energies

DONE

8 allow -> allows

DONE

Introduction.

The authors should consider adding a last paragraph after L61 to “motivate” why AGILE was needed to continue these explorations.

A: We added a paragraph following the referee's comment.

Split into two sentences from l44 to 46

DONE

57 Galactic ->galactic

DONE

Sec 2: The AGILE Mission (probably change to past tense some of the sentences, since it just stopped operating). for example:

(2007-2023)

was a mission

it combined

had the possibility

was in a low-Earth … 85 was down-linked

ALL DONE

86 to 89. Since you mention that ASi Malandi is in Kenya, you should also mention the location of Fucino and the ADC locations as a country, country and city or just city but all the same format.

DONE

103 very-high

DONE

103 to 106. Either this sentence is incorrect or prone to misunderstandings. The Crab Nebula has been detected up to tens of TeVs with MAGIC and H.E.S.S. (see arxiv-1905.12518) 

A: The referee is right, we rephrased this sentence.

103-111. I suggest shortening this paragraph as I find it miss-placed to mention CTA prototypes and leaving behind other IACTs; it is only needed to know that the Crab has been detected at VHEs by Chrenkov Telescopes and past 100 TeV by HAWC/LHAASO.

A: We rephrased this paragraph.

L110. This belongs to a new paragraph, as this is starting to talk about models of its emission.

DONE

L119. Missing a “.” after the citations.

DONE

L125 was awarded. PI abbreviation not introduced before in the text.

DONE

Was also confirmed the day following AGILE’s discovery (otherwise it reads as if it was discovered by Fermi the day after the award)
I suggest restructuring this paragraph 123 to 134. You start to talk again about the same flare in L129 and becomes temporarly confusing since you already stated the discovery by Fermi before etc. A solution is to move L129 at the begining. and remove the repeated mention of Fermi.

DONE

L132 observatory → satellite (?)

DONE

Figure 2. Could you indicate in the caption if the red/black lines are fitted functions. Also the plots are missing axis labels. In the right panel, could you expand the x-axis so that the green bow-tie is fully shown. I encourage to add labels in the plot.

A: The original of figure 2 (left) does not have the axis marked, so we put the information in the caption and made the figure a little bigger. Unfortunately we were not the authors of Figure 2 (right panel) and we cannot modify it.
Moreover, black/red lines are fitted functions and we added a sentence in the text.

L148 to 150 Sentence a bit hard to understand.

A: Since the detailed treatment of the different theoretical models is beyond the scope of this manuscript, we added a reference to a recent review which describes them in detail. 

Probes of cosmic-ray acceleration.
L158 use SNR

DONE

L160 add citation to W28 detection by H.E.S.S?

A: We added the appropriate H.E.S.S. reference 

L165. I understand that the authors are part of the AGILE collaboration but the use of “we” in this text should be avoided as this is a review paper from a short set of authors. Same L183, L201 we demonstrate → it was demonstrated

DONE

Remove from L187 until the end of the paragraph. It is just an explanation of the colors in the figure.

DONE

Replace with some discussion on the figure, implications etc. UL_TeV is not explained, the caption says TeV data only. Could you mention where the radio data comes from? Same for TeV data?. Maybe one can rephrase “Figure 2 shows the multi-wavelenght SED of W44 with observations in Radio up to TeV. A model described by a synchrotron, brems. and IC contribution is shown and constitutes the first evidence of hadronic cosmic-ray acceleration in the 50-100 MeVs”

A: We added the references for the radio and TeV data and rephrased the sentence according to the referee's suggestion.

L197 of exposure towards this source

DONE

The Cygnus Region
227 one-day timescales

DONE

L234 There is no explanation in the text about the “spinning” mode of AGILE.

A: We added a footnote explaining the spinning mode and providing references.

6 . The Crazy Diamond 3C 454.3

248 reknown → renown

DONE

L260 we

DONE

The sentence starting at L267 could be rewritten to improve readability.

A: We rephrased the whole sentence.

High-redshift sources (attention to the hyphen here)
L273 the sentence lacks something to make it readable.

A: We rephrased the whole sentence.

L283 we

DONE

L286 we

DONE

L314 one-component

DONE

From MeV to TeV
L336 two regions near the Galactic Plane”:”

DONE

L341 we

DONE

Figure 13 and related to Line 360. You mention that the dominance of SSC + external Compton is the favoured model, but this is not shown/supported by the figure. I think either a) add the figure with the modelling or b) at the end of this sentence L362 cite and refer to a specific figure of the bibliography

A: We added a reference to the Figure describing the SED.

L373, This scenario is quite “accepted”, but could you add a reference supporting this statement?

A: We added proper references to this scenario.

GRBs and MM astrophysics
L382 executed and “were available” in the same sentence

DONE

L384 reacted

DONE

The paragraph of L391 is hard to understand. I am unsure if you want to say that the variation is only temporal between the two bands (delay onset of the emission) or if there is also a flux-ratio evolution between the distinct temporal phases. Tried to follow the paper cited but realised that your text is an almost exact reproduction of the abstract. Since this is another “first” of AGILE, I suggest the authors rewrite and improve the readability.

A: We rephrased most of this subsection. 

L408 It is important to note that the duration here refers to the duration of the prompt emission.

DONE

L410 are spectrally hard or have a harder spectrum compared to long GRBs.

DONE

I suggest restructuring this section a bit. The features and classification of Long and Short GRBs should open this section. The part of “Today, the AGILE and Fermi…” might be better suited for the end of this section. 

A: We have restructured the text to better the flow and distinction between the two samples of GRBs observed 

L438 Add citation to the LHAASO paper

DONE

The AGILE legacy: the Catalogs
The section is missing a reference to table 4

DONE

Conclusion.
I suggest adding a sentence here and in the abstract about the end of the operations of AGILE.

DONE

Round 2

Reviewer 1 Report

Everything is clear and in my opinion, it could be accepted in its current form.

just one very, very small correction:
- line 314 pp 13 "the AGILE..." should be changed to "The AGILE.."
- in the text unified use "Panel" or "panel". 

Reviewer 2 Report

Dear authors,

I thank you for your replies. I have no further comments to the paper.

all the best,