The Structure of Gamma Ray Burst Jets

Round 1

Reviewer 1 Report

The authors have given a timely and comprehensive review on the jet structure of gamma-ray bursts, which I believe will benefit the GRB community. The manuscript is well-organized, however, I still got some comments and suggestions before a recommendation for publication. The details are given as follows.

1.      In page 3, line 104-109 the authors have just briefly mentioned the BNS merger event GW170817. However, it is such an important event that has played a crucial role in our understanding of structured jet. I think it is necessary to give it more credit by introducing more details about it, including the extensive theoretical modeling (both radial & angular structure) and the latest numerical simulation results. Also, establishing a typical case will help the readers that are new to this area.

2.      In page 8, section 2.4 the authors mainly focused on the propagation of hydrodynamic jets. In order to be consistent with the two jet-launching models in section 2.3, the propagation of Poynting-flux dominated jets needs to be mentioned for completeness. It differs from that of hydrodynamic jets in many ways, for instance, recollimation shock cannot be formed effectively and magnetic hoop stress is important instead. The opening angle of magnetized jets might be much smaller than hydrodynamic jets after collimation (e.g., Bromberg et al. 2014, 2016 MNRAS).

3.      There is an excess of GRB rate over star-formation rate at low redshift (Yu &Wang 2015 ApJS 218, 13; Petrosian et al. 2015 ApJ 806, 44). I wonder to what extent jet structure can influence this result, because those off-axis jets can only be observed at low redshift. Could the authors give some comments on this? Is it possible to reconcile this discrepancy by considering structured jets?

4.      While I understand that the jets may reach a quasi-universal structure after similar jet launching and propagation process, I’m still astonished at the narrowly-distributed jet parameters (energy, Lorentz factor and opening angle) in Figure 7. The jet power and density profile of progenitor envelope can be very distinct from case to case. So, what is the underlying physical reason for these narrowly-distributed parameters? Is there any self-modulation for GRB jets?

5.      The last Section 5 looks oversimplified. I suggest the authors add some more discussion on future prospect and give their own predictions for next ten to twenty years. What is the most promising way to pinpoint/diagnose the jet structure in the future? What kind of observation is needed?

Some minor typos:

1) Page 3, line 104, GRB 170718A à GRB 170817A

2) Page 5, line 130, jeton à jet on

3) Page 7, line 197, references are missing

4) Page 12, line 412, appearent à apparent

Author Response

Dear referee,

We are thankful for the comments provided, which are significant and helped in improving our manuscript. We highlighted the changes in the manuscript in boldface (except for the new section on GW170817). Below we provide point-by-point answers.

R = referee; A = authors

R: In page 3, line 104-109 the authors have just briefly mentioned the BNS merger event GW170817. However, it is such an important event that has played a crucial role in our understanding of structured jet. I think it is necessary to give it more credit by introducing more details about it, including the extensive theoretical modeling (both radial & angular structure) and the latest numerical simulation results. Also, establishing a typical case will help the readers that are new to this area.
A: We thank the referee for raising this point. We added an entirely new section about GW170817/GRB17017A at the end of the manuscript, which indeed helps much in giving context and concreteness to the theory described in the rest of the text.

R: In page 8, section 2.4 the authors mainly focused on the propagation of hydrodynamic jets. In order to be consistent with the two jet-launching models in section 2.3, the propagation of Poynting-flux dominated jets needs to be mentioned for completeness. It differs from that of hydrodynamic jets in many ways, for instance, recollimation shock cannot be formed effectively and magnetic hoop stress is important instead. The opening angle of magnetized jets might be much smaller than hydrodynamic jets after collimation (e.g., Bromberg et al. 2014, 2016 MNRAS).
A: We thank the referee for this comment. We added the suggested references along with a comment. We prefer not to add many details for the sake of simplicity.

R: There is an excess of GRB rate over star-formation rate at low redshift (Yu &Wang 2015 ApJS 218, 13; Petrosian et al. 2015 ApJ 806, 44). I wonder to what extent jet structure can influence this result, because those off-axis jets can only be observed at low redshift. Could the authors give some comments on this? Is it possible to reconcile this discrepancy by considering structured jets?
A: This is an interesting point, but it is out of the scope of this review. However, for the sake of discussion, let us not that several papers published in the last several years (e.g. Pescalli et al. 2016, A&A, 547, 40; Le et al. 2020, MNRAS, 4793, 1479; Bryant et al. 2021, MNRAS, 504, 4192) demonstrated that such an excess (as found originally by Yu et al. and Petrosian et al.) is due to severe incompleteness of the samples they use. Recently, Ghirlanda & Salvaterra 2022 confirmed that there is no excess at low redshift but rather there is a dearth of long GRBs with respect to the star-formation rate (in agreement with the low metallicity bias as derived from GRB host studies). 

R: While I understand that the jets may reach a quasi-universal structure after similar jet launching and propagation process, I’m still astonished at the narrowly-distributed jet parameters (energy, Lorentz factor and opening angle) in Figure 7. The jet power and density profile of progenitor envelope can be very distinct from case to case. So, what is the underlying physical reason for these narrowly-distributed parameters? Is there any self-modulation for GRB jets?
A: The peak luminosities of GRB-supernovae have a dispersion of around one dex, while the corresponding GRBs have isotropic equivalent energies tha span ~6 orders of magnitude (e.g. figure 8 in Lu et al. 2018, ApJ 862, 2, 130). This is one of the lines of evidence that suggests that the GRB progenitor parameter space is relatively narrow, and that to a large extent their diversity is to be attributed to extrinsic sources (e.g. the viewing angle). To date, this remains only a suggestion, and the viability of a quasi-universal jet structure unification of GRBs remains an open field of research. We added a comment about this on page 17.

R: The last Section 5 looks oversimplified. I suggest the authors add some more discussion on future prospect and give their own predictions for next ten to twenty years. What is the most promising way to pinpoint/diagnose the jet structure in the future? What kind of observation is needed?
A: We added some additional remarks about the possibility to learn about the jet structure from observations in section 4.2 and in section 5 (now section 6).

R: Minors:
Page 3, line 104, GRB 170718A à GRB 170817A
Page 5, line 130, jeton à jet on
Page 7, line 197, references are missing
Page 12, line 412, appearent à apparent

A: Typos corrected, thanks.

Reviewer 2 Report

No comments, the paper is complete and self sustaining.

A good review which shall be useful for future research works.

Author Response

We thank the referee for their kind comments.

Reviewer 3 Report

This paper summarizes the history of the GRB jet structure research and describes the current understanding of the jet structure formation and its effect on the prompt/afterglow emission and on the luminosity function. This paper deserves publication once the following comments are properly addressed.

In page 2, the authors said the jet angles are estimated from 4 degree to 16 degree. However in fact some GRBs with jet opening angle smaller than 1 degree have been found (e.g. Lloyd-Ronning et al. 2020, MNRAS, 494, 4371), which should be mentioned in the paper.

In page 3, for power law jet model the reference Wei & Jin (2003, A&A, 400, 415) should be cited.

In page 3, for structured jet model of GRB 170817A, the following papers should be cited: Ryan et al. 2020, ApJ, 896, 166; Alexander et al. 2018, ApJL, 863, L18; Wu & MacFadyen, 2018, ApJ, 869, 55

In page 3: “GRB170718A” should be “GRB 170817A”

In page 7: the end of the first paragraph: what’s the “[…]”?

Author Response

Dear referee,

We are thankful for the comments provided, which are significant and helped in improving our manuscript. We highlighted the changes in the manuscript in boldface (except for the new section on GW170817). Below we provide point-by-point answers.

R = referee; A = authors

R:  In page 2, the authors said the jet angles are estimated from 4 degree to 16 degree. However in fact some GRBs with jet opening angle smaller than 1 degree have been found (e.g. Lloyd-Ronning et al. 2020, MNRAS, 494, 4371), which should be mentioned in the paper.

A: It is difficult to trust these estimates, which trace back to Lu et al. 2012 (ApJ 745, 168) and Chandra & Frail 2012 (ApJ 746, 156). These are bulk studies which analyze large numbers of GRBs. The study from Lu et al. assumes a fixed gamma-ray efficiency and ISM density to estimate all the opening angles, which introduces a systematic error which is not included in their error estimates. Chandra & Frail collect ISM density estimates from other studies in the literature (not sure how to assess the reliability of these), but some of the jet break times they report are unrealistic (e.g. a jet break at 0.004 days - i.e. 346 seconds post-trigger - for GRB 080603B, which translates to an opening angle of 0.45 deg according to their estimate). We added a footnote about this.
 
R: In page 3, for power law jet model the reference Wei & Jin (2003, A&A, 400, 415) should be cited.
A: We thank the referee for pointing out this reference. We think it is more relevant to cite it in section 4.2. We added a citation there.
 
R: In page 3, for structured jet model of GRB 170817A, the following papers should be cited: Ryan et al. 2020, ApJ, 896, 166; Alexander et al. 2018, ApJL, 863, L18; Wu & MacFadyen, 2018, ApJ, 869, 55
A: Thank you, we added the suggested citations.
 
R: In page 3: “GRB170718A” should be “GRB 170817A”; In page 7: the end of the first paragraph: what’s the “[…]”?
A: Typos corrected, thanks.

Reviewer 4 Report

The article “The structure of Gamma-ray Burst jets” by Salafia and Ghirlanda is an interesting, timely and comprehensive pedagogical review on this topic related to Gamma-ray bursts (GRBs). I believe it should be accepted for publication after some minor revisions. I present some of these necessary revisions below.

1. The introduction should include references to a few recent review articles related to GRBs. It should also include some references to a few recent review articles on electromagnetic signatures of neutron star mergers and GW170817 (e.g., Margutti & Chornock 2021, ARA&A 59, 155, and Nakar 2020, PhR 886, 1). 

2. There should be a note when referring to Fig. 1 (and the NASA ADS search the authors have conducted, page 3 line 67) that the exercise in Fig. 1 should only be taken as a guideline, since it is possible that other papers may have referred to a structured jet using other terms.  In other words, this exercise represents only a lower limit.

3. In page 3 there is a mention of orphan afterglows in line 99. A reference to Granot et al. (2002, ApJ 570, L61) should be included here. In fact, it seems to me that already in this paper there is an attempt of using a more realistic jet profile obtained from simulations where “there is some material at the sides of the jet with a moderate Lorentz factor” (their model 3) although the word “structured” is not explicitly used. 

4. In line 197 page 7 there is a missing reference “[…]”.

5. Line 249 in page 8 discusses the pair annihilation mechanism to launch a jet, but it should be noted that the total jet power in this scenario may not be sufficient to power a GRB (e.g., Kawanaka et al. 2013, ApJ, 766, 31).

6. In page 9 (in the line before equation 4), there is a reference to refs. 78 and 79. Other relevant references should be included here like ref. 80 (Begelman & Cioffi 1989) and also ref. 81 (Bromberg et al. 2011).

7. In line 336: there are 2D MHD simulations of short GRB jets (e.g., Kathirgamaraju et al. 2018, MNRAS 473, 121) and 3D MHD simulations of long GRB jets (e.g., Bromberg & Tchekhovskoy 2016, MNRAS, 456, 1739) that consistently consider jet launch.

8. In line 352, there are other realistic 3D GRMHD simulations that consistently consider jet launch (e.g., Christie et al. 2019 MNRAS 490, 4811; Kathirmaraju et al. 2019, MNRAS, 484, 98; Fernandez et al. 2019, MNRAS, 482, 3373).

9. In line 506, a reference to other papers when computing the average Liso(theta_v) for the prompt emission can be added (e.g., Kathirgamaraju et al. 2018, MNRAS 473, 121; Kathirmaraju et al. 2019, MNRAS, 484, 98; Beniamini et al. 2019, MNRAS, 483, 840).

10. In line 619, it is interesting to note that Sari & Piran 1995 (ref. 190) reached a similar conclusion to that of Figure 8 using similar arguments, but they did not consider a structured jet. 

Author Response

Dear referee,

We are thankful for the comments provided, which are significant and helped in improving our manuscript. We highlighted the changes in the manuscript in boldface (except for the new section on GW170817). Below we provide point-by-point answers.

R = referee; A = authors

R: The introduction should include references to a few recent review articles related to GRBs. It should also include some references to a few recent review articles on electromagnetic signatures of neutron star mergers and GW170817 (e.g., Margutti & Chornock 2021, ARA&A 59, 155, and Nakar 2020, PhR 886, 1).
A: We thank the referee for pointing out these missing references. We added them.

R: There should be a note when referring to Fig. 1 (and the NASA ADS search the authors have conducted, page 3 line 67) that the exercise in Fig. 1 should only be taken as a guideline, since it is possible that other papers may have referred to a structured jet using other terms.  In other words, this exercise represents only a lower limit.
A: We added a note in the figure caption.

R: In page 3 there is a mention of orphan afterglows in line 99. A reference to Granot et al. (2002, ApJ 570, L61) should be included here. In fact, it seems to me that already in this paper there is an attempt of using a more realistic jet profile obtained from simulations where “there is some material at the sides of the jet with a moderate Lorentz factor” (their model 3) although the word “structured” is not explicitly used. 
A: Reference added, thanks.

R: In line 197 page 7 there is a missing reference “[…]”.
A: Fixed, thanks.

R: Line 249 in page 8 discusses the pair annihilation mechanism to launch a jet, but it should be noted that the total jet power in this scenario may not be sufficient to power a GRB (e.g., Kawanaka et al. 2013, ApJ, 766, 31).
A: We thank the referee for pointing this out. We added a sentence about the difficulties faced by the neutrino-antineutrino scenario at the end of sec. 2.3.

R: In page 9 (in the line before equation 4), there is a reference to refs. 78 and 79. Other relevant references should be included here like ref. 80 (Begelman & Cioffi 1989) and also ref. 81 (Bromberg et al. 2011).
A: References added, thanks.

R: In line 336: there are 2D MHD simulations of short GRB jets (e.g., Kathirgamaraju et al. 2018, MNRAS 473, 121) and 3D MHD simulations of long GRB jets (e.g., Bromberg & Tchekhovskoy 2016, MNRAS, 456, 1739) that consistently consider jet launch.
R: In line 352, there are other realistic 3D GRMHD simulations that consistently consider jet launch (e.g., Christie et al. 2019 MNRAS 490, 4811; Kathirmaraju et al. 2019, MNRAS, 484, 98; Fernandez et al. 2019, MNRAS, 482, 3373).
A: References added, thanks.

R: In line 506, a reference to other papers when computing the average Liso(theta_v) for the prompt emission can be added (e.g., Kathirgamaraju et al. 2018, MNRAS 473, 121; Kathirmaraju et al. 2019, MNRAS, 484, 98; Beniamini et al. 2019, MNRAS, 483, 840).
A: References added in the appropriate places, thanks.

R: In line 619, it is interesting to note that Sari & Piran 1995 (ref. 190) reached a similar conclusion to that of Figure 8 using similar arguments, but they did not consider a structured jet. 
A: We thank the referee for this comment. We read carefully the paper by Sari and Piran, but we could not find statements that suggest they get to a similar conclusion.

Round 2

Reviewer 1 Report

The authors have addressed all my questions properly.