Off-Axis Color Characteristics of Binary Neutron Star Merger Events: Applications for Space Multi-Band Variable Object Monitor and James Webb Space Telescope

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper titled "Off-Axis Color Characteristics of Binary Neutron Star Merger Events: Applications for SVOM and JWST" proposes an empirical method for distinguishing between electromagnetic components generated as a consequence of neutron star mergers, i.e., between off-axis afterglows and kilonovae.

The work explores different results predicted by numerical models and contrasts them with the multi-frequency observational capabilities of SVOM/VT and JWST/NIRCam.

The paper is relevant due to its potential impact in the coming months/years. Additionally, the authors present the work within a well-suited context in the introduction, provide a clear methodology, concise results, and a high-quality discussion. The references used are necessary and appropriate.


General Comments:

- The quality of Figures 6 and 7, specifically the light curves (green lines), appears very saturated, making it difficult to clearly appreciate what is being illustrated.
- The empirical functions presented by the authors (Eq. 6-9 and Eq. 10-12) should be interpreted physically. I suggest adding a discussion of how the parameters increase/decrease and how this relates to the 0.5 magnitude delta in magnitude (Eq. 6-9). Also, consider discussing the parameter degeneracy in Eq. 10-12.
- To give the work a little more impact, I suggest adding how much time SVOM/JWST dedicates/will dedicate to following up on these events, or what their priority is.

Specific Comments (by line):

- L17: “accepted that short gamma-ray burst” —> "accepted that at least some short gamma-ray bursts”
- L23: Update the BNS rate with data from O4.
- L27: “...of GRB170817A/GW170817/AT2017gfo confirmed that BNS mergers…” —> “...of GRB170817A/GW170817/AT2017gfo confirmed by a simultaneous detection that BNS mergers…”
- L29: [7-17] —> [see e.g. 7-17].
- L34: Rewrite the last sentence. I suggest: “Due to the single detection associated with a BNS merger, it is impossible to rule out any of these models.”
- L42: "many transient sources" —> I suggest focusing the attention only on GRBs.
- L130: "the thermal emission"
- L131: Define “late time.”
- L146: Log (\epsilon_B\)
- L182-186: The description of the missions should be in the introduction.
- Figure 6: “difficult to detect” —> "difficult to distinguish”
- L220: Explain how you obtained Eq. 6-9.
- L232: “Detect” —> “Distinguish”
- L265: Same as L220, explain the functions in Eq. 10-12.

Author Response

Dear reviewer:

 

Thank you for your suggestions and help with the paper. Below is our response:

 

General Comments:

- The quality of Figures 6 and 7, specifically the light curves (green lines), appears very saturated, making it difficult to clearly appreciate what is being illustrated.

 

We have revised Figures 6 and 7, replacing the multiple lines with shaded areas to improve clarity and presentation.

 

- The empirical functions presented by the authors (Eq. 6-9 and Eq. 10-12) should be interpreted physically. I suggest adding a discussion of how the parameters increase/decrease and how this relates to the 0.5 magnitude delta in magnitude (Eq. 6-9). Also, consider discussing the parameter degeneracy in Eq. 10-12.

 

When the viewing angle exceeds the jet’s half-opening angle, the afterglow brightness is initially dim and then gradually brightens due to the increasing Doppler factor. A larger viewing angle combined with a narrower jet core results in a weaker afterglow contribution at early times. For the circumburst density, a higher density naturally leads to a stronger interaction between the ejecta and the surrounding medium, thereby boosting the afterglow’s luminosity. This leads to the result of Eq. 6-9 and Eq. 10-12. We have added the description into L213-221.

 

- To give the work a little more impact, I suggest adding how much time SVOM/JWST dedicates/will dedicate to following up on these events, or what their priority is.

 

If the inclination of a merger event is inferred from gravitational waves, and combined with constraints on the range of θ_c and n₀, one can estimate the optimal follow-up time required for kilonova detection. For example, in an on-axis event where θ_obs = 0, equations (6-7) cannot be satisfied, indicating that the window for identifying the kilonova is likely to be less than 2 days. We have added the discussion into L248-252.

 

Specific Comments (by line):

- L17: “accepted that short gamma-ray burst” —> "accepted that at least some short gamma-ray bursts”

We have revised it.

 

- L23: Update the BNS rate with data from O4.

O4 is still ongoing, and no publicly released data on BNS detections are available yet. We have updated the BNS merger rate by citing the latest published results from Abbott et al. (2023).

 

- L27: “...of GRB170817A/GW170817/AT2017gfo confirmed that BNS mergers…” —> “...of GRB170817A/GW170817/AT2017gfo confirmed by a simultaneous detection that BNS mergers…”

We have revised it.

 

- L29: [7-17] —> [see e.g. 7-17].

We have revised it.

 

- L34: Rewrite the last sentence. I suggest: “Due to the single detection associated with a BNS merger, it is impossible to rule out any of these models.”

We have rewritten it.

 

- L42: "many transient sources" —> I suggest focusing the attention only on GRBs.

We have revised the sentence to be “which has been used to fit and explain numerous afterglows associated with GRBs”, and removed the citation Ye et al. 2024.

 

- L130: "the thermal emission" and - L131: Define “late time.”

We have rewritten the sentence to be: “If the kilonova model considers only the thermal emission, the predicted flux will become inaccurate once the ejecta transitions to the nebular phase.”

 

- L146: Log (\epsilon_B\)

We have revised it.

 

- L182-186: The description of the missions should be in the introduction.

We have moved the description to the 6^th paragraph of the introduction.

 

- Figure 6: “difficult to detect” —> "difficult to distinguish”

We have revised it.

 

- L220: Explain how you obtained Eq. 6-9.

In the parameter space of θ_obs, θc, and log₁₀n₀, we separated all of the samples into four groups. These groups represent samples for which the color evolution deviates from that of a single kilonova by less than 0.5 magnitudes within two weeks, one week, two days, and a group that is not easily distinguished, respectively. We noted that the four groups can be separated by three planes in the parameter space.

 

The equation of the plane can be expressed as Aθ_obs+Bθ_c+log₁₀n₀+C=0. For example, to ensure that as many samples from group 1 as possible lie on one side of the plane, without including samples from other groups on the same side, the values of the parameters A, B, and C can be determined accordingly. Using this method, all three plane equations, corresponding to Eqs. 6-9, can be derived.

 

We have revised the description into L227-230.

 

- L232: “Detect” —> “Distinguish”

We have revised it.

 

- L265: Same as L220, explain the functions in Eq. 10-12.

To avoid repetitive descriptions, we revise it to be “Similar to samples of SVOM, these groups can also be delineated by planes in the parameter space of $\theta_{\rm obs}$, $\theta_{\rm c}$, and $\log_{10} n_{0}$.”

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The result of the article is fascinating, and the presentation of the article is quite well-organized.

However, I have comments below:

1. Line 22, please put a reference  after the sentence "constrain the equation of state of NS"

2. Please read and check the article again and please add references to sentences that are needed a reference.

Author Response

Dear reviewer:

 

Thank you for your suggestions and help with the paper. Below is our revised response:

 

1. Line 22, please put a reference after the sentence "constrain the equation of state of NS"

The references have been added:

 

Bauswein, A.; Goriely, S.; Janka, H.T. Systematics of Dynamical Mass Ejection, Nucleosynthesis, and Radioactively Powered Electromagnetic Signals from Neutron-star Mergers. The Astrophysical Journal 2013, 773, 78, [arXiv:astro-ph.SR/1302.6530]. https://doi.org/10.1088/0004-637X/773/1/78

 

Rosswog, S.; Korobkin, O.; Arcones, A.; Thielemann, F.K.; Piran, T. The long-term evolution of neutron star merger remnants - I. The impact of r-process nucleosynthesis. Monthly Notices of the Royal Astronomical Society 2014, 439, 744–756, [arXiv:astro-ph.HE/1307.2939]. https://doi.org/10.1093/mnras/stt2502.

 

2. Please read and check the article again and please add references to sentences that are needed a reference.

We have checked and added the 2 references:

L39:

Mészáros, P.; Rees, M.J. Optical and Long-Wavelength Afterglow from Gamma-Ray Bursts. The Astrophysical Journal 1997, 428 476, 232–237, [arXiv:astro-ph/astro-ph/9606043]. https://doi.org/10.1086/303625.

 

L55:

Huang, Y.F.; Dai, Z.G.; Lu, T. Failed gamma-ray bursts and orphan afterglows. Monthly Notices of the Royal Astronomical Society 458 2002, 332, 735–740, [arXiv:astro-ph/astro-ph/0112469]. https://doi.org/10.1046/j.1365-8711.2002.05334.x.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors perform simulations of the kilonova+afterglow emission lightcurves of NS-NS mergers using the afterglow py public code to predict the observations by SVOM and JWST. The research methodology is well explained and designed, and the results are clearly presented. However, after reading the paper, the question of what the paper's result is remains. I couldn't find a comparison of SVOM and JWST NS-NS merger observation capabilities relative to detectors observing these sources (e.g., Swift-UVOT, Hubble ST, and ground-based telescopes). Moreover, JWST has been operative and has released data for more than two years, so for JWST, these predictions could have already been analyzed in light of what JWST has (e.g., GRB 230307A) or has not observed. Therefore, what does this analysis tell us? We know that SVOM and JWST can detect these sources. Therefore, I strongly suggest the authors analyze (or reformulate or add conclusions) what SVOM and JWST can say more (or less) about NS-NS mergers. This information is not transparent in the current analysis presentation and the discussion and conclusions section. Further, the analysis is similar to that of Zhu et al., so I suggest clearly stating what this paper adds relative to that publication.  

Based on the above, I suggest the authors present a revised version of the paper before recommending it for publication. 

Author Response

Dear reviewer:

 

Thank you for your suggestions and help with the paper. Below is our revised response:

 

The authors perform simulations of the kilonova+afterglow emission lightcurves of NS-NS mergers using the afterglow py public code to predict the observations by SVOM and JWST. The research methodology is well explained and designed, and the results are clearly presented. However, after reading the paper, the question of what the paper's result is remains. I couldn't find a comparison of SVOM and JWST NS-NS merger observation capabilities relative to detectors observing these sources (e.g., Swift-UVOT, Hubble ST, and ground-based telescopes). Moreover, JWST has been operative and has released data for more than two years, so for JWST, these predictions could have already been analyzed in light of what JWST has (e.g., GRB 230307A) or has not observed. Therefore, what does this analysis tell us? We know that SVOM and JWST can detect these sources. Therefore, I strongly suggest the authors analyze (or reformulate or add conclusions) what SVOM and JWST can say more (or less) about NS-NS mergers. This information is not transparent in the current analysis presentation and the discussion and conclusions section. Further, the analysis is similar to that of Zhu et al., so I suggest clearly stating what this paper adds relative to that publication.  

 

Based on the above, I suggest the authors present a revised version of the paper before recommending it for publication. 

 

Zhu et al. (2022) calculated the color evolution of kilonova based on the POSSIS code. In this paper, we calculate the kilonova emission using the three-dimensional model described by Gong et al. (2024), in which the wavelength-dependent opacity and the evolution of thermal efficiency are considered, we find that our results are consistent with that of Zhu et al.(2022). In addition, this work provides a simple method to preliminary determine whether the kilonova component is present in the observed data. Usually the kilonova component can be identified only through multi-wavelength light curves fitting or obtaining the kilonova spectra, which would cost many observation time, while here we propose that we can search for kilonova component just through measuring the color evolution, which would greatly reduce the observation time. This method is suitable for SVOM VT, a new telescope for GRB follow-up observations. Once SVOM detected a short GRB, VT can quickly perform follow-up observation and preliminary determine whether there is the kilonova signal. For dim GRBs, JWST may take the follow-up observation to search for the kilonova signal.

 

We have added the above discussion in Section 4 and highlighted it in blue.

Author Response File: Author Response.pdf