Peer review of: The photospheric origin of the Yonetoku relation in gamma-ray bursts (2019)



Reviewer #3 (Remarks to the Author): 

I have read with great interest the manuscript by Ito et al. entitled 

"The photospheric origin of the Yonetoku relation in Gamma-Ray Bursts". 

I find it very well written, and containing excellent and innovative 

work. It definitely deserves publication. However, I do not consider it 

to be at the level of a Nature Communication publication. The reason for 

such consideration is twofold. First, the idea is not new, as the 

potential for photospheric emission to be able to reproduce the varius 

ensamble correlations in prompt GRB emission has been already discussed 

in many papers beforehand (which are all correctly cited). The authors 

argue that their numerical treatment is so superior to what previously 

accomplished that their results are much more credible. I do not agree 

with such an assessment, and that's my second reason for not considering 

this a Nature Communications worth manuscript. 

While it is true that extending the domain of the calculations makes 

them more credible and allows the authors to explore viewing geometries 

at large off-axis angles, it is also true that the increase of the 

domain comes at a price. The authors had to inject their jets at a 

somewhat large radius (10^10cm, for a star with radius 4x10^10cm) and 

accept a resolution that at large radii is comparable, if not larger 

than, R/Gamma^2. I am particularly concerned with the inner boundary. 

The progenitor they use is 16TI from Woosley and Heger. It has a mass of 

14.15 M_sun at pre-supernova. The mass at radii larger than 1e10 cm, 

however, is only 3 solar masses. The authors are therefore neglecting 

all the interaction that the remaining 10 solar masses have with the 

jet. Their 3D jets, at least the one shown in Figure 1, look indeed 

fairly smooth, with no strong shock, despite the role that the authors 

claim the shocks to have in the text. 

In summary, I think this manuscript presents a significant advantage 

with respect to previous MC radiation transfer results in the size of 

the domain of the hydro simulation, but an equally considerable 

disadvantage in treating the star-jet interaction accurately and in 

resolving shocks at the distances at which the photosphere is located. 

It certainly deserves publication. Just not in a Nature journal. Even if 

the authors would be able to perform a higher resolution simulation 

injecting the jet at smaller radii, I think they should still be well 

aware of the limitations of a post-processing radiation models, and 

understand that they is not the ultimate answer. First, they do not 

capture adequately the jet dynamics in regions for which the optical 

depth is of order unity; second, they do not include particle cooling, 

so that an overheated region can give infinite energy to the photons 

rather than be cooled down as a consequence of the interaction with the 

radiation field. 



Reviewer #4 (Remarks to the Author): 

This article lays out a detailed case for photospheric emission being the dominant process in the 
production of GRB prompt emission, based on comparing results from numerical simulation to the 
Yonetoku relation between photon peak energy E_p and peak luminosity L_p. The simulations 
presented do not suffer some limitations of previous works in that they are 3-dimensional; they run 
to a larger radius so that radiation can be calculated at a wider range of angles; and the photon 
spectrum and peak energy are calculated via Monte Carlo simulations of photon-electron collisions 
from photons injected at an optical depth of 100, rather than using an analytic model for photon 
peak energy. These simulations combine the best features of a number of previous approaches to 
produce the best hydrodynamic photospheric emission models to date. This article largely confirms 
the results of previous, less sophisticated approaches such as Lazzati et al. 2013, Lopez-Camara et al. 
2014, Parsotan & Lazzati 2018, among many others, that show photospheric emission is consistent 
with the Amati, Yonetoku, and Golenetskii relations. 


Comments on the article: 

1. The article does a reasonably good job of address the shortcomings of it's radiation model. The 
section "On the validity of the numerical setup" makes the point that the total energy in photons will 
not be significantly impacted by dissipation so long as thermal energy is >> than dissipation energy. 
However, even in this limit the number of photons produced could be an issue. For example, a large 
number of photons could be produced at low energy via synchrotron radiation, which could then be 
upscattered and impact the peak photon energy. The choice of location of photon injection should 
also impact the number of photons and peak photon energy. 

I am not clear on why injecting photons at an optical depth of 1e5 vs. 100 does not significantly 
change E_p. I would think the smaller number of photons would increase E_p by a factor of several 
(about 20? for injection radius of 1e10 vs. 1e12 cm) but I may not be understanding how photon 
number is calculated? Is there some mechanism for producing additional photons beyond the 
injection radius? 

2. It may also be worthwhile to compare the method used in this paper directly with the analytic 
formula of Giannios 2012 (used in Lazzati et al. 2013 and subsequent works). It should be possible to 
calculate the same quantities (for the Yonetoku relation) with this formula for the simulation 
presented in this article. 

I'm a bit worried that the choice of tau = 100 for photon injection happens to give the right E_p. A 
comparison with radiation based on the Giannios formula would help. Picking a larger optical depth 
for photon injection, e.g. tau = 1000, might also provide reassurance. 

3. I am worried that the dynamic range of the hydrodynamic simulations inside the star is only 4 
(1e10 to 4e10) compared to the larger range in other simulations, e.g. 40 in Lazzati et al. 2013. 
Although this will likely have a large effect on the particular evolution of each simulation (breakout 
time, energy distribution vs. angle, time to emergence of unshocked jet), it probably does not have a 
large impact on the correlation found in the modeled radiation. A brief comment on this limitation 
should be sufficient. 


4. A number of important papers in the development of the photospheric model seem not to be 
cited. E.g. Giannios et al. 2012 and Beloborodov 2010 for the photospheric spectrum, Lazzati et al. 
2009 and 2011 for hydrodynamic photosphere models, Golenetskii et al. 1983 for the Golenetskii 
correlation. This isn't an exhaustive list, and there may be additional papers that should be cited.