Peer review of: Measuring the Hubble constant with a sample of kilonovae (2020)

Reviewers' comments:  

Reviewer #1 (Remarks to the Author):  

In the current paper, authors present an interesting method to find the distance to kilonovae without using GW or host galaxy observations. They apply to previous sGRB afterglows and claim to obtain most precise Hubble-Lemaitre constant using these four events.  

However, to the best of my knowledge this idea was first proposed by Kashyap+2019 
(doi:10.3847/2041-8213/ab543f) to the community using a very similar methodology although only 
using bolometric luminosity. Please cite them appropriately.  

Although the methodology is not novel (presented before in Couhglin et. al. 2019; 
arXiv:1908.00889), the application of such methodology to past SGRB afterglows is presented for the 
first time here. Several additional investigations are required to establish the claim presented in the 
paper.  

General Comments:  

(1.) Authors have not provided the details of sampling over parameter space of ejecta mass and 
velocity, composition and angle. If sampling is done uniformly over all values possible for kilonovae 
to obtain a mapping between parameter values and peak absolute magnitude in K-band (Methods 
section), there are two problems that arise because of this--  


(1a) Numerical simulations show that not all combinations of parameter values are possible. For 
example, Radice et. al. 2018 and Coughlin et. al. 2019 show that large ejecta mass and large ejecta 
velocities are not supported. How will it change your conclusions?  

Alternatively, you can adopt the fitting formulae in two references above for your Eqn-2,3 (Methods 
section) mapping where they properly take into account the non-linearities of numerical simulations.  


(1b) Later for some of the GRB, a different band (e.g. r-band) is used for H0 measurement. This 
seems inconsistent at first thought because the original interpolation was derived in a different 
band.  

 (2.) Please note that the use of all bands for testing the kilonova hypothesis and parameter 
estimation is an orthogonal exercise than the use of observations of one particular band in a 
standardization method obtained from theoretical model. The use of one of the kilonovae such as 
GW170817 to inform standardization could be used however with unknown systematic errors.  

 

(3.) Kilonovae could also be observed from NS-BH binary system. Does your methodology work for 
those systems as well? Please show how and the implications of this fact.  

 

(4.) H0 measurement is presented here is highly peaked. It appears to be more accurate and in 
contrast to both Planck and SHOES data. This bold claim is very hard to digest based on sparse 
afterglows data that are not established definitively as kilonovae.  

Furthermore, please quote the results from LIGO GW only and GW-EM measurement of H0 
compared to your results possibly in figure-2 (arXiv:1710.05835,1908.06060; please cite them).  

 

(5.) Authors should note that kilonovae being less brighter than SNe Ia, could be more useful for 
constraining other distance indicators rather than as cosmological probes.  

 

;Detailed Comments:  

'23-24': The process described here and as compared to SNe Ia makes kilonovae standardizable 
candle and not a standard candle. Please note the difference and provide the correction.  


'31-32:' The phrase "...systematic uncertainty as large as statistical..." appears to show that this is 
the largest error but, it may as well be true that the actual error could be much larger than the both.  


'81-82:' I provide here comments on the use of each of the sGRB data. The primary concern is the 
sparsity of data in K-band which has been used for standardization. I quote references regarding 
each of the data. Please cite them appropriately.  


(*) GRB 150101: K-band observation is only an upper limit at day >1 day which is post-peak.  

(*) GRB 050709: Here as well, there are only two K-band observations at day 5.6 and 16.6 which is 
significantly post-peak.  
(*) GRB 160821B: There is only one K-band observation available with two upper limits. I believe this 
gives a very skewed measurement of peak value and decline rate which is very crucial to inferences 
obtained here. Please cite (10.3847/1538-4357/ab38bb) for their analysis of afterglow observation 
data for this event.  

(*) The progenitors of GRB060614 is not settled to be BNS system. It could also be tidal disruption 
event (doi:10.1086/590899) or, even a WD-NS system (10.1051/0004-6361/200810676). In fact, Xu 
et.al. 2009 (doi: 10.1088/0004-637X/696/1/971) analyzed the multiband temporal and spectral 
properties of the two afterglows and found that their afterglows were consistent with the long GRB. 
I find the use of this event questionable. It could also be an NS-BH merger, in which case please 
provide details in light of General Comments-(2) above.   

'87-89:' I believe the main results in the paper hinges upon this assumption which is very tenuous in 
the present case.  

'93-94:' It was not clear how did you use the color evolution. Please present clearly.   

'98-99:' We don't yet know if it's possible to standardize kilonovae; we can only check it provided we 
have sufficiently large sample of good quality kilonovae observations.  

'104-109:' It's not clear why distance to GW is obtained. Which GW and sGRB source are referred to 
here.  

'110-111:' Please show the distance estimate for AT2017gfo using your method of standardization. 
How does it compare with the distance to the host galaxy?  

'117-118:' Again this line is not clear. Please remove if it's not necessary for the main idea of the 
paper.  

'121:' How did the authors come up with the estimate of the systematic error bars? Please provide 
details in the Methods section.  
'line-145-146:' It is important to capture the peak of kilonova during future observation to make this 
method usable. Multiple kilonovae observation but, post-peak will not be much useful. Kindly make 
this distinction in the text.  

'line-295:' There seems to be some an ad hoc tuning of interpolator whose sensitivity is not shown. 
Please clarify this.  

'281,285' It is not clear whether for "each simulation", you have " a specific set of intrinsic 
parameters" or, "a grid of parameters". Please elaborate if it's not a contradictory statement.  

'312-315:' this statement is not exact. Please provide some quantification.  

'322:' Please cite the sources and whether they are public or, protected.  



Reviewer #2 (Remarks to the Author):  

I have read “Measuring the Hubble Constant with a sample of Kilonovae” by Coughlin et al. The 
authors demonstrate that the Hubble constant is measured by using the observed kilonovae 
associated with short GRBs as a standard candle. The authors evaluate the statistical and systematic 
errors. The Hubble constant obtained here is consistent with those measured by Planck and SH0ES 
within uncertainties. Once we have more kilonovae associated with a gravitational-wave event as 
well as with a short GRB, the observations of kilonovae can become a powerful method to prove the 
cosmological parameters. Therefore I think this paper is worth being published in Nature 
Communications. However, there are several things in the analysis that are not sufficiently clear to 
me. I request the authors to address the following concerns.  


1. The authors calculate the absolute magnitude at a given band based on the theoretical modelings, 
which depend on a few model parameters. I think I understand the method of Couglin et al 17 and 
Ascenzi et al 19. In this paper, however, it is not clear to me how these parameters are chosen for 
each kilonova event without knowing the distance. In other wards, how can it be possible that the 
model parameters are independent of the distance moduli? Are they calibrated by the kilonova in 
GW170817? I request the authors to describe a bit more details and show the posterior distribution 
of the model parameters and distance moduli for short GRB kilonovae.  


2. This is related to the question 1. The Bulla model depends on the viewing angle. It is not 
sufficiently clear how the viewing angles for the short GRB kilonovae are chosen. As the authors 
mention, the association of GRB 170817A with GW suggests that short GRBs are produced by 
merger. However, this particular GRB is very different from the short GRBs associated with 
kilovnoae. One of likely explanations for this difference is that the viewing angle is different between 
GRB170817A and the others. For short GRBs, the viewing angle is much closer to the polar axis and 
therefore kilonovae seen in GRBs are likely to be quite biased toward small viewing angles. I suggest 
the authors to explicitly mention how the viewing angle is determined and what prior distribution is 
used. In addition, it would be great if the authors add some discussion about how the unknown 
distribution of the viewing angles of sGRB kilonovae, which is likely quite anisotropic, causes the bias 
in the H0 measurement.  

3. The authors include GW170817 to get the combined posterior distribution of the Hubble constant. 
Although the combined analysis is more constraining H0, this adds more systematic uncertainties 
because the kilonova standard candle method and standard siren method have a different type of 
systematic errors. Specifically, GW170817 is so close that the H0 measurement depends on our 
guess of the peculiar velocity of the host. This problem perhaps will remain even for future events if 
only close GW-EM events are observed. However, the kilonova standard candle method uses only 
the distance measurement and does not relay on the peculiar motion of local galaxies. So, I think it 
will be better not to include the H0 measurements from GW events in the future (once we have a 
sufficient number of kilonovae).  

I suggest to show the current H0 posteriors only with short GRB kilonovae. (This is just a suggestion 
that may be somewhat biased towards my opinion. So please ignore this suggestion if the authors do 
not want to add).