Bulk Viscosity of Relativistic npeμ Matter in Neutron-Star Mergers

Round 1

Reviewer 1 Report

The article discusses the effect of direct Urca processes (DU) on bulk viscosity and damping of the density oscillation of the neutron star matter. The article provides a clearly written essential review of the authors’ earlier work and presents new results on the damping of isentropic oscillations and the case of neutrino-transparent matter. The authors have shown that the DU processes rates are dominant in the temperature-density range under consideration and that they are responsible for a small oscillation damping timescale in small density-temperatures region. Therefore these processes can affect the NS merger remnant evolution and may be essential for numerical modelling of such transients.
The study is a logical and important continuation of the authors’ work and is definitely recommended for publication after a minor fix: Figs. 4, 5 and 7 contain a shaded area which is not commented. It is introduced only in Fig.8. Does it have the same meaning? Please explain it in the earlier figure captions.

Author Response

Reviewer 1. We thank the reviewer for noticing the issues of shading in several figures. This is now 
corrected in the manuscript, see colored text in the captions.

Reviewer 2 Report

In the present article, the authors discuss the bulk viscosity of the dense nuclear matter relevant to the binary neutron star merger scenario. In particular, they study the effect of neutrino trapping within a density-dependent relativistic mean field framework. Finally, they discuss the effect of the bulk viscosity on the dynamics of the evolution of the postmerger hypermassive compact object.

The article is well-written and rigorous. The content of the article is timely. The only criticism can be that the authors did not provide any qualitative information regarding the results if they had chosen any other form of RMF parameterization. This article should be published when the authors include some texts explaining this point.

Author Response

We thank the reviewer for bringing up the issue of using different functionals. We have responded to this issue on page 1 (colored text). We have been using DDME2 functional in the present paper, but results for an alternative NL3 functional are given in our previous paper and one can assess the uncertainty due to the choice of functional from this work. We agree that at some point it would be useful to carry out a sensitivity study of the results on the input functional.