Instabilities in neutron-star postmerger remnants

Abstract

Using nonlinear, fully relativistic, simulations we investigate the dynamics and gravitationalwave signature associated with instabilities in neutron star postmerger remnants. For simplified models of the remnant we establish the presence of an instability in stars with moderate T /|W |, the ratio between the kinetic and the gravitational potential energies. Detailed analysis of the density oscillation pattern reveals a local instability in the inner region of the more realistic differential rotation profile. We apply Rayleigh's inflection theorem and Fjørtoft's theorem to analyze the stability criteria concluding that this inner local instability originates from a shear instability close to the peak of the angular velocity profile, and that it later evolves into a fast-rotating m = 2 oscillation pattern. We discuss the importance of the presence of a corotation point in the fluid, its connection with the shear instability, and comparisons to the Rossby wave and Papaloizou Pringle instabilities considered in the wider literature.

Thermal aspects of neutron star mergers

“…Much of this work took the barotropic model as starting point and introduced an effective model for the internal energy and the heat. As this approach is still used in simulations (see for example [17,18]), it is useful to explain the logic involved.…”

Equilibrium sequences of differentially rotating stars with post-merger-like rotational profiles

“…The original work of Uryū et al focused on presenting a limited number of selected models. Subsequent studies used the new 4parameter Uryu+ rotation law for describing rotating strange stars (Zhou et al 2019), explored the new 3-parameter Uryu+ rotation law in a Newtonian framework , and evolved in GR a selected model constructed with the new 3-parameter Uryu+ rotation law (Xie et al 2020). This was a gap that we tried to partially fill, for the case of the new 4-parameter Uryu+ rotation law, with our present contribution.…”

A gravitational-wave perspective on neutron-star seismology

“…And let us not forget the phenomenology that comes into play when we add differential rotation to the mix. In particular, suggestions that the so-called low-T/W instability [120,121] may act in merger remnants are intriguing [122][123][124]. Again, this is a mechanism that is much less well understood than it ought to be.…”