Projecting the likely importance of weak-interaction-driven bulk\n viscosity in neutron star mergers

Abstract

In this work, we estimate how much bulk viscosity driven by Urca processes is\nlikely to affect the gravitational wave signal of a neutron star coalescence.\nIn the late inspiral, we show that bulk viscosity affects the binding energy at\nfourth post-Newtonian (PN) order. Even though this effect is enhanced by the\nsquare of the gravitational compactness, the coefficient of bulk viscosity is\nlikely too small to lead to observable effects in the waveform during the late\ninspiral, when only considering the orbital motion itself. In the post-merger,\nhowever, the characteristic time-scales and spatial scales are different,\npotentially leading to the opposite conclusion. We post-process data from a\nstate-of-the-art equal-mass binary neutron star merger simulation to estimate\nthe effects of bulk viscosity (which was not included in the simulation\nitself). In that scenario, we find that bulk viscosity can reach high values in\nregions of the merger. We compute several estimates of how much it might\ndirectly affect the global dynamics of the considered merger scenario, and find\nthat it could become significant. Even larger effects could arise in different\nmerger scenarios or in simulations that include non-linear effects. This\nassessment is reinforced by a quantitative comparison with relativistic\nheavy-ion collisions where such effects have been explored extensively.\n