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

Understand

In this work, we estimate how much bulk viscosity driven by Urca processes is likely to affect the gravitational wave signal of a neutron star coalescence.

• In the late inspiral, we show that bulk viscosity affects the binding energy at fourth post-Newtonian (PN) order.
• Even though this effect is enhanced by the square of the gravitational compactness, the coefficient of bulk viscosity is likely too small to lead to observable effects in the waveform during the late inspiral, when only considering the orbital motion itself.
• In the post-merger, however, the characteristic time-scales and spatial scales are different, potentially leading to the opposite conclusion.