Bulk viscosity of 2SC quark matter in neutron star mergers

Mark Alford, Arus Harutyunyan, Armen Sedrakian, Stefanos Tsiopelas

Published 2024-07-17Version 1

We study the bulk viscosity of moderately hot and dense, neutrino-transparent color superconducting quark matter arising from weak-interaction-driven direct Urca processes. The quark matter is modeled using Nambu--Jona-Lasinio (NJL) model improved to account for vector and t'Hooft interactions as well as antisymmetric pairing among the red/green up and down quarks. The unpaired excitations are the strange quarks and the blue up or down quarks. We compute the relaxation rates associated with $d$ and $s$-quark decay and electron capture processes on $u$ quark for blue color. The resulting bulk viscosity for density oscillations in the 1 to 10\,kHz range shows a resonant peak at $T\sim 5\,\MeV$, and the damping time may drop below 10 ms. This is short enough to affect the post-merger evolution and is very similar to the damping predicted in nuclear matter.