The Influence of Quark Matter at High Densities on Binary Neutron Star   Mergers
  We consider the influence of potential quark matter existing at high densities in neutron star interiors on gravitational waves (GW) emitted in a binary neutron star merger event. Two types of equations of state (EoS) at zero temperatures are used, one describing pure nuclear matter, the other nuclear matter with a phase transition to quark matter at very high densities. Binary equilibrium sequences close to the innermost stable circular orbit (ISCO) are calculated to determine the GW frequencies just before merger. It is found that EoS effects begin to play a role for gravitational masses larger than $M_\infty\simeq1.5M_\odot$. The difference in the gravitational wave frequency at the ISCO grows to up to $\simeq 10%$ for the maximal allowed mass given by the EoSs used. Then, we perform 3D hydrodynamic simulations for each EoS varying the initial mass and determine the characteristic GW frequencies of the merger remnants. The implications of quark matter show up mainly in a different collapse behaviour of the merger remnant. If the collapse does not take place immediately after merger, we find a phase difference between two EoS's in the post-merger GW signal. We also compare the GW frequencies emitted by the merger remnant to values from simulations using a polytropic EoS and find an imprint of the non-constant adiabatic index of our EoSs. All calculations are based on the conformally flat (CF) approximation to general relativity and the GW signal from the merger simulation is extracted up to quadrupole order. 
R. Oechslin, K. Ury\=u, G. Poghosyan, F. K. Thielemann