Post-merger Gravitational Wave Research Articles

Numerical relativity simulations of neutron star merger remnants using conservative mesh refinement

We study equal and unequal-mass neutron star mergers by means of new numerical relativity simulations in which the general relativistic hydrodynamics solver employs an algorithm that guarantees mass conservation across the refinement levels of the computational mesh. We consider eight binary configurations with total mass $M=2.7\,M_\odot$, mass-ratios $q=1$ and $q=1.16$, and four different equation of states (EOSs), and one configuration with a stiff EOS, $M=2.5M_\odot$ and $q=1.5$. We focus on the post-merger dynamics and study the merger remnant, dynamical ejecta and the postmerger gravitational wave spectrum. Although most of the merger remnants form a hypermassive neutron star collapsing to a black hole+disk system on dynamical timescales, stiff EOSs can eventually produce a stable massive neutron star. Ejecta are mostly emitted around the orbital plane; favored by large mass ratios and softer EOS. The postmerger wave spectrum is mainly characterized by non-axisymmetric oscillations of the remnant. The stiff EOS configuration consisting of a $1.5M_\odot$ and a $1.0M_\odot$ neutron star shows a rather peculiar dynamics. During merger the companion star is very deformed; about~$\sim0.03M_\odot$ of rest-mass becomes unbound from the tidal tail due torque; and the merger remnant forms stable neutron star surrounded by a massive accretion disk $\sim0.3M_\odot$. Similar configurations might be particularly interesting for electromagnetic counterparts. Comparing results obtained with and without the conservative mesh refinement algorithm, we find that post-merger simulations can be affected by systematic errors if mass conservation is not enforced in the mesh refinement strategy. However, mass conservation also depends on grid details and on the artificial atmosphere setup. [abridged]

Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers

Neutron star (NS) merger simulations are conducted for 38 representative microphysical descriptions of high-density matter in order to explore the equation-of-state (EoS) dependence of the postmerger ring-down phase. The formation of a deformed, oscillating, differentially rotating very massive NS is the typical outcome of the coalescence of two stars with $1.35{M}_{\ensuremath{\bigodot}}$ for most candidate EoSs. The oscillations of this object imprint a pronounced peak in the gravitational wave (GW) spectra, which is used to characterize the emission for a given model. The peak frequency of this postmerger GW signal correlates very well with the radii of nonrotating NSs, and thus allows us to constrain the high-density EoS by a GW detection. In the case of $1.35\mathrm{\text{\ensuremath{-}}}1.35{M}_{\ensuremath{\bigodot}}$ mergers the peak frequency scales particularly well with the radius of an NS with $1.6{M}_{\ensuremath{\bigodot}}$, where the maximum deviation from this correlation is only 60 m for fully microphysical EoSs which are compatible with NS observations. Combined with the uncertainty in the determination of the peak frequency it appears likely that a GW detection can measure the radius of a $1.6{M}_{\ensuremath{\bigodot}}$ NS with an accuracy of about 100--200 m. We also uncover relations of the peak frequency with the radii of nonrotating NSs with $1.35{M}_{\ensuremath{\bigodot}}$ or $1.8{M}_{\ensuremath{\bigodot}}$, with the radius or the central energy density of the maximum-mass Tolman-Oppenheimer-Volkoff configuration, and with the pressure or sound speed at a fiducial rest mass density of about twice the nuclear saturation density. Furthermore, it is found that a determination of the dominant postmerger GW frequency can provide an upper limit for the maximum mass of nonrotating NSs. The effect of variations of the binary setup are investigated and corresponding functional dependences between the peak frequency and radii of nonrotating NSs are derived. With higher total binary masses, correlations are tighter for radii of nonrotating NSs with higher masses. The prospects for a detection of the postmerger GW signal and a determination of the dominant GW frequency are estimated to be in the range of 0.015--1.2 events per year with the upcoming Advanced Laser Interferometer Gravitational Wave Observatory detector.

Measuring Neutron-Star Properties via Gravitational Waves from Neutron-Star Mergers

We demonstrate by a large set of merger simulations for symmetric binary neutron stars (NSs) that there is a tight correlation between the frequency peak of the postmerger gravitational-wave (GW) emission and the physical properties of the nuclear equation of state (EoS), e.g., expressed by the radius of the maximum-mass Tolman-Oppenheimer-Volkhoff configuration. Therefore, a single measurement of the peak frequency of the postmerger GW signal will constrain the NS EoS significantly. For optimistic merger-rate estimates a corresponding detection with Advanced LIGO is expected to happen within an operation time of roughly a year.

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.