Binary Neutron Star Inspirals Research Articles

Detecting short-term gravitational waves from post-merger hyper-massive neutron stars with a kilohertz detector

Abstract Gravitational waves emanating from binary neutron star inspirals, alongside electromagnetic transients resulting from the aftermath of the GW170817 merger, have been successfully detected. However, the intricate post-merger dynamics that bridge these two sets of observables remain enigmatic. This includes if, and when, the post-merger remnant star collapses to a black hole, and what are the necessary conditions to power a short gamma-ray burst, and other observed electromagnetic counterparts. Our focus is on the detection of GW emissions from hyper-massive neutron stars (NSs) formed through binary neutron star (BNS) mergers. Utilizing several kilohertz GW detectors, we simulate BNS mergers within the detection limits of LIGO-Virgo-KARGA O4. Our objective is to ascertain the fraction of simulated sources that may emit detectable post-merger GW signals. For kilohertz detectors equipped with a new cavity design, we estimate that approximately 1.1%-32% of sources would emit a detectable post-merger GW signal. This fraction is contingent on the mass converted into gravitational wave energy, ranging from 0.01M ⊙ to 0.1M ⊙. Furthermore, by evaluating other well-regarded proposed kilohertz GW detectors, we anticipate that the fraction can increase to as much as 2.1%-61% under optimal performance conditions.

Prospects for a Precise Equation of State Measurement from Advanced LIGO and Cosmic Explorer

Gravitational-wave observations of neutron star mergers can probe the nuclear equation of state by measuring the imprint of the neutron star’s tidal deformability on the signal. We investigate the ability of future gravitational-wave observations to produce a precise measurement of the equation of state from binary neutron star inspirals. Because measurability of the tidal effect depends on the equation of state, we explore several equations of state that span current observational constraints. We generate a population of binary neutron stars as seen by a simulated Advanced LIGO–Virgo network, as well as by a planned Cosmic Explorer observatory. We perform Bayesian inference to measure the parameters of each signal, and we combine measurements across each population to determine R 1.4, the radius of a 1.4 M ⊙ neutron star. We find that, with 321 signals, the LIGO–Virgo network is able to measure R 1.4 to better than 2% precision for all equations of state we consider; however, we also find that achieving this precision could take decades of observation, depending on the equation of state and the merger rate. On the other hand, we find that with one year of observation, Cosmic Explorer will measure R 1.4 to better than 0.6% precision. In both cases, we find that systematic biases, such as from an incorrect mass prior, can significantly impact measurement accuracy, and efforts will be required to mitigate these effects.

Degeneracy in the Inference of Phase Transitions in the Neutron Star Equation of State from Gravitational Wave Data.

Gravitational wave (GW) detections of binary neutron star inspirals will be crucial for constraining the dense matter equation of state (EOS). We demonstrate a new degeneracy in the mapping from tidal deformability data to the EOS, which occurs for models with strong phase transitions. We find that there exists a new family of EOS with phase transitions that set in at different densities and that predict neutron star radii that differ by up to ∼500 m but that produce nearly identical tidal deformabilities for all neutron star masses. Next-generation GW detectors and advances in nuclear theory may be needed to resolve this degeneracy.

Unstable modes of hypermassive compact stars driven by viscosity and gravitational radiation

ABSTRACT We study the oscillation modes of differential rotating remnants of binary neutron star inspirals by modelling them as incompressible Riemann ellipsoids parametrized by the ratio f of their internal circulation to the rotation frequency. The effects of viscosity and gravitational radiation on the modes are studied, and it is shown that these bodies exhibit generic instability towards gravitational radiation akin to the Chandrasekhar–Friedman–Schutz instability for uniformly rotating stars. The odd-parity modes are unstable for all values of f (except for the spherical model) and deformations, whereas the even parity unstable modes appear only in highly eccentric ellipsoids. We quantify the modification of the modes with the varying mass of the model and the magnitude of the viscosity. The modes are weakly dependent on the range of the masses relevant to binary neutron star mergers. Large turbulent viscosity can lead to a suppression of the gravitational-radiation instability, whereas kinematical viscosity has a negligible influence on the modes and their damping time-scales.

Probing hybrid stars with gravitational waves via interfacial modes

One of the uncertainties in nuclear physics is whether a phase transition between hadronic nuclear matter to quark matter exists in supranuclear matter equations of state. Such a feature can be probed via gravitational-wave signals from binary neutron star inspirals that contain information of the induced tides. The dynamical part of the tides is caused by the resonance of pulsation modes of stars, which causes a shift in the gravitational-wave phase. In this paper, we investigate the dynamical tides of the interfacial mode ($i$-mode) of spherical degree $l=2$, a non-radial mode caused by an interface associated with a quark-hadron phase transition inside a hybrid star. In particular, we focus on hybrid stars with a crystalline quark matter core and a fluid hadronic envelope. We find that the resonant frequency of such $i$-modes typically ranges from 300Hz to 1500Hz, and the frequency increases as the shear modulus of the quark core increases. We next estimate the detectability of such a mode with existing and future gravitational-wave events from the inspiral waveform with a Fisher analysis. We find that GW170817 and GW190425 have the potential to detect the $i$-mode if the quark-hadron phase transition occurs at sufficiently low pressure and the shear modulus of the quark matter phase is large enough. We also find that the third-generation gravitational-wave detectors can further probe the $i$-mode with intermediate transition pressure. This finding opens a new, interesting direction for probing the existence of quark core inside a neutron star.

Toward observing neutron star collapse with gravitational wave detectors

Gravitational waves from binary neutron star inspirals have been detected along with the electromagnetic transients coming from the aftermath of the merger in GW170817. However, much is still unknown about the post-merger dynamics that connects these two sets of observables. This includes if, and when, the post-merger remnant star collapses to a black hole, and what are the necessary conditions to power a short gamma-ray burst, and other observed electromagnetic counterparts. Observing the collapse of the post-merger neutron star would shed led on these questions, constraining models for the short gamma-ray burst engine and the hot neutron star equation of state. In this work, we explore the scope of using gravitational wave detectors to measure the timing of the collapse either indirectly, by establishing the shut-off of the post-merger gravitational emission, or---more challengingly---directly, by detecting the collapse signal. For the indirect approach, we consider a kilohertz high-frequency detector design that utilises a previously studied coupled arm cavity and signal recycling cavity resonance. This design would give a signal-to-noise ratio of 0.5\,-\,8.6 (depending on the variation of waveform parameters) for a collapse gravitational wave signal occurring at 10\,ms post-merger of a binary at 50\,Mpc and with total mass $2.7 M_\odot$. For the direct approach, we propose a narrow-band detector design, utilising the sensitivity around the frequency of the arm cavity free spectral range. The proposed detector achieves a signal-to-noise ratio of 0.3\,-\,1.9, independent of the collapse time. This detector is limited by both the fundamental classical and quantum noise with the arm cavity power chosen as 10\,MW.

Role of dense matter in tidal deformations of inspiralling neutron stars and in gravitational waveforms with unified equations of state

The role of dense-matter properties in the tidal deformability of a cold nonaccreted neutron star is further investigated. Using the set of Brussels-Montreal unified equations of state, we have computed the gravitoelectric Love numbers ${k}_{\ensuremath{\ell}}$ and the gravitomagnetic Love numbers ${j}_{\ensuremath{\ell}}$ up to $\ensuremath{\ell}=5$. Their relative importance and their sensitivity to the symmetry energy and the neutron-matter stiffness are numerically assessed. Their impact on the phase of the gravitational-wave signal emitted by binary neutron star inspirals is also discussed.

Systematic Uncertainty of Standard Sirens from the Viewing Angle of Binary Neutron Star Inspirals.

The independent measurement of the Hubble constant with gravitational-wave standard sirens will potentially shed light on the tension between the local distance ladders and Planck experiments. Therefore, thorough understanding of the sources of systematic uncertainty for the standard siren method is crucial. In this Letter, we focus on two scenarios that will potentially dominate the systematic uncertainty of standard sirens. First, simulations of electromagnetic counterparts of binary neutron star mergers suggest aspherical emissions, so the binaries available for the standard siren method can be selected by their viewing angles. This selection effect can lead to ≳2% bias in Hubble constant measurement even with mild selection. Second, if the binary viewing angles are constrained by the electromagnetic counterpart observations but the bias of the constraints is not controlled under ∼10°, the resulting systematic uncertainty in the Hubble constant will be >3%. In addition, we find that both of the systematics cannot be properly removed by the viewing angle measurement from gravitational-wave observations. Comparing to the known dominant systematic uncertainty for standard sirens, the ≤2% gravitational-wave calibration uncertainty, the effects from the viewing angle appear to be more significant. Therefore, the systematic uncertainty from the viewing angle might be a major challenge before the standard sirens can resolve the tension in the Hubble constant, which is currently ∼9%.

Spontaneous Creation of Circularly Polarized Photons in Chiral Astrophysical Systems.

This work establishes a relation between chiral anomalies in curved spacetimes and the radiative content of the gravitational field. In particular, we show that a flux of circularly polarized gravitational waves triggers the spontaneous creation of photons with net circular polarization from the quantum vacuum. Using waveform catalogs, we identify precessing binary black holes as astrophysical configurations that emit such gravitational radiation and then solve the fully nonlinear Einstein's equations with numerical relativity to evaluate the net effect. The quantum amplitude for a merger is comparable to the Hawking emission rate of the final black hole and small to be directly observed. However, the implications for the inspiral of binary neutron stars could be more prominent, as argued on symmetry grounds.

Searching for Exotic Cores with Binary Neutron Star Inspirals

Abstract We study the feasibility of detecting exotic cores in merging neutron stars with ground-based gravitational-wave detectors. We focus on models with a sharp nuclear/exotic matter interface, and assume a uniform distribution of neutron stars in the mass range [1, 2] M ⊙. We find that the existence of exotic cores can be confirmed at the 70% confidence level with as few as several tens of detections. Likewise, with such a sample, we find that some models of exotic cores can be excluded with high confidence.

Studying strong phase transitions in neutron stars with gravitational waves

The composition of neutron stars at the extreme densities reached in their cores is currently unknown. Besides nuclear matter of normal neutrons and protons, the cores of neutron stars might harbor exotic matter such as deconfined quarks. In this paper we study strong hadron-quark phase transitions in the context of gravitational wave observations of inspiraling neutron stars. We consider upcoming detections of neutron star coalescences and model the neutron star equations of state with phase transitions through the Constant-Speed-of-Sound parametrization. We use the fact that neutron star binaries with one or more hadron-quark hybrid stars can exhibit qualitatively different tidal properties than binaries with hadronic stars of the same mass, and hierarchically model the masses and tidal properties of simulated populations of binary neutron star inspiral signals. We explore the parameter space of phase transitions and discuss under which conditions future observations of binary neutron star inspirals can identify this effect and constrain its properties, in particular the threshold density at which the transition happens and the strength of the transition. We find that if the detected population of binary neutron stars contains both hadronic and hybrid stars, the onset mass and strength of a sufficiently strong phase transition can be constrained with 50-100 detections. If the detected neutron stars are exclusively hadronic or hybrid, then it is possible to place lower or upper limits on the transition density and strength.

Using negative-latency gravitational wave alerts to detect prompt radio bursts from binary neutron star mergers with the Murchison Widefield Array

ABSTRACT We examine how fast radio burst (FRB)-like signals predicted to be generated during the merger of a binary neutron star (BNS) may be detected in low-frequency radio observations triggered by the aLIGO/Virgo gravitational-wave detectors. The rapidity, directional accuracy, and sensitivity of follow-up observations with the Murchison Widefield Array (MWA) are considered. We show that with current methodology, the rapidity criterion fails for triggered MWA observations above 136 MHz for BNS mergers within the aLIGO/Virgo horizon, for which little dispersive delay is expected. A calculation of the expected reduction in response time by triggering on ‘negative latency’ alerts from aLIGO/Virgo observations of gravitational waves generated by the BNS inspiral is presented. This allows for observations up to 300 MHz where the radio signal is expected to be stronger. To compensate for the poor positional accuracy expected from these alerts, we propose a new MWA observational mode that is capable of viewing one-quarter of the sky. We show the sensitivity of this mode is sufficient to detect an FRB-like burst from an event similar to GW 170817 if it occurred during the ongoing aLIGO/Virgo third science run (O3).

Effect of spin on the inspiral of binary neutron stars

We perform long-term simulations of spinning binary neutron stars, with our highest dimensionless spin being $\chi \sim 0.32$. To assess the importance of spin during the inspiral we vary the spin, and also use two equations of state, one that consists of plain nuclear matter and produces compact stars (SLy), and a hybrid one that contains both nuclear and quark matter and leads to larger stars (ALF2). Using high resolution that has grid spacing $\Delta x\sim 98$ m on the finest refinement level, we find that the effects of spin in the phase evolution of a binary system can be larger than the one that comes from tidal forces. Our calculations demonstrate explicitly that although tidal effects are dominant for small spins ($\lesssim 0.1$), this is no longer true when the spins are larger, but still much smaller than the Keplerian limit.

Surrogate model for an aligned-spin effective-one-body waveform model of binary neutron star inspirals using Gaussian process regression

Fast and accurate waveform models are necessary for measuring the properties of inspiraling binary neutron star systems such as GW170817. We present a frequency-domain surrogate version of the aligned-spin binary neutron star waveform model using the effective-one-body formalism known as SEOBNRv4T. This model includes the quadrupolar and octopolar adiabatic and dynamical tides. The version presented here is improved by the inclusion of the spin-induced quadrupole moment effect and completed by a prescription for tapering the end of the waveform to qualitatively reproduce numerical relativity simulations. The resulting model has 14 intrinsic parameters. We reduce its dimensionality by using universal relations that approximate all matter effects in terms of the leading quadrupolar tidal parameters. The implementation of the time-domain model can take up to an hour to evaluate using a starting frequency of 20 Hz, and this is too slow for many parameter estimation codes that require $\mathcal{O}({10}^{7})$ sequential waveform evaluations. We therefore construct a fast and faithful frequency-domain surrogate of this model using Gaussian process regression. The resulting surrogate has a maximum mismatch of $4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ for the Advanced LIGO detector and requires $\ensuremath{\sim}0.13\text{ }\text{ }\mathrm{s}$ to evaluate for a waveform with a starting frequency of 20 Hz. Finally, we perform an end-to-end test of the surrogate with a set of parameter estimation runs and find that the surrogate accurately recovers the parameters of injected waveforms.

Tests of General Relativity with GW170817.

The recent discovery by Advanced LIGO and Advanced Virgo of a gravitational wave signal from a binary neutron star inspiral has enabled tests of general relativity (GR) with this new type of source. This source, for the first time, permits tests of strong-field dynamics of compact binaries in the presence of matter. In this Letter, we place constraints on the dipole radiation and possible deviations from GR in the post-Newtonian coefficients that govern the inspiral regime. Bounds on modified dispersion of gravitational waves are obtained; in combination with information from the observed electromagnetic counterpart we can also constrain effects due to large extra dimensions. Finally, the polarization content of the gravitational wave signal is studied. The results of all tests performed here show good agreement with GR.

Sub-threshold Binary Neutron Star Search in Advanced LIGO’s First Observing Run

Abstract We present a search for gravitational waves from double neutron star binaries inspirals in Advanced Laser Interferometer Gravitational-Wave Observatory’s (LIGO’s) first observing run. The search considers a narrow range of binary chirp masses motivated by the population of known double neutron-star binaries in the nearby universe. This search differs from previously published results by providing the most sensitive published survey of neutron stars in Advanced LIGO’s first observing run within this narrow mass range, and also including times when only one of the two LIGO detectors was in operation in the analysis. The search was sensitive to binary neutron star (BNS) inspirals to an average distance of ∼85 Mpc over 93.2 days. We do not identify any unambiguous gravitational wave signals in our sample of 103 sub-threshold candidates with false-alarm rates of less than one per day. However, given the expected BNS merger rate of , we expect gravitational-wave events within our candidate list. This suggests the possibility that one or more of these candidates is in fact a BNS merger. Although the contamination fraction in our candidate list is ∼99%, it might be possible to correlate these events with other messengers to identify a potential multi-messenger signal. We provide an online candidate list with the times and sky locations for all events in order to enable multi-messenger searches.

The symmetry energy and incompressibility constrained by the observations of glitching pulsars

We investigate the masses of glitching pulsars in order to constrain their equation of state (EOS). The observations of glitches (sudden jumps in rotational frequency) may provide information on the interior physics of neutron stars. With the assumption that glitches are triggered by superfluid neutrons, the masses of glitching neutron stars can be estimated using observations of maximum glitches. Together with the observations of thermal emission from glitching pulsars Vela and J1709–4429, the slope of symmetry energy and incompressibility of nuclear matter at saturation density can be constrained. The slope of symmetry energy L should be larger than 67 MeV while the lower limit of incompressibility for symmetric nuclear matter K0 is 215 MeV. We also obtain a relationship between L and K0: 6.173 MeV + 0.283 K0 ≤ L ≤ 7.729 MeV + 0.291 K0. The restricted EOSs are consistent with the observations of 2-solar-mass neutron stars and gravitational waves from a binary neutron star inspiral.

Urca reactions during neutron star inspiral

Abstract We study the impact of Urca reactions driven by tidally induced fluid motion during binary neutron star inspiral. Fluid compression is computed for low radial order oscillation modes through an adiabatic, time-dependent solution for the mode amplitudes. Optically thin neutrino emission and heating rates are then computed from this adiabatic fluid motion. Calculations use direct and modified Urca reactions operating in a $M=1.4\, \mathrm{ M}_\odot$ neutron star, which is constructed using the Skyrme Rs equation of state. We find that the energy pumped into low-order oscillation modes is not efficiently thermalized even by direct Urca reactions, with core temperatures reaching only T ≃ 108 K during the inspiral. Although this is an order of magnitude larger than the heating due to shear viscosity considered by previous studies, it reinforces the result that the stars are quite cold at merger. Upon excitation of the lowest order g mode, the chemical potential imbalance reaches $\beta \gtrsim 1\, \rm MeV$ at orbital frequencies $\nu _{\rm orb} \gtrsim 200\, \rm Hz$, implying significant charged-current optical depths and Fermi-blocking. To assess the importance of neutrino degeneracy effects, the neutrino transfer equation is solved in the static approximation for the three-dimensional density distribution, and the reaction rates are then computed including Fermi-blocking. We find that the heating rate is suppressed by a factor of a ∼2 for $\nu _{\rm orb} \gtrsim 200\, \rm Hz$. The spectrum of emitted νe and $\bar{\nu }_e$, including radiation transfer effects, is presented for a range of orbital separations.

Search for High-Energy Neutrinos from GW170817 with the Baikal-GVD Neutrino Telescope

The Advanced LIGO and Advanced Virgo observatories recently discovered gravitational waves from a binary neutron star inspiral. A short gamma-ray burst (GRB) that followed the merger of this binary was also recorded by Fermi-GBM and INTEGRAL, indicating particle acceleration by the source. The precise location of the event was determined by optical detections of emission following the merger. We searched for high-energy neutrinos from the merger in the TeV - 100 PeV energy range using Baikal-GVD. No neutrinos directionally coincident with the source were detected within $\pm$500 s around the merger time, as well as during a 14-day period after the GW detection. We derived 90% confidence level upper limits on the neutrino fluence from GW170817 during a $\pm$500 s window centered on the GW trigger time, and a 14-day window following the GW signal under the assumption of an $E^{-2}$ neutrino energy spectrum.

Indications of an unexpected signal associated with the GW170817 binary neutron star inspiral

We report experimental evidence at the 2.5σ level for an unexpected signal associated with the GW170817 binary neutron star inspiral. This evidence derives from a laboratory experiment simultaneously measuring the β-decay rates of Si-32 and Cl-36 in a common detector. Whereas the Si-32 and Cl-36 decay rates show no statistical correlation before or after the inspiral, they are highly correlated (∼95%) in the 5-h time interval immediately following the inspiral. If we interpret this correlation as arising from the influence of particles emitted during the inspiral, then we can estimate the mass mx of these particles from the time delay between the gravity-wave signal and a peak in the β-decay data. We find for particles of energy 10 MeV, mx ≲ 16 eV which includes the neutrino mass region mν ≲ 2 eV. The latter is based on existing limits for the masses mi of the three known neutrino flavors. Additionally, we find that the correlation is even stronger if we include data in the 80 minute period before the arrival of the gravity wave signal. Given the large number of radionuclides whose decays are being monitored at any given time, we conjecture that other groups may also be in a position to search for statistically suggestive fluctuations of radionuclide decay rates associated with the GW170817 inspiral, and possibly with other future inspirals.