Search

We introduce a classification scheme of the postmerger dynamics and gravitational wave emission in binary neutron star mergers, after identifying a new mechanism by which a secondary peak in the gravitational wave spectrum is produced. It is caused by a spiral deformation, the pattern of which rotates slower with respect to the double-core structure in the center of the remnant. This secondary peak is typically well separated in frequency from the secondary peak produced by a nonlinear interaction between a quadrupole and a quasiradial oscillation. The new mechanism allows for an explanation of low-frequency modulations seen in a number of physical characteristics of the remnant, such as the central lapse function, the maximum density and the separation between the two cores, but also in the gravitational wave amplitude. We find empirical relations for both types of secondary peaks between their gravitational wave frequency and the compactness of nonrotating individual neutron stars, that exist for fixed total binary masses. These findings are derived for equal-mass binaries without intrinsic neutron star spin analyzing hydrodynamical simulations without magnetic field effects. Our classification scheme may form the basis for the construction of detailed gravitational wave templates of the postmerger phase. We find that the quasiradial oscillation frequency of the remnant decreases with the total binary mass. For a given merger event, our classification scheme may allow one to determine the proximity of the measured total binary mass to the threshold mass for prompt black hole formation, which can, in turn, yield an estimate of the maximum neutron star mass. [ABSTRACT FROM AUTHOR]

The equation of state plays a critical role in the physics of the merger of two neutron stars. Recent numerical simulations with a microphysical equation of state suggest the outcome of such events depends on the mass of the neutron stars. For less_ massive systems, simulations favor the formation of a hypermassive, quasistable neutron star, for which the oscillations produce a short, high-frequency burst of gravitational radiation. Its dominant frequency content is tightly correlated with the radius of the neutron star, and its measurement can be used to constrain the supranuclear equation of state. In contrast, the merger of higher mass systems results in prompt gravitational collapse to a black hole. We have developed an algorithm that combines waveform reconstruction from a morphology-independent search for gravitational wave transients with the Bayesian model selection to discriminate between postmerger scenarios and accurately measure the dominant oscillation frequency. We demonstrate the efficacy of the method using a catalog of simulated binary merger signals in data from LIGO and Virgo, and we discuss the prospects for this analysis in advanced ground-based gravitational wave detectors. From the waveforms considered in this work, we find that the postmerger neutron star signal may be detectable by this technique to ~4-12 Mpc, for sources with random sky locations and orientations with respect to the Earth. We also find that we successfully discriminate between the postmerger scenarios with ~95% accuracy and determine the dominant oscillation frequency of surviving postmerger neutron stars to within ~10 Hz, averaged over all detected signals. This leads to an uncertainty in the estimated radius of a nonrotating 1.6 M0 reference neutron star of ~100 m. [ABSTRACT FROM AUTHOR]

We present a novel method for revealing the equation of state of high-density neutron star matter through gravitational waves emitted during the postmerger phase of a binary neutron star system. The method relies on a small number of detections of the peak frequency in the postmerger phase for binaries of different (relatively low) masses, in the most likely range of expected detections. From such observations, one can construct the derivative of the peak frequency vs the binary mass, in this mass range. Through a detailed study of binary neutron star mergers for a large sample of equations of state, we show that one can extrapolate the above information to the highest possible mass (the threshold mass for black hole formation in a binary neutron star merger). In turn, this allows for an empirical determination of the maximum mass of cold, nonrotating neutron stars to within 0.1 M0, while the corresponding radius is determined to within a few percent. Combining this with the determination of the radius of cold, nonrotating neutron stars of 1.6M0 [to within a few percent, as was demonstrated in Bauswein et al. Phys. Rev. D 86, 063001 (2012)], allows for a clear distinction of a particular candidate equation of state among a large set of other candidates. Our method is particularly appealing because it reveals simultaneously the moderate and very high-density parts of the equation of state, enabling the distinction of mass-radius relations even if they are similar at typical neutron star masses. Furthermore, our method also allows us to deduce the maximum central energy density and maximum central rest-mass density of cold, nonrotating neutron stars with an accuracy of a few percent. [ABSTRACT FROM AUTHOR]

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 1.35Mʘ 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 1.35-1.35Mʘ mergers the peak frequency scales particularly well with the radius of an NS with 1.6Mʘ, 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 1.6Mʘ NS with an accuracy of about 100-200 m. We also uncover relations of the peak frequency with the radii of nonrotating NSs with 1.35Μʘ or 1.8Mʘ, 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. [ABSTRACT FROM AUTHOR]

We relate the fundamental quadrupolar fluid mode of isolated nonrotating neutron stars (NSs) and the dominant oscillation frequency of neutron star merger remnants. Both frequencies individually are known to correlate with certain stellar parameters like radii or the tidal deformability, which we further investigate by constructing fit formulas and quantifying the scatter of the data points from those relations. Furthermore, we compare how individual data points deviate from the corresponding fit to all data points. Considering this point-to-point scatter we uncover a striking similarity between the frequency deviations of perturbative data for isolated NSs and of oscillation frequencies of rapidly rotating, hot, massive merger remnants. The correspondence of frequency deviations in these very different stellar systems points to an underlying mechanism and equation of state information being encoded in the frequency deviation. We trace the frequency scatter back to deviations of the tidal Love number from an average tidal Love number for a given stellar compactness. Our results thus indicate a possibility to break the degeneracy between NS radii, tidal deformability, and tidal Love number. We also relate frequency deviations to the derivative of the tidal deformability with respect to mass. Our findings generally highlight a possibility to improve gravitational wave (GW) asteroseismology relations where the systematic behavior of frequency deviations is employed to reduce the scatter in such relationships and consequently increase the measurement accuracy. In addition, we relate the f-mode frequency of static stars and the dominant GW frequency of merger remnants. We find an analytic mapping to connect the masses of both stellar systems, which yields particularly accurate mass-independent relations between both frequencies and between the postmerger frequency and the tidal deformability. [ABSTRACT FROM AUTHOR]

This study addresses the collapse behavior of neutron star (NS) mergers expressed through the binary threshold mass Mthres for prompt black-hole (BH) formation, which we determine by relativistic hydrodynamical simulations for a set of 40 equation of state (EOS) models of NS matter. Mthres can be well described by various fit formulas involving stellar parameters of nonrotating NSs, which are employed to characterize the EOS models. Using these relations we compute which constraints on NS radii and the tidal deformability are set by current and future merger detections that reveal information about the merger product. We systematically investigate the impact of the binary mass ratio q=M1/M2 and assemble various fits, which make different assumptions about a-priori knowledge. This includes fit formulas for Mthres for a fixed mass ratio or a range of q if this parameter is not known very well. Also, we construct relations describing the threshold to prompt collapse for different classes of candidate EOSs, which for instance do or do not include models with a phase transition to quark matter. In particular, we find fit formulas for Mthres including an explicit q dependence, which are valid in a broad range of 0.7≤q≤1 and which are nearly as tight as relations for fixed mass ratios. For most EOS models except for some extreme cases the threshold mass of asymmetric mergers is equal or smaller than the one of equal-mass binaries. Generally, the impact of the binary mass asymmetry on Mthres becomes stronger with more extreme mass ratios, while Mthres is approximately constant for small deviations from q=1, i.e., for 0.85≤q≤1. The magnitude of the reduction of Mthres with the binary mass asymmetry follows a systematic EOS dependence. We also describe in more detail that a phase transition to deconfined quark matter can leave a characteristic imprint on the collapse behavior of NS mergers. The occurrence of quark matter can reduce the stability of the remnant and thus the threshold mass relative to a purely hadronic reference model. Comparing specifically the threshold mass and the combined tidal deformability ˜Λthres of a system with Mthres can yield peculiar combinations of those two quantities, where Mthres is particularly small in relation to ˜Λthres. Since no purely hadronic EOS can yield such a combination of Mthres and ˜Λthres, a combined measurement or a constraint on both quantities can indicate the onset of quark deconfinement. Finally, we point out new univariate relations between Mthres and stellar properties of high-mass NSs, which can be employed for direct EOS constraints or consistency checks in combination with other measurements. [ABSTRACT FROM AUTHOR]

We study the possible occurrence of the hadron-quark phase transition (PT) during the merging of neutron star binaries by hydrodynamical simulations employing a set of temperature-dependent hybrid equations of state (EOSs). Following previous work, we describe an unambiguous and measurable signature of deconfined quark matter in the gravitational-wave (GW) signal of neutron star binary mergers including equal-mass and unequal-mass systems of different total binary mass. The softening of the EOS by the PT at higher densities, i.e., after merging, leads to a characteristic increase of the dominant postmerger GW frequency f peak relative to the tidal deformability Λ inferred during the premerger inspiral phase. Hence, measuring such an increase of the postmerger frequency provides evidence for the presence of a strong PT. If the postmerger frequency and the tidal deformability are compatible with results from purely baryonic EOS models yielding very tight relations between f peak and Λ, a strong PT can be excluded up to a certain density. We find tight correlations of f peak and Λ with the maximum density during the early postmerger remnant evolution. These GW observables thus inform about the density regime which is probed by the remnant and its GW emission. Exploiting such relations, we devise a directly applicable, concrete procedure to constrain the onset density of the QCD PT from future GW measurements. We point out two interesting scenarios: if no indications for a PT are inferred from a GW detection, our procedure yields a lower limit on the onset density of the hadron-quark PT. On the contrary, if a merger event reveals evidence for the occurrence of deconfined quark matter, the inferred GW parameters set an upper limit on the PT onset density. Both scenarios would thus have strong implications for high-density matter physics, e.g., determining the range of validity of nuclear physics and constraining the properties for quark deconfinement. These prospects demonstrate the importance of simultaneously measuring pre- and postmerger GW signals to exploit the complementarity of the information encoded in both phases. Hence, our work stresses the value added by dedicated high-frequency GW instruments. [ABSTRACT FROM AUTHOR]

We construct new, multivariate empirical relations for measuring neutron star radii and tidal deformabilities from the dominant gravitational wave frequency in the postmerger phase of binary neutron star mergers. The relations determine neutron star radii and tidal deformabilities for specific neutron star masses with consistent accuracy and depend only on two observables: the postmerger peak frequency fpeak and the chirp mass Mchirp. The former could be measured with good accuracy from gravitational waves emitted in the postmerger phase using next-generation detectors, whereas the latter is already obtained with good accuracy from the inspiral phase with present-day detectors. Our main dataset consists of a gravitational wave catalog obtained with smoothed-particle hydrodynamics simulations within the spatial conformal flatness approximation. We also extract the fpeak frequency from the publicly available CoRe data set, obtained through grid-based general-relativistic hydrodynamical simulations and find good agreement between the extracted frequencies of the two datasets. As a result, we can construct empirical relations for the combined datasets. Furthermore, we investigate empirical relations for two secondary peaks, f2-0 and fspiral, and show that these relations are distinct in the whole parameter space, in agreement with a previously introduced spectral classification scheme. Finally, we show that the spectral classification scheme can be reproduced using machine-learning techniques. [ABSTRACT FROM AUTHOR]

The recent detection of a neutron star binary through gravitational waves, GW170817, has offered another source of information about the properties of cold supranuclear matter. Information from the signal emitted before the neutron stars merged has been used to study the equation of state of these bodies, however, any complementary information included in the signal emitted after the merger has been lost in the detector noise. In this paper we investigate the prospects of studying GW170817-like post-merger signals with future gravitational-wave detectors. We first compute the expected properties of the possible GW170817 post-merger signal using information from pre-merger analyses. We then quantify the required improvement in detector sensitivity in order to extract key features of the post-merger signal. We find that if we observe a signal of similar strength to GW170817 when the aLIGO detectors have been improved by ∼2-3 times over their design sensitivity in the kHz regime, we will be able to extract the dominant frequency component of the post-merger. With further improvements and next-generation detectors we will also be able to extract subdominant frequencies. We conclude that post-merger signals could be brought within our reach in the coming years given planned detector upgrades, such as A+, Voyager, and the next-generation detectors. [ABSTRACT FROM AUTHOR]

We present a robust method to characterize the gravitational wave emission from the remnant of a neutron star coalescence. Our approach makes only minimal assumptions about the morphology of the signal and provides a full posterior probability distribution of the underlying waveform. We apply our method on simulated data from a network of advanced ground-based detectors and demonstrate the gravitational wave signal reconstruction. We study the reconstruction quality for different binary configurations and equations of state for the colliding neutron stars. We show how our method can be used to constrain the yet-uncertain equation of state of neutron star matter. The constraints on the equation of state we derive are complementary to measurements of the tidal deformation of the colliding neutron stars during the late inspiral phase. In the case of nondetection of a post-merger signal following a binary neutron star inspiral, we show that we can place upper limits on the energy emitted. [ABSTRACT FROM AUTHOR]

The physics of neutron stars can be studied with gravitational waves emitted from coalescing binary systems. Tidal effects become significant during the last few orbits and can be visible in the gravitational wave spectrum above 500 Hz. After the merger, the neutron star remnant oscillates at frequencies above 1 kHz and can collapse into a black hole. Gravitational wave detectors with a sensitivity of ≃10-24 strain/√Hz at 2-4 kHz can observe these oscillations from a source which is approximately 100 Mpc away. The current observatories, such as LIGO and Virgo, are limited by shot noise at high frequencies and have a sensitivity of greater than or equal to 2×10-23 strain/√Hz at 3 kHz. In this paper, we propose an optical configuration of gravitational wave detectors, which can be set up in present facilities using the current interferometer topology. This scheme has the potential to reach 7×10-25 strain/√Hz at 2.5 kHz without compromising the detector sensitivity to black hole binaries. We argue that the proposed instruments have the potential to detect similar amount of postmerger neutron star oscillations as the next generation detectors, such as Cosmic Explorer and Einstein Telescope. We also optimize the arm length of the future detectors for neutron star physics and find that the optimal arm length is ≈20 km. These instruments have the potential to observe neutron star postmerger oscillations at a rate of approximately 30 events per year with a signal-to-noise ratio of 5 or more. [ABSTRACT FROM AUTHOR]