arXiv subjects

Nikolaos Stergioulas

Publications and source records attributed to Nikolaos Stergioulas.

At least 55 records · Page 3Linked to original sources

arXiv2022

Astrophysical Implications of Neutron Star Inspiral and Coalescence

The first inspiral of two neutron stars observed in gravitational waves was remarkably close, allowing the kind of simultaneous gravitational wave and electromagnetic observation that had not been expected for several years. Their merger, followed by a gamma-ray burst and a kilonova, was observed across the spectral bands of electromagnetic telescopes. These GW and electromagnetic observations have led to dramatic advances in understanding short gamma-ray bursts; determining the origin of the heaviest elements; and determining the maximum mass of neutron stars. From the imprint of tides on the gravitational waveforms and from observations of X-ray binaries, one can extract the radius and deformability of inspiraling neutron stars. Together, the radius, maximum mass, and causality constrain the neutron-star equation of state, and future constraints can come from observations of post-merger oscillations. We selectively review these results, filling in some of the physics with derivations and estimates.

astro-ph.HE↗

arXiv2021

Frequency deviations in universal relations of isolated neutron stars and postmerger remnants

We relate the fundamental quadrupolar fluid mode of isolated non-rotating 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 formulae 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 EoS 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 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.

astro-ph.HE↗

arXiv2021

GW190814: On the properties of the secondary component of the binary

We show that the odds of the mass-gap (secondary) object in GW190814 being a neutron star (NS) improve if one allows for a stiff high-density equation of state (EoS) or a large spin. Since its mass is $\in (2.50,2.67) M_{\odot}$, establishing its true nature will make it either the heaviest NS or the lightest black hole (BH), and can have far-reaching implications on NS EoS and compact object formation channels. When limiting oneself to the NS hypothesis, we deduce the secondary's properties by using a Bayesian framework with a hybrid EoS formulation that employs a parabolic expansion-based nuclear empirical parameterization around the nuclear saturation density augmented by a generic 3-segment piecewise polytrope (PP) model at higher densities and combining a variety of astrophysical observations. For the slow-rotation scenario, GW190814 implies a very stiff EoS and a stringent constraint on the EoS specially in the high-density region. On the other hand assuming the secondary object is a rapidly rotating NS, we constrain its rotational frequency to be $f=1170^{+389}_{-495}$ Hz, within a $90\%$ confidence interval. In this scenario, the secondary object in GW190814 would qualify as the fastest rotating NS ever observed. However, for this scenario to be viable, rotational instabilities would have to be suppressed both during formation and the subsequent evolution until merger, otherwise the secondary of GW190814 is more likely to be a BH.

astro-ph.HE↗

arXiv2021

Autoencoder-driven Spiral Representation Learning for Gravitational Wave Surrogate Modelling

Recently, artificial neural networks have been gaining momentum in the field of gravitational wave astronomy, for example in surrogate modelling of computationally expensive waveform models for binary black hole inspiral and merger. Surrogate modelling yields fast and accurate approximations of gravitational waves and neural networks have been used in the final step of interpolating the coefficients of the surrogate model for arbitrary waveforms outside the training sample. We investigate the existence of underlying structures in the empirical interpolation coefficients using autoencoders. We demonstrate that when the coefficient space is compressed to only two dimensions, a spiral structure appears, wherein the spiral angle is linearly related to the mass ratio. Based on this finding, we design a spiral module with learnable parameters, that is used as the first layer in a neural network, which learns to map the input space to the coefficients. The spiral module is evaluated on multiple neural network architectures and consistently achieves better speed-accuracy trade-off than baseline models. A thorough experimental study is conducted and the final result is a surrogate model which can evaluate millions of input parameters in a single forward pass in under 1ms on a desktop GPU, while the mismatch between the corresponding generated waveforms and the ground-truth waveforms is better than the compared baseline methods. We anticipate the existence of analogous underlying structures and corresponding computational gains also in the case of spinning black hole binaries.

cs.LG↗

arXiv2021

Systematics of prompt black-hole formation in neutron star mergers

This study addresses the collapse behavior of neutron star (NS) mergers expressed through the binary threshold mass M_thr for prompt black hole (BH) formation, which we determine by relativistic hydrodynamical simulations for 40 equation of state (EoS) models. M_thr can be well described by various fit formulae involving stellar parameters of nonrotating NSs. Using these relations we compute which constraints on NS radii and the tidal deformability are set by current and future merger detections revealing information about the merger product. We systematically investigate the impact of the binary mass ratio q=M_1/M_2 and assemble different fits, which make different assumptions about a-priori knowlegde. We find fit formulae for M_thr 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 extreme cases M_thr of asymmetric mergers is equal or smaller than the one of equal-mass binaries. The impact of the binary mass asymmetry on M_thr becomes stronger with more extreme mass ratios, while M_thr is approximately constant for small deviations from q=1. We describe that a phase transition to deconfined quark matter can leave a characteristic imprint on the collapse behavior. The presence of quark matter can reduce the stability of the remnant and thus M_thr relative to a purely hadronic reference model. Comparing the threshold mass and the tidal deformability Lambda_thr of a system with M_thr can yield peculiar combinations of those two quantities, where M_thr is particularly small in relation to Lambda_thr. Hence, a combined measurement of both quantities can indicate the onset of quark deconfinement. We point out new univariate relations between M_thr and stellar properties of high-mass NSs, which can be employed for direct EoS constraints or consistency checks. (abridged)

astro-ph.HE↗

arXiv2021

Equilibrium sequences of differentially rotating stars with post-merger-like rotational profiles

We present equilibrium sequences of rotating relativistic stars, constructed with a new rotation law that was proposed by Uryu et al. (2017). We choose rotational parameters motivated by simulations of binary neutron star merger remnants, but otherwise adopt a cold, relativistic N=1 polytropic EOS, in order to perform a detailed comparison to published equilibrium sequences that used the Komatsu, Eriguchi and Hachisu (1989) rotation law. We find a small influence of the choice of rotation law on the mass of the equilibrium models and a somewhat larger influence on their radius. The versatility of the new rotation law allows us to construct models that have a similar rotational profile and axis ratio as observed for merger remnants, while at the same time being quasi-spherical. More specifically, we construct equilibrium sequence variations with different degrees of differential rotation and identify type A and type C solutions, similar to the corresponding types in the classification of Ansorg, Gondek-Rosinska and Villain (2009). While our models are highly accurate solutions of the fully general relativistic structure equations, we demonstrate that for models relevant to merger remnants the IWM-CFC approximation still maintains an acceptable accuracy.

gr-qc↗

arXiv2020

Equation of state constraints from the threshold binary mass for prompt collapse of neutron star mergers

Using hydrodynamical simulations for a large set of high-density matter equations of state (EoSs) we systematically determine the threshold mass M_thres for prompt black-hole formation in equal-mass and asymmetric neutron star (NS) mergers. We devise the so far most direct, general and accurate method to determine the unknown maximum mass of nonrotating NSs from merger observations revealing M_thres. Considering hybrid EoSs with hadron-quark phase transition, we identify a new, observable signature of quark matter in NS mergers. Furthermore, our findings have direct applications in gravitational wave searches, kilonova interpretations and multi-messenger constraints on NS properties.

astro-ph.HE↗

arXiv2020

A Parametrized Equation of State for Neutron Star Matter with Continuous Sound Speed

We present a generalized piecewise polytropic parameterization for the neutron-star equation of state using an ansatz that imposes continuity in not only pressure and energy density, but also in the speed of sound. The universe of candidate equations of state is shown to admit preferred dividing densities, determined by minimizing an error norm consisting of integral astrophysical observables. Generalized piecewise polytropes accurately reproduce astrophysical observables, such as mass, radius, tidal deformability and mode frequencies, as well as thermodynamic quantities, such as the adiabatic index. This makes the new EOS useful for Bayesian parameter estimation from gravitational waveforms. Moreover, since they are differentiable, generalized piecewise polytropes can improve pointwise convergence in numerical relativity simulations of neutron stars. Existing implementations of piecewise polytropes can easily accommodate this generalization.

astro-ph.HE↗

arXiv2020

Numerical-relativity simulations of long-lived remnants of binary neutron star mergers

We analyze the properties of the gravitational wave signal emitted after the merger of a binary neutron star system when the remnant survives for more than a 80 ms (and up to 140ms). We employ four different piecewise polytropic equations of state supplemented by an ideal fluid thermal component. We find that the post-merger phase can be subdivided into three phases: an early post-merger phase (where the quadrupole mode and a few subdominant features are active), the intermediate post-merger phase (where only the quadrupole mode is active) and the late post-merger phase (where convective instabilities trigger inertial modes). The inertial modes have frequencies somewhat smaller than the quadrupole modes. In one model, we find an interesting association of a corotation of the quadrupole mode in parts of the star with a revival of its amplitude. The gravitational wave emission of inertial modes in the late post-merger phase is concentrated in a narrow frequency region and is potentially detectable by the planned third-generation detectors. This allows for the possibility of probing not only the cold part of the equation of state, but also its dependence on finite temperature. In view of these results, it will be important to investigate the impact of various type of viscosities on the potential excitation of inertial modes in binary neutron star merger remnants.

gr-qc↗

arXiv2020

Spectral classification of gravitational-wave emission and equation of state constraints in binary neutron star mergers

The first detection of gravitational waves from a binary neutron star merger (GW170817) and the accompanying electromagnetic emission has impressively advanced our understanding of the merger process and has set some first constraints on the macroscopic properties of neutron stars, with direct implications for the high-density equation of state. We discuss work on neutron star mergers focusing on the postmerger gravitational-wave emission. These studies are based on numerical simulations of the merger and survey a large sample of candidate equations of state for neutron star matter. The goal is to connect observables with the underlying physics questions. This offers a way to constrain the properties of high-density matter through the determination of neutron star radii, as inferred by an empirical relation connecting the dominant gravitational wave frequency peak in the postmerger phase to the radius of nonrotating neutron stars of a certain mass. We clarify the physical origin of secondary peaks and discuss a spectral classification scheme, based on their relative strength. Observational prospects for the dominant and the secondary peaks are also discussed. The threshold mass to black hole collapse is connected by another empirical relation to the maximum mass and compactness of nonrotating neutron stars, which can be derived semi-analytically. The observation of GW170817 then sets an absolute minimum radius for neutron stars of typical masses, based only on a minimal number of assumptions. We discuss future prospects, in light of the planned upgrades of the current gravitational wave detectors.

gr-qc↗

arXiv2019

Empirical relations for gravitational-wave asteroseismology of binary neutron star mergers

We construct new, multivariate empirical relations for measuring neutron star radii and tidal deformabilities from the dominant gravitational wave frequency in the post-merger 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 post-merger peak frequency $f_{\rm peak}$ and the chirp mass $M_{\rm chirp}$. The former could be measured with good accuracy from gravitational waves emitted in the post-merger phase using next-generation detectors, whereas the latter is already obtained with good accuracy from the inspiral phase with present-day detectors. Our main data set consists of a gravitational wave catalogue obtained with CFC/SPH simulations. We also extract the $f_{\rm peak}$ frequency from the publicly available CoRe data set, obtained through grid-based GRHD simulations and find good agreement between the extracted frequencies of the two data sets. As a result, we can construct empirical relations for the combined data sets. Furthermore, we investigate empirical relations for two secondary peaks, $f_{2-0}$ and $f_{\rm spiral}$, 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.

gr-qc↗

arXiv2019

Probing the Nature of Black Holes: Deep in the mHz Gravitational-Wave Sky

Black holes are unique among astrophysical sources: they are the simplest macroscopic objects in the Universe, and they are extraordinary in terms of their ability to convert energy into electromagnetic and gravitational radiation. Our capacity to probe their nature is limited by the sensitivity of our detectors. The LIGO/Virgo interferometers are the gravitational-wave equivalent of Galileo's telescope. The first few detections represent the beginning of a long journey of exploration. At the current pace of technological progress, it is reasonable to expect that the gravitational-wave detectors available in the 2035-2050s will be formidable tools to explore these fascinating objects in the cosmos, and space-based detectors with peak sensitivities in the mHz band represent one class of such tools. These detectors have a staggering discovery potential, and they will address fundamental open questions in physics and astronomy. Are astrophysical black holes adequately described by general relativity? Do we have empirical evidence for event horizons? Can black holes provide a glimpse into quantum gravity, or reveal a classical breakdown of Einstein's gravity? How and when did black holes form, and how do they grow? Are there new long-range interactions or fields in our universe, potentially related to dark matter and dark energy or a more fundamental description of gravitation? Precision tests of black hole spacetimes with mHz-band gravitational-wave detectors will probe general relativity and fundamental physics in previously inaccessible regimes, and allow us to address some of these fundamental issues in our current understanding of nature.

astro-ph.HE↗

arXiv2019

Equation-of-state Constraints and the QCD Phase Transition in the Era of Gravitational-Wave Astronomy

We describe a multi-messenger interpretation of GW170817, which yields a robust lower limit on NS radii. This excludes NSs with radii smaller than about 10.7 km and thus rules out very soft nuclear matter. We stress the potential of this type of constraints when future detections become available. A very similar argumentation may yield an upper bound on the maximum mass of nonrotating NSs. We also discuss simulations of NS mergers, which undergo a first-order phase transition to quark matter. We point out a different dynamical behavior. Considering the gravitational-wave signal, we identify an unambiguous signature of the QCD phase transition in NS mergers. The occurrence of quark matter through a strong first-order phase transition during merging leads to a characteristic shift of the dominant postmerger frequency. The frequency shift is indicative for a phase transition if it is compared to the postmerger frequency which is expected for purely hadronic EoS models. A very strong deviation of several 100 Hz is observed for hybrid EoSs in an otherwise tight relation between the tidal deformability and the postmerger frequency. We address the potential impact of a first-order phase transition on the electromagnetic counterpart of NS mergers. Our simulations suggest that there would be no significant qualitative differences between a system undergoing a phase transition to quark matter and purely hadronic mergers. The quantitative differences are within the spread which is found between different hadronic EoS models. This implies on the one hand that GW170817 is compatible with a possible transition to quark matter. On the other hand these considerations show that it may not be easy to identify quantitative differences between purely hadronic mergers and events in which quark matter occurs considering solely their electromagnetic counterpart or their nucleosynthesis products. (abridged)

astro-ph.HE↗

arXiv2019

Rotating Black Holes in Higher Order Gravity

We develop a new technique for finding black hole solutions in modified gravity that have "stealth" hair, i.e., hair whose only gravitational effect is to tune the cosmological constant. We consider scalar-tensor theories in which gravitational waves propagate at the speed of light, and show that Einstein metrics can be painted with stealth hair provided there exists a family of geodesics always normal to spacelike surfaces. We also present a novel scalar-dressed rotating black hole that has finite scalar field at both the black hole and cosmological event horizons.

hep-th↗

arXiv2018

Differentially rotating neutron stars in scalar-tensor theories of gravity

We present the first numerical models of differentially rotating stars in alternative theories of gravity. We chose a particular class of scalar-tensor theories of gravity that is indistinguishable from GR in the weak field regime but can lead to significant deviations when strong fields are considered. We show that the maximum mass that a differentially rotating neutron star can sustain increases significantly for scalarized solutions and such stars can reach larger angular momenta. In addition, the presence of a nontrivial scalar field has the effect of increasing the required axis ratio for reaching a given value of angular momentum, when compared to a corresponding model of same rest mass in general relativity. We find that the scalar field also makes rapidly rotating models less quasi-toroidal than their general-relativistic counterparts. For large values of the angular momentum and values of the coupling parameter that are in agreement with the observations, we find a second turning point for scalarized models along constant angular momentum sequences, which could have interesting implications for the stability of remnants created in a binary neutron star merger.

gr-qc↗

arXiv2018

Convective Excitation of Inertial Modes in Binary Neutron Star Mergers

We present the first very long-term simulations (extending up to ~140 ms after merger) of binary neutron star mergers with piecewise polytropic equations of state and in full general relativity. Our simulations reveal that at a time of 30-50 ms after merger, parts of the star become convectively unstable, which triggers the excitation of inertial modes. The excited inertial modes are sustained up to several tens of milliseconds and are potentially observable by the planned third-generation gravitational-wave detectors at frequencies of a few kilohertz. Since inertial modes depend on the rotation rate of the star and they are triggered by a convective instability in the postmerger remnant, their detection in gravitational waves will provide a unique opportunity to probe the rotational and thermal state of the merger remnant. In addition, our findings have implications for the long-term evolution and stability of binary neutron star remnants

gr-qc↗

arXiv2018

Universal and approximate relations for the gravitational-wave damping timescale of $f$-modes in neutron stars

Existing estimates of the gravitational-wave damping timescale of the dominant quadrupole oscillation mode in the case of rapidly rotating stars are based on using a Newtonian estimate for the energy of the mode, in combination with the lowest-order post-Newtonian quadrupole formula for estimating the gravitational-wave luminosity. We investigate a number of other choices for estimating the gravitational-wave damping timescale in the nonrotating limit and construct a highly accurate, empirically corrected formula that has a maximum relative error of only 3% with respect to the perturbative result in full general relativity. The expressions involved are sufficiently general to be extended to the case of rapidly rotating stars. We also present a new higher-order empirical relation for the gravitational-wave damping timescale of quadrupole oscillations that is accurate in the whole range of expected values for the compactness of neutron stars, without the need for involving the moment of inertia.

gr-qc↗

arXiv2017

Rotating Stars in Relativity

Rotating relativistic stars have been studied extensively in recent years, both theoretically and observationally, because of the information they might yield about the equation of state of matter at extremely high densities and because they are considered to be promising sources of gravitational waves. The latest theoretical understanding of rotating stars in relativity is reviewed in this updated article. The sections on equilibrium properties and on nonaxisymmetric oscillations and instabilities in $f$-modes and $r$-modes have been updated. Several new sections have been added on equilibria in modified theories of gravity, approximate universal relationships, the one-arm spiral instability, on analytic solutions for the exterior spacetime, rotating stars in LMXBs, rotating strange stars, and on rotating stars in numerical relativity including both hydrodynamic and magnetohydrodynamic studies of these objects.

astro-ph.HE↗