arXiv subjects

Matteo Breschi

Publications and source records attributed to Matteo Breschi.

At least 19 recordsLinked to original sources

arXiv2025

Kilohertz Gravitational Waves from Binary Neutron Star Mergers: Full Spectrum Analyses and High-density Constraints on Neutron Star Matter

We demonstrate Bayesian analyses of the complete gravitational-wave spectrum of binary neutron star mergers events with the next-generation detector Einstein Telescope. Our mock analyses are performed for 20 different signals using the TEOBResumSPA_NRPMw waveform that models gravitational-waves from the inspiral to the postmerger phase. They are employed to validate a pipeline for neutron star's extreme matter constraints with prospective detections and under minimal hypotheses on the equation of state. The proposed analysis stack delivers inferences for the mass-radius curve, the mass dependence of the quadrupolar tidal polarizability parameter, the neutron star's maximum density, the maximum mass and the relative radius, and the pressure-density relation itself. We show that a single event at a signal-to-noise ratio close to the minimum threshold for postmerger detection is sufficient to tightly constrain all the above relations as well as quantities like the maximum mass (maximum density) to precision of ${\sim}6$% (${\sim}10$%) at 90% credibility level. We also revisit inferences of prompt black hole formation with full spectrum signals and find that the latter can be robustly identified, even when the postmerger is not detectable due to a low signal-to-noise ratio. New results on the impact of the initial signal frequency and of the detector configuration (triangular vs. two-L) on the source's parameters estimation are also reported. An improvement of approximately one order of magnitude in the precision of the chirp mass and mass ratio can be achieved by lowering the initial frequency from 20 Hz to 2 Hz. The two-L configuration shows instead significant improvements on the inference of the source declination, due to geographical separation of the two detectors.

gr-qc↗

arXiv2025

Gravitational waves from eccentric binary neutron star mergers: Systematic biases and inadequacy of quasicircular templates

The use of quasicircular waveforms in matched-filter analyses of signals from eccentric binary neutron star mergers can lead to biases in the source's parameter estimation. We demonstrate that significant biases can be present already for moderate eccentricities $e_{0} \gtrsim 0.05$ and signals detected by LIGO-Virgo-KAGRA with signal-to-noise ratio $\gtrsim 12$. We perform systematic Bayesian mock analyses of unequal-mass nonspinning binary neutron star signals up to eccentricities $e_0 \sim 0.1$ using quasicircular effective-one-body waveforms with spins. We find fractional signal-to-noise ratio losses up to tens of percent and up to 16$\sigma$ deviations in the inference of the chirp mass. The latter effect is sufficiently large to lead to an incorrect (and ambiguous) source identification. The inclusion of spin precession in the quasicircular waveform does not capture eccentricity effects. We conclude that high-precision observations with advanced (and next generation) detectors are likely to require standardized, accurate, and fast eccentric waveforms.

gr-qc↗

arXiv2025

The Science of the Einstein Telescope

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

gr-qc↗

arXiv2024

Bayesian evidence estimation from posterior samples with normalizing flows

We propose a novel method ($floZ$), based on normalizing flows, to estimate the Bayesian evidence (and its numerical uncertainty) from a pre-existing set of samples drawn from the unnormalized posterior distribution. We validate it on distributions whose evidence is known analytically, up to 15 parameter space dimensions, and compare with two state-of-the-art techniques for estimating the evidence: nested sampling (which computes the evidence as its main target) and a $k$-nearest-neighbors technique that produces evidence estimates from posterior samples. Provided representative samples from the target posterior are available, our method is more robust to posterior distributions with sharp features, especially in higher dimensions. For a simple multivariate Gaussian, we demonstrate its accuracy for up to 200 dimensions with $10^5$ posterior samples. $floZ$ has wide applicability, e.g., to estimate evidence from variational inference, Markov Chain Monte Carlo samples, or any other method that delivers samples and their likelihood from the unnormalized posterior density. As a physical application, we use $floZ$ to compute the Bayes factor for the presence of the first overtone in the ringdown signal of the gravitational wave data of GW150914, finding good agreement with nested sampling.

stat.ML↗

arXiv2024

Bayesian inference of multimessenger astrophysical data: Joint and coherent inference of gravitational waves and kilonovae

We present a Bayesian framework for joint and coherent analyses of multimessenger binary neutron star signals. The method, implemented in our bajes infrastructure, incorporates a joint likelihood for multiple datasets, support for various semi-analytical kilonova models and numerical-relativity (NR) informed relations for the mass ejecta, as well as a technique to include and marginalize over modeling uncertainties. As a first application, we analyze the gravitational-wave GW170817 and the kilonova AT2017gfo data. These results are then combined with the most recent X-ray pulsars analyses of PSR J0030+0451 and PSR J0740+6620 to obtain EOS constraints.Various constraints on the mass-radius diagram and neutron star properties are then obtained by resampling over a set of ten million parametrized EOS built under minimal assumptions. We find that a joint and coherent approach improves the inference of the extrinsic parameters (distance) and, among the instrinc parameters, the mass ratio. The inclusion of NR informed relations strongly improves over the case of using an agnostic prior on the intrinsic parameters. Comparing Bayes factors, we find that the two observations are better explained by the common source hypothesis only by assuming NR-informed relations. These relations break some of the degeneracies in the employed kN models. The EOS inference folding-in PSR J0952-0607 minimum-maximum mass, PSR J0030+0451 and PSR J0740+6620 data constrains, among other quantities, the neutron star radius to $R_{1.4}={12.30}^{+0.81}_{-0.56}$ km ($R_{1.4}={13.20}^{+0.91}_{-0.90}$ km) and the maximum mass to $M_{max}={2.28}^{+0.25}_{-0.17}~{\rm M_\odot}$ ($M_{max}={2.32}^{+0.30}_{-0.19}~{\rm M_\odot}$) where the ST+PDT (PDT-U) analysis of Vinciguerra et a (2023) for PSR J0030+0451 is employed. Hence, the systematics on PSR J0030+0451 data reduction currently dominate the mass-radius diagram constraints.

astro-ph.HE↗

arXiv2023

Detectability of QCD phase transitions in binary neutron star mergers: Bayesian inference with the next generation gravitational wave detectors

We study the detectability of postmerger QCD phase transitions in neutron star binaries with next-generation gravitational-wave detectors Cosmic Explorer and Einstein Telescope. We perform numerical relativity simulations of neutron star mergers with equations of state that include a quark deconfinement phase transition through either a Gibbs or Maxwell construction. These are followed by Bayesian parameter estimation of the associated gravitational-wave signals using the $\tt{NRPMw}$ waveform model, with priors inferred from the analysis of the inspiral signal. We assess the ability of the model to measure the postmerger peak frequency $f_2^{\rm peak}$ and identify aspects that should be improved in the model. We show that, even at postmerger signal to noise ratios as low as 10, the model can distinguish (at the 90% level) $f_2^{\rm peak}$ between binaries with and without a phase transition in most cases. Phase-transition induced deviations in the $f_2^{\rm peak}$ from the predictions of equation-of-state insensitive relations can also be detected if they exceed $1.6\,\sigma$. Our results suggest that next-generation gravitational wave detectors can measure phase transition effects in binary neutron star mergers. However, unless the phase transition is ``strong'', disentangling it from other hadronic physics uncertainties will require significant theory improvements.

gr-qc↗

arXiv2023

Analytically improved and numerical-relativity informed effective-one-body model for coalescing binary neutron stars

Gravitational wave astronomy pipelines rely on template waveform models for searches and parameter estimation purposes. For coalescing binary neutron stars (BNS), such models need to accurately reproduce numerical relativity (NR) up to merger, in order to provide robust estimate of the stars' equation of state - dependent parameters. In this work we present an improved version of the Effective One Body (EOB) model $\tt TEOBResumS$ for gravitational waves from BNS systems. Building upon recent post-Newtonian calculations, we include subleading order tidal terms in the waveform multipoles and EOB metric potentials, as well as add up to 5.5PN terms in the gyro-gravitomagnetic functions entering the spin-orbit sector of the model. In order to further improve the EOB-NR agreement in the last few orbital cycles before merger, we introduce next-to-quasicircular corrections in the waveform -- informed by a large number of BNS NR simulations -- and introduce a new NR-informed parameter entering the tidal sector of our conservative dynamics. The performance of our model is then validated against 14 new eccentricity reduced simulations of unequal mass, spinning binaries with varying equation of state. A time-domain phasing analysis and mismatch computations demonstrate that the new model overall improves over the previous version of $\tt TEOBResumS$. Finally, we present a closed-form frequency domain representation of the (tidal) amplitude and phase of the new model. This representation accounts for mass-ratio, aligned spin and (resummed) spin-quadrupole effects in the tidal phase and -- within the calibration region -- it is faithful to the original model.

gr-qc↗

arXiv2023

Thermal Effects in Binary Neutron Star Mergers

We study the impact of finite-temperature effects in numerical-relativity simulations of binary neutron star mergers with microphysical equations of state and neutrino transport in which we vary the effective nucleon masses in a controlled way. We find that, as the specific heat is increased, the merger remnants become colder and more compact due to the reduced thermal pressure support. Using a full Bayesian analysis, we demonstrate that this effect will be measurable in the postmerger gravitational wave signal with next-generation observatories at signal-to-noise ratios of 15.

astro-ph.HE↗

arXiv2023

Pre/post-merger consistency test for gravitational signals from binary neutron star mergers

Gravitational waves from binary neutron star (BNS) mergers can constrain nuclear matter models predicting the neutron star's equation of state (EOS). Matter effects on the inspiral-merger signal are encoded in the multipolar tidal polarizability parameters, whose leading order combination is sufficient to capture to high accuracy the key features of the merger waveform (e.g.~the merger frequency). Similar EOS-insensitive relations exist for the post-merger signal and can be used to model the emission from the remnant. Several works suggested that the appearance of new degrees of freedom or phase transitions in high-density post-merger matter can be inferred by observing a violation of these EOS-insensitive relations. Here, we demonstrate a Bayesian method to test such an EOS-insensitive relation between the tidal polarizability parameters (or any other equivalent parameter) and the dominant post-merger frequency, using information either up to merger or from the post-merger signal. Technically, the method is similar to tests of General Relativity with binary black holes that verify the inspiral-merger-ringdown consistency. However, differently from the latter, BNS pre/post-merger consistency tests are conceptually less informative and they only address the consistency (or the breaking) of the assumed EOS-insensitive relation. Specifically, we discuss how such tests cannot conclusively discriminate between an EOS not respecting such relation and the appearance of new degrees of freedom (or phase transitions) in high-density matter.

gr-qc↗

arXiv2022

Numerical-Relativity-Informed Effective-One-Body model for Black-Hole-Neutron-Star Mergers with Higher Modes and Spin Precession

We present the first effective-one-body (EOB) model for generic-spins quasi-circular black-hole--neutron-star (BHNS) inspiral-merger-ringdown gravitational waveforms (GWs). Our model is based on a new numerical-relativity (NR) informed expression of the BH remnant and its ringdown. It reproduces the NR $(\ell,m)=(2,2)$ waveform with typical phase agreement of ${\lesssim0.5}\,$rad (${\lesssim1}\,$rad) to merger (ringdown). The maximum (minimum) mismatch between the $(2,2)$ and the NR data is 4% (0.6%). Higher modes (HMs) $(2,1)$, $(3,2)$, $(3,3)$, $(4,4)$ and $(5,5)$ are included and their mismatch with the available NR waveforms are up to (down to) a 60% (1%) depending on the inclination. Phase comparison with a 16 orbit precessing simulation shows differences within the NR uncertainties. We demonstrate the applicability of the model in GW parameter estimation by perfoming the first BHNS Bayesian analysis with HMs (and non-precessing spins) of the event GW190814, together with new $(2,2)$-mode analysis of GW200105 and GW200115. For the GW190814 study, the inclusion of HMs gives tighter parameter posteriors. The Bayes factors of our analyses on this event show decisive evidence for the presence of HMs, but no clear preference for a BHNS or a binary black hole (BBH) source. Similarly, we confirm GW200105 and GW200115 show no evidence for tidal effects.

gr-qc↗

arXiv2022

Second release of the CoRe database of binary neutron star merger waveforms

We present the second data release of gravitational waveforms from binary neutron star merger simulations performed by the Computational Relativity (CoRe) collaboration. The current database consists of 254 different binary neutron star configurations and a total of 590 individual numerical-relativity simulations using various grid resolutions. The released waveform data contain the strain and the Weyl curvature multipoles up to $\ell=m=4$. They span a significant portion of the mass, mass-ratio,spin and eccentricity parameter space and include targeted configurations to the events GW170817 and GW190425. CoRe simulations are performed with 18 different equations of state, seven of which are finite temperature models, and three of which account for non-hadronic degrees of freedom. About half of the released data are computed with high-order hydrodynamics schemes for tens of orbits to merger; the other half is computed with advanced microphysics. We showcase a standard waveform error analysis and discuss the accuracy of the database in terms of faithfulness. We present ready-to-use fitting formulas for equation of state-insensitive relations at merger (e.g. merger frequency), luminosity peak, and post-merger spectrum.

gr-qc↗

arXiv2022

Combining effective-one-body accuracy and reduced-order-quadrature speed for binary neutron star merger parameter estimation with machine learning

We present mlgw-bns, a gravitational waveform surrogate that allows for a significant improvement in the generation speed of frequency-domain waveforms for binary neutron star mergers, at a negligible cost in accuracy. This improvement is achieved by training a machine-learning model on a dataset of waveforms generated with an accurate but comparatively costlier approximant: the state-of-the-art effective-one-body model TEOBResumSPA. When coupled to a reduced-order scheme, mlgw-bns can accelerate waveform generation up to a factor of ~35, outperforming all other approximants of similar accuracy. By analyzing GW170817 in realistic parameter estimation settings with our scheme, we showcase an overall speedup against TEOBResumSPA greater than an order of magnitude. Our methodology will bear a significant impact on the scientific program of next generation detectors by allowing routine usage of accurate effective-one-body models.

gr-qc↗

arXiv2022

Inferring eccentricity evolution from observations of coalescing binary black holes

The origin and formation of stellar-mass binary black holes remains an open question that can be addressed by precise measurements of the binary and orbital parameters from their gravitational-wave signal. Such binaries are expected to circularize due to the emission of gravitational waves as they approach merger. However, depending on their formation channel, some binaries could have a non-negligible eccentricity when entering the frequency band of current gravitational-wave detectors. In order to measure eccentricity in an observed gravitational-wave signal, accurate waveform models that describe binaries in eccentric orbits are necessary. In this work we demonstrate the efficacy of the improved TEOBResumS waveform model for eccentric coalescing binaries with aligned spins. We first validate the model against mock signals of aligned-spin binary black hole mergers and quantify the impact of eccentricity on the estimation of other intrinsic binary parameters. We then perform a fully Bayesian reanalysis of GW150914 with the eccentric waveform model. We find (i) that the model is reliable for aligned-spin binary black holes and (ii) that GW150914 is consistent with a non-eccentric merger although we cannot rule out small values of initial eccentricity at a reference frequency of $20$ Hz. Finally, we present a systematic method to measure the eccentricity and its evolution directly from the gravitational-wave posterior samples. Such an estimator is useful when comparing results from different analyses as the definition of eccentricity may differ between models. Our scheme can be applied even in the case of small eccentricities and can be adopted straightforwardly in post-processing to allow for direct comparison between models.

gr-qc↗

arXiv2022

Kilohertz Gravitational Waves from Binary Neutron Star Mergers: Inference of Postmerger Signals with the Einstein Telescope

Next-generation detectors are expected to be sensitive to postmerger signals from binary neutron star coalescences and thus to directly probe the remnant dynamics. We investigate the scientific potential of postmerger detections with the Einstein Telescope using full Bayesian analyses with the state-of-the-art waveform model ${\tt NRPMw}$. We find that: (i) Postmerger signals with signal-to-noise ratio (SNR) ${\sim}7$ can be confidently detected with a Bayes' factor of $\log{\cal B}\simeq 5$ ($\rm e$-folded) and the posterior distributions report informative measurements already at SNR ${\sim}6$ for some noise realizations. (ii) The postmerger peak frequency $f_2$ can be confidently identified at SNR $7$ with errors of $O(1~{\rm kHz})$, that decrease below $O(100~{\rm Hz})$ for SNR 10. (iii) The remnant's time of collapse to black hole can be constrained to $O(20~{\rm ms})$ at SNR 10. However, the inference can be biased by noise fluctuationsif the latter exceed the signal's amplitude before collapse. (iv) Violations of the EOS-insentive relations for $f_2$ can be detected at SNR $\gtrsim 8$ if the frequency shifts are $\gtrsim 500~{\rm Hz}$; they can be smoking guns for EOS softening effects at extreme densities. However, the $f_2$ measurement can be significantly biased by subdominant frequency components for short-lived remnants. In these cases, an EOS softening might be better inferred from the remnant's earlier collapse.

gr-qc↗

arXiv2022

Kilohertz Gravitational Waves From Binary Neutron Star Mergers: Numerical-relativity Informed Postmerger Model

We present ${\tt NRPMw}$, an analytical model of gravitational-waves from neutron star merger remnants informed using 618 numerical relativity (NR) simulations. ${\tt NRPMw}$ is designed in the frequency domain using a combination of complex Gaussian wavelets. The wavelet's parameters are calibrated to equations of state (EOS) insensitive relations from NR data. The NR simulations are computed with 21 EOS (7 of which are finite-temperature microphysical models, and 3 of which contain quark phase transitions or hyperonic degrees of freedom) and span total binary masses $M\in[2.4,3.4]~{\rm M}_\odot$, mass ratios up to $q=2$, and (nonprecessing) dimensionless spins magnitudes up to ${0.2}$. The theoretical uncertainties of the EOS-insensitive relations are incorporated in ${\tt NRPMw}$ using recalibration parameters that enhance the flexibility and accuracy of the model. ${\tt NRPMw}$ is NR-faithful with fitting factors ${\gtrsim}0.9$ computed on an independent validation set of 102 simulations.

gr-qc↗

arXiv2021

Constraints on the maximum densities of neutron stars from postmerger gravitational waves with third-generation observations

Using data from 289 numerical relativity simulations of merging binary neutron stars, we identify, for the first time, a robust quasi-universal relation connecting the postmerger peak gravitational-wave frequency and the value of the density at the center of the maximum mass nonrotating neutron star. This relation offers a new possibility for precision equation-of-state constraints with next-generation ground-based gravitational-wave interferometers. Mock Einstein Telescope observations of fiducial events indicate that Bayesian inferences can constrain the maximum density to ${\sim}15\%$ ($90\%$ confidence level) for a single signal at the minimum sensitivity threshold for a detection. If the postmerger signal is included in a full-spectrum (inspiral-merger-postmerger) analysis of such signal, the pressure-density function can be tightly constrained up to the maximum density, and the maximum neutron star mass can be measured with an accuracy better than $12\%$ ($90\%$ confidence level).

gr-qc↗

arXiv2021

GW190521 as a dynamical capture of two nonspinning black holes

Gravitational waves from $\sim 90$ black holes binary systems have currently been detected by the LIGO and Virgo experiments, and their progenitors' properties inferred. This allowed the scientific community to draw conclusions on the formation channels of black holes in binaries, informing population models and -- at times -- defying our understanding of black hole astrophysics. The most challenging event detected so far is the short duration gravitational-wave transient GW190521. We analyze this signal under the hypothesis that it was generated by the merger of two nonspinning black holes on hyperbolic orbits. The best configuration matching the data corresponds to two black holes of source frame masses of $81^{+62}_{-25}M_\odot$ and $52^{+32}_{-32}M_\odot$ undergoing two encounters and then merging into an intermediate-mass black hole. We find that the hyperbolic merger hypothesis is favored with respect to a quasi-circular merger with precessing spins with Bayes' factors larger than 4300 to 1, although this number will be reduced by the currently uncertain prior odds. Our results suggest that GW190521 might be the first gravitational-wave detection from the dynamical capture of two stellar-mass nonspinning black holes.

gr-qc↗

arXiv2021

TEOBResumS: assessment of consistent next-to-quasicircular corrections and post-adiabatic approximation in multipolar binary black holes waveforms

The use of effective-one-body (EOB) waveforms for black hole binaries analysis in gravitational-wave astronomy requires faithful models and fast generation times. A key aspect to achieve faithfulness is the inclusion of numerical-relativity (NR) informed next-to-quasicircular corrections(NQC), dependent on the radial momentum, to the waveform and radiation reaction. A robust method to speed up the waveform generation is the post-adiabatic iteration to approximate the solution of the EOB Hamiltonian equations. In this work, we assess the performances of a fast NQC prescription in combination to the post-adiabatic method for generating multipolar gravitational waves. The outlined approach allows a consistent treatment of NQC in both the waveform and the radiation-reaction, does not require iterative procedures to achieve high faithfulness, and can be efficiently employed for parameter estimation. Comparing to 611 NR simulations, for total mass $10M_\odot\leq M \leq 200M_\odot$ and using the Advanded LIGO noise, the model has EOB/NR unfaithfulness well below $0.01$, with 78.5\% of the cases below $0.001$. We apply the model to the parameter estimation of GW150914 exploring the impact of the new NQC and of the higher modes up to $\ell=m=8$.

gr-qc↗