arXiv subjects

Walter Del Pozzo

Publications and source records attributed to Walter Del Pozzo.

At least 55 records · Page 3Linked to original sources

arXiv2020

Machine Learning Gravitational Waves from Binary Black Hole Mergers

We apply machine learning methods to build a time-domain model for gravitational waveforms from binary black hole mergers, called mlgw. The dimensionality of the problem is handled by representing the waveform's amplitude and phase using a principal component analysis. We train mlgw on about $\mathcal{O}(10^3)$ TEOBResumS and SEOBNRv4 effective-one-body waveforms with mass ratios $q\in[1,20]$ and aligned dimensionless spins $s\in[-0.80,0.95]$. The resulting models are faithful to the training sets at the ${\sim}10^{-3}$ level (averaged on the parameter space). The speed up for a single waveform generation is a factor 10 to 50 (depending on the binary mass and initial frequency) for TEOBResumS and approximately an order of magnitude more for SEOBNRv4. Furthermore, mlgw provides a closed form expression for the waveform and its gradient with respect to the orbital parameters; such an information might be useful for future improvements in GW data analysis. As demonstration of the capabilities of mlgw to perform a full parameter estimation, we re-analyze the public data from the first GW transient catalog (GWTC-1). We find broadly consistent results with previous analyses at a fraction of the cost, although the analysis with spin aligned waveforms gives systematic larger values of the effective spins with respect to previous analyses with precessing waveforms. Since the generation time does not depend on the length of the signal, our model is particularly suitable for the analysis of the long signals that are expected to be detected by third-generation detectors. Future applications include the analysis of waveform systematics and model selection in parameter estimation.

gr-qc↗

arXiv2020

Massive black hole binary systems and the NANOGrav 12.5 year results

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has recently reported evidence for the presence of a common stochastic signal across their array of pulsars. The origin of this signal is still unclear. One of the possibilities is that it is due to a stochastic gravitational wave background (SGWB) in the $\sim 1-10\,{\rm nHz}$ frequency region. Taking the NANOGrav observational result at face value, we show that this signal would be fully consistent with a SGWB produced by an unresolved population of in-spiralling massive black hole binaries (MBHBs) predicted by current theoretical models. Considering an astrophysically agnostic model we find that the MBHB merger rate is loosely constrained to the range $10^{-11} - 2$ $\mathrm{Mpc}^{-3}\,\mathrm{Gyr}^{-1}$. Including additional constraints from galaxy pairing fractions and MBH-bulge scaling relations, we find that the MBHB merger rate is $10^{-5} - 5\times10^{-4}$ $\mathrm{Mpc}^{-3}\,\mathrm{Gyr}^{-1}$, the MBHB merger time-scale is $\le 3\,\mathrm{Gyr}$ and the norm of the $M_\mathrm{BH}-M_\mathrm{bulge}$ relation $\ge 1.2\times 10^{8}\,M_\odot$ (all intervals quoted at 90\% confidence). Regardless of the astrophysical details of MBHB assembly, this result would imply that a sufficiently large population of massive black holes pair up, form binaries and merge within a Hubble time.

astro-ph.HE↗

arXiv2020

Constraining black hole mimickers with gravitational wave observations

LIGO and Virgo have recently observed a number of gravitational wave (GW) signals that are fully consistent with being emitted by binary black holes described by general relativity. However, there are theoretical proposals of exotic objects that can be massive and compact enough to be easily confused with black holes. Nevertheless, these objects differ from black holes in having nonzero tidal deformabilities, which can allow one to distinguish binaries containing such objects from binary black holes using GW observations. Using full Bayesian parameter estimation, we investigate the possibility of constraining the parameter space of such "black hole mimickers" with upcoming GW observations. Employing perfect fluid stars with a polytropic equation of state as a simple model that can encompass a variety of possible black hole mimickers, we show how the observed masses and tidal deformabilities of a binary constrain the equation of state. We also show how such constraints can be used to rule out some simple models of boson stars.

gr-qc↗

arXiv2019

Observational Black Hole Spectroscopy: A time-domain multimode analysis of GW150914

The detection of the least damped quasi-normal mode from the remnant of the gravitational wave event GW150914 realised the long sought possibility to observationally study the properties of quasi-stationary black hole spacetimes through gravitational waves. Past literature has extensively explored this possibility and the emerging field has been named "black hole spectroscopy". In this study, we present results regarding the ringdown spectrum of GW150914, obtained by application of Bayesian inference to identify and characterise the ringdown modes. We employ a pure time-domain analysis method which infers from the data the time of transition between the non-linear and quasi-linear regime of the post-merger emission in concert with all other parameters characterising the source. We find that the data provides no evidence for the presence of more than one quasi-normal mode. However, from the central frequency and damping time posteriors alone, no unambiguous identification of a single mode is possible. More in-depth analysis adopting a ringdown model based on results in perturbation theory over the Kerr metric, confirms that the data do not provide enough evidence to discriminate among an $l=2$ and the $l=3$ subset of modes. Our work provides the first comprehensive agnostic framework to observationally investigate astrophysical black holes' ringdown spectra.

gr-qc↗

arXiv2019

GW150914 peak frequency: a novel consistency test of strong-field General Relativity

We introduce a novel test of General Relativity in the strong-field regime of a binary black hole coalescence. Combining information coming from Numerical Relativity simulations of coalescing black hole binaries with a Bayesian reconstruction of the gravitational wave signal detected in LIGO-Virgo interferometric data, allows one to test theoretical predictions for the instantaneous gravitational wave frequency measured at the peak of the gravitational wave signal amplitude. We present the construction of such a test and apply it on the first gravitational wave event detected by the LIGO and Virgo Collaborations, GW150914. The $p$-value obtained is $p=0.48$, to be contrasted with an expected value of $p=0.5$, so that no signs of violations from General Relativity were detected.

gr-qc↗

arXiv2018

Empirical tests of the black hole no-hair conjecture using gravitational-wave observations

We show that second-generation gravitational-wave detectors at their design sensitivity will allow us to directly probe the ringdown phase of binary black hole coalescences. This opens the possibility to test the so-called black hole no-hair conjecture in a statistically rigorous way. Using state-of-the-art numerical relativity-tuned waveform models and dedicated methods to effectively isolate the quasi-stationary perturbative regime where a ringdown description is valid, we demonstrate the capability of measuring the physical parameters of the remnant black hole, and subsequently determining parameterized deviations from the ringdown of Kerr black holes. By combining information from $\mathcal{O}(5)$ binary black hole mergers with realistic signal-to-noise ratios achievable with the current generation of detectors, the validity of the no-hair conjecture can be verified with an accuracy of $\sim 1.5\%$ at $90\%$ confidence.

gr-qc↗

arXiv2018

Time-domain effective-one-body gravitational waveforms for coalescing compact binaries with nonprecessing spins, tides and self-spin effects

We present TEOBResumS, a new effective-one-body (EOB) waveform model for nonprecessing (spin-aligned) and tidally interacting compact binaries.Spin-orbit and spin-spin effects are blended together by making use of the concept of centrifugal EOB radius. The point-mass sector through merger and ringdown is informed by numerical relativity (NR) simulations of binary black holes (BBH) computed with the SpEC and BAM codes. An improved, NR-based phenomenological description of the postmerger waveform is developed.The tidal sector of TEOBResumS describes the dynamics of neutron star binaries up to merger and incorporates a resummed attractive potential motivated by recent advances in the post-Newtonian and gravitational self-force description of relativistic tidal interactions. Equation-of-state dependent self-spin interactions (monopole-quadrupole effects) are incorporated in the model using leading-order post-Newtonian results in a new expression of the centrifugal radius. TEOBResumS is compared to 135 SpEC and 19 BAM BBH waveforms. The maximum unfaithfulness to SpEC data $\bar{F}$ -- at design Advanced-LIGO sensitivity and evaluated with total mass $M$ varying between $10M_\odot \leq M \leq 200 M_\odot$ --is always below $2.5 \times 10^{-3}$ except for a single outlier that grazes the $7.1 \times 10^{-3}$ level. When compared to BAM data, $\bar{F}$ is smaller than $0.01$ except for a single outlier in one of the corners of the NR-covered parameter space, that reaches the $0.052$ level.TEOBResumS is also compatible, up to merger, to high end NR waveforms from binary neutron stars with spin effects and reduced initial eccentricity computed with the BAM and THC codes. The model is designed to generate accurate templates for the analysis of LIGO-Virgo data through merger and ringdown. We demonstrate its use by analyzing the publicly available data for GW150914.

gr-qc↗

arXiv2018

Dirichlet Process Gaussian-mixture model: An application to localizing coalescing binary neutron stars with gravitational-wave observations

We reconstruct posterior distributions for the position (sky area and distance) of a simulated set of binary neutron-star gravitational-waves signals observed with Advanced LIGO and Advanced Virgo. We use a Dirichlet Process Gaussian-mixture model, a fully Bayesian non-parametric method that can be used to estimate probability density functions with a flexible set of assumptions. The ability to reliably reconstruct the source position is important for multimessenger astronomy, as recently demonstrated with GW170817. We show that for detector networks comparable to the early operation of Advanced LIGO and Advanced Virgo, typical localization volumes are $\sim10^4$--$10^5~\mathrm{Mpc^3}$ corresponding to $\sim10^2$--$10^3$ potential host galaxies. The localization volume is a strong function of the network signal-to-noise ratio, scaling roughly $\propto \varrho_{net}^{-6}$. Fractional localizations improve with the addition of further detectors to the network. Our Dirichlet Process Gaussian-mixture model can be adopted for localizing events detected during future gravitational-wave observing runs, and used to facilitate prompt multimessenger follow-up.

astro-ph.IM↗

arXiv2018

Testing general relativity using gravitational wave signals from the inspiral, merger and ringdown of binary black holes

Advanced LIGO's recent observations of gravitational waves (GWs) from merging binary black holes have opened up a unique laboratory to test general relativity (GR) in the highly relativistic regime. One of the tests used to establish the consistency of the first LIGO event with a binary black hole merger predicted by GR was the inspiral-merger-ringdown consistency test. This involves inferring the mass and spin of the remnant black hole from the inspiral (low-frequency) part of the observed signal and checking for the consistency of the inferred parameters with the same estimated from the post-inspiral (high-frequency) part of the signal. Based on the observed rate of binary black hole mergers, we expect the advanced GW observatories to observe hundreds of binary black hole mergers every year when operating at their design sensitivities, most of them with modest signal to noise ratios (SNRs). Anticipating such observations, this paper shows how constraints from a large number of events with modest SNRs can be combined to produce strong constraints on deviations from GR. Using kludge modified GR waveforms, we demonstrate how this test could identify certain types of deviations from GR if such deviations are present in the signal waveforms. We also study the robustness of this test against reasonable variations of a variety of different analysis parameters.

gr-qc↗

arXiv2018

Matter imprints in waveform models for neutron star binaries: tidal and self-spin effects

The combined observation of gravitational and electromagnetic waves from the coalescence of two neutron stars marks the beginning of multi-messenger astronomy with gravitational waves (GWs). The development of accurate gravitational waveform models is a crucial prerequisite to extract information about the properties of the binary system that generated a detected GW signal. In binary neutron star systems (BNS), tidal effects also need to be incorporated in the modeling for an accurate waveform representation. Building on previous work [Phys.Rev.D96 121501], we explore the performance of inspiral-merger waveform models that are obtained by adding a numerical relativity (NR) based approximant for the tidal part of the phasing (NRTidal) to existing models for nonprecessing and precessing binary black hole systems (SEOBNRv4, PhenomD and PhenomPv2), as implemented in the LSC Algorithm Library Suite. The resulting BNS waveforms are compared and contrasted to target waveforms hybridizing NR waveforms, covering the last approx. 10 orbits up to merger and extending through the postmerger phase, with inspiral waveforms calculated from 30Hz obtained with TEOBResumS. The latter is a state-of-the-art effective-one-body waveform model that blends together tidal and spin effects. We probe that the combination of the PN-based self-spin terms and of the NRTidal description is necessary to obtain minimal mismatches (< 0.01) and phase differences (< 1 rad) with respect to the target waveforms. However, we also discuss possible improvements and drawbacks of the NRTidal approximant in its current form, since we find that it tends to overestimate the tidal interaction with respect to the TEOBResumS model during the inspiral.

gr-qc↗

arXiv2018

Parameterized tests of the strong-field dynamics of general relativity using gravitational wave signals from coalescing binary black holes: Fast likelihood calculations and sensitivity of the method

Thanks to the recent discoveries of gravitational wave signals from binary black hole mergers by Advanced Laser Interferometer Gravitational Wave Observatory and Advanced Virgo, the genuinely strong-field dynamics of spacetime can now be probed, allowing for stringent tests of general relativity (GR). One set of tests consists of allowing for parametrized deformations away from GR in the template waveform models and then constraining the size of the deviations, as was done for the detected signals in previous work. In this paper, we construct reduced-order quadratures so as to speed up likelihood calculations for parameter estimation on future events. Next, we explicitly demonstrate the robustness of the parametrized tests by showing that they will correctly indicate consistency with GR if the theory is valid. We also check to what extent deviations from GR can be constrained as information from an increasing number of detections is combined. Finally, we evaluate the sensitivity of the method to possible violations of GR.

gr-qc↗

arXiv2017

No tension between assembly models of supermassive black hole binaries and pulsar observations

Pulsar timing arrays (PTAs) are presently the only means to search for the gravitational wave stochastic background from supermassive black hole binary populations, considered to be within the grasp of current or near future observations. However, the stringent upperlimit set by the Parkes PTA (Shannon et al. 2013, 2015) has been interpreted as excluding at $> 90\%$ confidence the current paradigm of binary assembly through galaxy mergers and hardening via stellar interactions, suggesting evolution is accelerated (by stars and/or gas) or stalled. Using Bayesian hierarchical modelling, we consider implications of this upperlimit for a comprehensive range of astrophysical scenarios, without invoking stalling nor more exotic physical processes. We find they are fully consistent with the upperlimit, but (weak) bounds on population parameters can be inferred. Bayes factors between models vary between $\approx 1.03$ -- $5.81$ and Kullback-Leibler divergences between characteristic amplitude prior and posterior lie between $0.37$ -- $0.85$. Considering prior astrophysical information on galaxy merger rates, recent upwards revisions of the black hole-galaxy bulge mass relation (Kormendy & Ho 2013) are disfavoured at $1.6σ$ against lighter models (e.g. Shankar et al. 2016). We also show, if no detection is achieved once sensitivity improves by an order of magnitude, the most optimistic scenario is disfavoured at $3.9σ$.

astro-ph.GA↗

arXiv2017

Efficient computation of the gravitational wave spectrum emitted by eccentric massive black hole binaries in stellar environments

We present a fast and versatile method to calculate the characteristic spectrum $h_c$ of the gravitational wave background (GWB) emitted by a population of eccentric massive black hole binaries (MBHBs). We fit the spectrum of a reference MBHB with a simple analytic function and show that the spectrum of any other MBHB can be derived from this reference spectrum via simple scalings of mass, redshift and frequency. We then apply our calculation to a realistic population of MBHBs evolving via 3-body scattering of stars in galactic nuclei. We demonstrate that our analytic prescription satisfactorily describes the signal in the frequency band relevant to pulsar timing array (PTA) observations. Finally we model the high frequency steepening of the GWB to provide a complete description of the features characterizing the spectrum. For typical stellar distributions observed in massive galaxies, our calculation shows that 3-body scattering alone is unlikely to affect the GWB in the PTA band and a low frequency turnover in the spectrum is caused primarily by high eccentricities.

astro-ph.CO↗

arXiv2017

Probing the assembly history and dynamical evolution of massive black hole binaries with pulsar timing arrays

We consider the inverse problem in pulsar timing array (PTA) analysis, investigating what astrophysical information about the underlying massive black hole binary (MBHB) population can be recovered from the detection of a stochastic gravitational wave background (GWB). We employ a physically motivated model that connects the GWB spectrum to a series of parameters describing the underlying redshift evolution of the MBHB mass function and to the typical eccentricity they acquire while interacting with the dense environment of post merger galactic nuclei. This allows the folding in of information about the spectral shape of the GWB into the analysis. The priors on the model parameters are assumed to be uninformative and consistent with the current lack of secure observations of sub-parsec MBHBs. We explore the implications of current upper limits as well as of future detections with a variety of PTA configurations. We confirm our previous finding that current upper limits can only place an upper bound on the overall MBHB merger rate. Depending on the properties of the array, future detections can also constrain several MBHB population models at different degrees of fidelity. In particular, a simultaneous detection of a steepening of the spectrum at high frequency and a bending at low frequency will place strong constraints on both the MBHB mass function and on the typical eccentricity of inspiralling MBHBs, providing insights on MBHB astrophysics unlikely to be achievable by any other means.

astro-ph.HE↗

arXiv2017

Probing Extreme-Density Matter with Gravitational Wave Observations of Binary Neutron Star Merger Remnants

We present a proof-of-concept study, based on numerical-relativity simulations, of how gravitational waves (GWs) from neutron star merger remnants can probe the nature of matter at extreme densities. Phase transitions and extra degrees of freedom can emerge at densities beyond those reached during the inspiral, and typically result in a softening of the equation of state (EOS). We show that such physical effects change the qualitative dynamics of the remnant evolution, but they are not identifiable as a signature in the GW frequency, with the exception of possible black-hole formation effects. The EOS softening is, instead, encoded in the GW luminosity and phase and is in principle detectable up to distances of the order of several Mpcs with advanced detectors and up to hundreds of Mpcs with third generation detectors. Probing extreme-density matter will require going beyond the current paradigm and developing a more holistic strategy for modeling and analyzing postmerger GW signals.

astro-ph.HE↗

arXiv2017

Stellar binary black holes in the LISA band: a new class of standard sirens

The recent Advanced LIGO detections of coalescing black hole binaries (BHBs) imply a large population of such systems emitting at milli-Hz frequencies, accessible to the Laser Interferometer Space Antenna (LISA). We show that these systems provide a new class of cosmological standard sirens. Direct LISA luminosity distance -$D_l$- measurements, combined with the inhomogeneous redshift -$z$- distribution of possible host galaxies provide an effective way to populate the $D_l-z$ diagram at $z<0.1$, thus allowing a precise local measurement of the Hubble expansion rate. To be effective, the method requires a sufficiently precise LISA distance determination and sky localization of a sizeable number of BHBs, which is best achieved for a 6-link detector configuration. We find that, for a BHB population consistent with current fiducial LIGO rates, the Hubble constant $H_0$ can be determined at the $\sim$5% and $\sim$2% level (68% confidence) assuming two and five million Km arm-length respectively.

astro-ph.CO↗

arXiv2017

An analytic family of post-merger template waveforms

Building on the analytical description of the post-merger (ringdown) waveform of coalescing, non-precessing, spinning, (BBHs) introduced in Phys. Rev. D 90, 024054 (2014), we propose an analytic, closed form, time-domain, representation of the $\ell=m=2$ gravitational radiation mode emitted after merger. This expression is given as a function of the component masses and dimensionless spins $(m_{1,2},χ_{1,2})$ of the two inspiralling objects, as well as of the mass $M_{\rm BH}$ and (complex) frequency $σ_{1}$ of the fundamental quasi-normal mode of the remnant black hole. Our proposed template is obtained by fitting the post-merger waveform part of several publicly available numerical relativity simulations from the Simulating eXtreme Spacetimes (SXS) catalog and then suitably interpolating over (symmetric) mass ratio and spins. We show that this analytic expression accurately reproduces ($\sim$~0.01 rad) the phasing of the post-merger data of other datasets not used in its construction. This is notably the case of the spin-aligned run SXS:BBH:0305, whose intrinsic parameters are consistent with the 90\% credible intervals reported by the parameter-estimation followup of GW150914 in Phys. Rev. Lett. 116 (2016) no.24, 241102. Using SXS waveforms as "experimental" data, we further show that our template could be used on the actual GW150914 data to perform a new measure the complex frequency of the fundamental quasi-normal mode so to exploit the complete (high signal-to-noise-ratio) post-merger waveform.

gr-qc↗

arXiv2017

Cosmological inference using only gravitational wave observations of binary neutron stars

[Abridged] This study presents the first Bayesian investigation of the accuracy with which the cosmological parameters can be measured using information coming \emph{only} from the gravitational wave observations of binary neutron star systems by Einstein Telescope. We find, by direct simulation of $10^3$ detections of binary neutron stars, that, within our simplifying assumptions, $H_0,Ω_m,Ω_Λ,w_0$ and $w_1$ can be measured at the $95\%$ level with an accuracy of $\sim 8\%,65\%,39\%,80\%$ and $90\%$, respectively. We also find, by extrapolation, that a measurement accuracy comparable with current measurements by Planck is possible if the number of gravitational wave events observed is $O(10^{6-7})$. We conclude that, while not competitive with electro-magnetic missions in terms of significant digits, gravitational wave alone are capable of providing a complementary determination of the dynamics of the Universe.

gr-qc↗