arXiv subjects

Simone Albanesi

Publications and source records attributed to Simone Albanesi.

29 records · Page 2Linked to original sources

arXiv2023

GR-Athena++: General-relativistic magnetohydrodynamics simulations of neutron star spacetimes

We present the extension of GR-Athena++ to general-relativistic magnetohydrodynamics (GRMHD) for applications to neutron star spacetimes. The new solver couples the constrained transport implementation of Athena++ to the Z4c formulation of the Einstein equations to simulate dynamical spacetimes with GRMHD using oct-tree adaptive mesh refinement. We consider benchmark problems for isolated and binary neutron star spacetimes demonstrating stable and convergent results at relatively low resolutions and without grid symmetries imposed. The code correctly captures magnetic field instabilities in non-rotating stars with total relative violation of the divergence-free constraint of $10^{-16}$. It handles evolutions with a microphysical equation of state and black hole formation in the gravitational collapse of a rapidly rotating star. For binaries, we demonstrate correctness of the evolution under the gravitational radiation reaction and show convergence of gravitational waveforms. We showcase the use of adaptive mesh refinement to resolve the Kelvin-Helmholtz instability at the collisional interface in a merger of magnetised binary neutron stars. GR-Athena++ shows strong scaling efficiencies above $80\%$ in excess of $10^5$ CPU cores and excellent weak scaling is shown up to $\sim 5 \times 10^5$ CPU cores in a realistic production setup. GR-Athena++ allows for the robust simulation of GRMHD flows in strong and dynamical gravity with exascale computers.

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arXiv2023

Faithful effective-one-body waveform of small-mass-ratio coalescing black hole binaries: the eccentric, nonspinning, case

We present a new effective-one-body (EOB) waveform for eccentric, nonspinning, binaries in the extreme mass ratio limit, with initial eccentricities up to $0.95$. The EOB analytical waveform, that includes noncircular corrections up to second post-Newtonian order, is completed by a phenomenological ringdown model that is informed by Regge-Wheeler-Zerilli (RWZ) type waveforms generated by a point-particle source. This model notably includes the beating between positive and negative frequency quasi-normal-modes (QNMs). We analyze various prescriptions to faithfully complete the analytical EOB waveform in the transition from plunge to merger. In particular, we systematically explore the effect of: (i) the generic Newtonian prefactor; (ii) next-to-quasi-circular (NQC) corrections to amplitude and phase; (iii) the point were NQC corrections are determined; (iv) the ringdown attachment point. This yields EOB/RWZ quadrupolar phase differences through merger and ringdown $\lesssim 0.01$~rad for the quasi-circular case and $\lesssim 0.05$~rad for the eccentric case. Higher modes are also modeled up to the $\ell=m=5$ multipole. We finally discuss the excitation of the QNMs and present a heuristic model to motivate it in correlation with the presence of a point-particle source.

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arXiv2023

Towards numerical-relativity informed effective-one-body waveforms for dynamical capture black hole binaries

Dynamical captures of black holes may take place in dense stellar media due to the emission of gravitational radiation during a close passage. Detection of such events requires detailed modelling, since their phenomenology qualitatively differs from that of quasi-circular binaries. Very few models can deliver such waveforms, and none includes information from Numerical Relativity (NR) simulations of non quasi-circular coalescences. In this study we present a first step towards a fully NR-informed Effective One Body (EOB) model of dynamical captures. We perform 14 new simulations of single and double encounter mergers, and use this data to inform the merger-ringdown model of the TEOBResumS-Dali approximant. We keep the initial energy approximately fixed to the binary mass, and vary the mass-rescaled, dimensionless angular momentum in the range $(0.6, 1.1)$, the mass ratio in $(1, 2.15)$ and aligned dimensionless spins in $(-0.5, 0.5)$. We find that the model is able to match NR to $97%$, improving previous performances, without the need of modifying the base-line template. Upon NR informing the model, this improves to $99%$ with the exception of one outlier corresponding to a direct plunge. The maximum EOBNR phase difference at merger for the uninformed model is of $0.15$ radians, which is reduced to $0.1$ radians after the NR information is introduced. We outline the steps towards a fully informed EOB model of dynamical captures, and discuss future improvements.

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arXiv2022

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.

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arXiv2022

New Avenue for Accurate Analytical Waveforms and Fluxes for Eccentric Compact Binaries

We introduce a new paradigm for constructing accurate analytic waveforms (and fluxes) for eccentric compact binaries. Our recipe builds on the standard Post-Newtonian (PN) approach but (i) retains implicit time-derivatives of the phase space variables in the instantaneous part of the noncircular waveform, and then (ii) suitably factorizes and resums this partly PN-implicit waveform using effective-one-body (EOB) procedures. We test our prescription against the exact results obtained by solving the Teukolsky equation with a test-mass source orbiting a Kerr black hole, and compare the use of the exact vs PN equations of motion for the time derivatives computation. Focusing only on the quadrupole contribution, we find that the use of the exact equations of motion yields an analytical/numerical agreement of the (averaged) angular momentum fluxes that is improved by $40\%$ with respect to previous work, with $4.5\%$ fractional difference for eccentricity $e=0.9$ and black hole dimensionless spin $-0.9\leq \hat{a}\leq +0.9$. We also find a remarkable convergence trend between Newtonian, 1PN and 2PN results. Our approach carries over to the comparable mass case using the resummed EOB equations of motion and paves the way to faithful EOB-based waveform model for long-inspiral eccentric binaries for current and future gravitational wave detectors.

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arXiv2022

Towards a gravitational self force-informed effective-one-body waveform model for nonprecessing, eccentric, large-mass-ratio inspirals

Building upon several recent advances in the development of effective-one-body models for spin-aligned eccentric binaries with individual masses $(m_1,m_2)$ we introduce a new EOB waveform model that aims at describing inspiralling binaries in the large mass-ratio regime, $m_1\gg m_2$. The model exploits the current state-of-the-art TEOBResumS-Dali model for eccentric binaries, but the standard EOB potentials $(A,\bar{D},Q)$, informed by Numerical Relativity (NR) simulations, are replaced with the corresponding functions that are linear in the symmetric mass ratio $ν\equiv m_1 m_2/(m_1+m_2)^2$ taken at 8.5PN accuracy. To improve their strong-field behavior, these functions are: (i) suitably factorized and resummed using Padé approximants and (ii) additionally effectively informed to state-of-the-art numerical results obtained by gravitational self-force theory (GSF). For simplicity, the spin-sector of the model is taken to be the one of TEOBResumS-Dali, though removing the NR-informed spin-orbit effective corrections. We propose the current GSF-informed EOB framework as a conceptually complete analytical tool to generate waveforms for eccentric Extreme (and Intermediate) Mass Ratio Inspirals for future gravitational wave detectors.

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arXiv2022

Fermion-axion stars: static solutions and dynamical stability

We construct spherically-symmetric static solutions of the Einstein-Klein-Gordon-Euler system involving a complex scalar field governed by a periodic potential which emerges in models of axion-like particles, and fermionic matter modeled by a perfect fluid with a polytropic equation of state. Such solutions describe gravitationally bound composites of fermions and axions which we dub as fermion-axion stars. Sequences of pure axion-stars in the existence domain may show the presence of multiple stable branches depending on the value of the decay constant parameter in the potential; this reflects in the appearance of multiple islands of stability in the 2-dimensional parameter space of fermion-axion configurations. We investigate the domain of existence for three different values of the decay constant, identifying one or more regions of linear stability making use of a method we already employed in previous works. We confirm the results from the linear analysis performing fully non-linear numerical relativity evolutions. In this context we perform several numerical simulations to identify regions in the parameter space where unstable models face different fates: the collapse to a Schwarzschild black hole, the migration to a stable model and finally the dispersion of the scalar field together with the dilution of the fermionic matter, which approaches a static fermion star model with very low mass. This latter scenario was never observed in previous models without the periodic potential.

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arXiv2022

Assessment of Effective-One-Body Radiation Reactions for Generic Planar Orbits

In this paper we assess the performances of different analytical prescriptions for the effective-one-body (EOB) radiation reaction along generic planar orbits using exact numerical result in the test-mass limit. We consider three prescriptions put forward in the recent literature: (i) the quasicircular prescription (QC), (ii) the QC with second post-Newtonian (2PN) order noncircular corrections (QC2PN), and (iii) the QC corrected by the noncircular Newtonian prefactor (NCN). The analytical fluxes are then compared against the exact fluxes that are computed by solving the Teukolsky equation with a test-mass source in geodesic motion. We find that the NCN prescription is the most accurate for both eccentric and hyperbolic orbits and it is in robust agreement also for large values of the eccentricity. This result carries over to the comparable masses, as we discuss for a numerical-relativity (NR) case study. We also demonstrate that, while the EOB/NR waveform unfaithfulness is a necessary check for the precision of EOB models, the direct comparison of EOB/NR fluxes is a more stringent and informative test to select the best prescription. Finally, we propose an improved radiation reaction, NCN2PN, that includes noncircular 2PN corrections, in resummed form, as a further multiplicative contribution and that is valid for any mass ratio.

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arXiv2022

Effective one-body model for extreme-mass-ratio spinning binaries on eccentric equatorial orbits: testing radiation reaction and waveform

We provide a systematic analysis of the multipolar gravitational waveform, energy and angular momentum fluxes emitted by a nonspinning test particle orbiting a Kerr black hole along equatorial, eccentric orbits. These quantities are computed by numerically solving the Teukolsky equation in the time domain and are then used to test and improve the radiation reaction (and waveform) of an effective-one-body (EOB) model. Eccentricity is incorporated into EOB by replacing the quasi-circular Newtonian (or leading-order) prefactors in the EOB-factorized multipolar waveform (and fluxes) with their generic counterparts. The comparison between numerical and analytical quantities is carried out over a large portion of the parameter space, notably for orbits close to the separatrix and with high eccentricities. The analytical model agrees to $\sim 1\%$ with the numerical data for orbits with moderate eccentricities ($e\lesssim 0.3$) and moderate spins ($\hat{a}\lesssim 0.5$), although this increases up to $\sim 33\%$ for large, positive, black hole spins ($\sim 0.9$) and large eccentricities ($\sim 0.9$). For moderate eccentricities, the new EOB fluxes can be used to drive the dynamics through the nonadiabatic transition from eccentric inspiral to plunge, merger and ringdown, thus providing accurate an description of the merger. We also show that the same technique can be applied to hyperbolic encounters. In general, our approach to radiation reaction for eccentric inspirals should be seen as a first step toward EOB modelization of extreme-mass-ratio-inspirals waveforms for LISA.

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arXiv2022

Exploiting Newton-factorized, 2PN-accurate, waveform multipoles in effective-one-body models for spin-aligned noncircularized binaries

We present a new approach to factorize and resum the post-Newtonian (PN) waveform for generic equatorial motion to be used within effective-one-body (EOB) based waveform models. The new multipolar waveform factorization improves previous prescriptions in that: (i) the generic Newtonian contribution is factored out from each multipole; (ii) the circular part is factored out and resummed using standard EOB methods and (iii) the residual, 2PN-accurate, noncircular part, and in particular the tail contribution, is additionally resummed using Padé approximants. The resulting waveform is validated in the extreme-mass-ratio limit by comparisons with nine (mostly nonspinning) numerical waveforms either from eccentric inspirals, with eccentricities up to $e=0.9$, or dynamical captures . The resummation of the noncircular tail contribution is found essential to obtain excellent (${\lesssim}0.05$~rad at periastron for $e=0.9$) analytical/numerical agreement and to considerably improve the prescription with just the Newtonian prefactor. In the comparable mass case, the new 2PN waveform shows only a marginal improvement over the previous Newtonian factorization, though yielding maximal unfaithfulness $\simeq 10^{-3}$ with the 28 publicly available numerical relativity simulations with eccentricity up to $\sim 0.3$ (except for a single outlier that grazes $10^{-2}$). We finally use test-particle data to validate the waveform factorization proposed by Khalil et al.~[Phys.~Rev.~104 (2021) 2, 024046] and conclude that its amplitude can be considered reliable (though less accurate, $\sim 6\%$ fractional difference versus $1.5\%$ of our method) only up to eccentricities $\sim 0.3$.

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arXiv2021

Waveforms and fluxes: Towards a self-consistent effective one body waveform model for nonprecessing, coalescing black-hole binaries for third generation detectors

We present a comprehensive comparison between numerical relativity (NR) angular momentum fluxes at infinity and the corresponding quantity entering the radiation reaction in TEOBResumS, an Effective-One-Body (EOB) waveform model for nonprecessing coalescing black hole binaries on quasi-circular orbits. This comparison prompted us to implement two changes in the model: (i) including Next-to-Quasi-Circular corrections in the $\ell=m$, $\ell\leq 5$ multipoles entering the radiation reaction and (ii) consequently updating the NR-informed spin-orbital sector of the model. This yields a new waveform model that presents a higher self-consistency between waveform and dynamics and an improved agreement with NR simulations. We test the model computing the EOB/NR unfaithfulness $\bar{F}_{\rm EOB/NR}$ over all 534 spin-aligned configurations available through the Simulating eXtreme Spacetime catalog, notably using the noise spectral density of Advanced LIGO, Einstein Telescope and Cosmic Explorer, for total mass up to $500M_\odot$. We find that the maximum unfaithfulness $\bar{F}^{\rm max}_{\rm EOB/NR}$ is mostly between $10^{-4}$ and $10^{-3}$, and the performance progressively worsens up to $\sim 5\times 10^{-3}$ as the effective spin of the system is increased. We perform similar analyses on the \SEOB{} model, that delivers $\bar{F}^{\rm max}_{\rm EOB/NR}$ values uniformly distributed versus effective spin and mostly between $10^{-3}$ and $10^{-2}$. We conclude that the improved TEOBResumS model already represents a reliable and robust first step towards the development of highly accurate waveform templates for third generation detectors.

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