arXiv subjects

Sarp Akcay

Publications and source records attributed to Sarp Akcay.

At least 19 recordsLinked to original sources

arXiv2024

Incorporation of model accuracy in gravitational wave Bayesian inference

Inferring the properties of colliding black holes from gravitational-wave observations is subject to systematic errors arising from modelling uncertainties. Although the accuracy of each model can be calculated through comparison to theoretical expectations from general relativity, Bayesian analyses are yet to incorporate this information. As such, a mixture model is typically used where results obtained with different gravitational-wave models are combined with either equal weight, or based on their relative Bayesian evidence. In this work we present a novel method to incorporate the accuracy of multiple models in gravitational-wave Bayesian analyses. By analysing simulated gravitational-wave signals in zero-noise, we show that our technique uses $30\%$ less computational resources, and more faithfully recovers the true parameters than existing techniques. We further apply our method to a real gravitational-wave signal and, when assuming the binary black hole hypothesis, demonstrate that the source of GW191109_010717 has unequal component masses, with the primary having a $69\%$ probability that it lies above the maximum black hole mass from stellar collapse. We envisage that this method will become an essential tool within ground-based gravitational-wave astronomy.

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arXiv2024

A Survey of Four Precessing Waveform Models for Binary Black Hole Systems

Angular momentum and spin precession are expected to be generic features of a significant fraction of binary black hole systems. As such, it is essential to have waveform models that faithfully incorporate the effects of precession. Here, we assess how well the current state of the art models achieve this for waveform strains constructed only from the $\ell=2$ multipoles.Specifically, we conduct a survey on the faithfulness of the waveform models %(approximants) \texttt{SEOBNRv5PHM}, \texttt{TEOBResumS}, \texttt{IMRPhenomTPHM}, \texttt{IMRPhenomXPHM} to the numerical relativity (NR) surrogate \texttt{NRSur7dq4} and to NR waveforms from the \texttt{SXS} catalog. The former assessment involves systems with mass ratios up to six and dimensionless spins up to 0.8. The latter employs $317$ short and $23$ long \texttt{SXS} waveforms. For all cases, we use reference inclinations of zero and $90^\circ$. We find that all four models become more faithful as the mass ratio approaches unity and when the merger-ringdown portion of the waveforms are excluded. We also uncover a correlation between the co-precessing $(2,\pm2)$ multipole mismatches and the overall strain mismatch. We additionally find that for high inclinations, precessing $(2,\pm 1)$ multipoles that are more faithful than their $(2,\pm2)$ counterparts, and comparable in magnitude, improve waveform faithfulness. As a side note, we show that use of uniformly-filled parameter spaces may lead to an overestimation of precessing model faithfulness. We conclude our survey with a parameter estimation study in which we inject two precessing \texttt{SXS} waveforms (at low and high masses) and recover the signal with \texttt{SEOBNRv5PHM}, \texttt{IMRPhenomTPHM} and \texttt{IMRPhenomXPHM}. As a bonus, we present preliminary multidimensional fits to model unfaithfulness for Bayesian model selection in parameter estimation studies.

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arXiv2021

A hybrid post-Newtonian -- effective-one-body scheme for spin-precessing compact-binary waveforms

We introduce \texttt{TEOBResumSP}: an efficient, accurate hybrid scheme for generating gravitational waveforms from spin-precessing compact binaries. The precessing waveforms are generated via the established technique of Euler rotating the non-precessing \texttt{TEOBResumS} waveforms from a precessing frame to an inertial frame. We obtain the Euler angles by solving the post-Newtonian precession equations expanded to second post-Newtonian order. Current version of \texttt{TEOBResumSP} produces precessing waveforms through the inspiral phase up to the onset of the merger. We compare \texttt{TEOBResumSP} to current state-of-the-art precessing approximants \texttt{NRSur7dq4}, \texttt{SEOBNRv4PHM}, and \texttt{IMRPhenomPv3HM} for 200 cases of precessing compact binary inspirals with orbital inclinations up to 90 degrees, mass ratios up to four, and the effective precession parameter $χ_p$ up to 0.75. We further provide an extended comparison with \texttt{SEOBNRv4PHM} involving 1030 more inspirals with $χ_p\le 1$ and mass ratios up to 10. We find that 91\% of the \texttt{TEOBResumSP}-\texttt{NRSur7dq4} matches, 85\% of the \texttt{TEOBResumSP}-\texttt{SEOBNRv4PHM} matches, and 77\% of the \texttt{TEOBResumSP}-\texttt{IMRPhenomPv3HM} matches are greater than $0.965$. Most disagreements occur for large mass ratios and $χ_p \gtrsim 0.6$. We identify the mismatch of the \emph{non}-precessing $(2,1)$ mode as one of the leading causes of disagreements. We also introduce a new parameter, $χ_{\perp,\text{max}}$, to measure the strength of precession and hint that the mismatch between the above approximants shows an exponential dependence on $χ_{\perp,\text{max}}$ though this requires further study. Our results indicate that \texttt{TEOBResumSP} is on its way to becoming a robust precessing approximant to be employed in the parameter estimation of generic-spin compact binaries.

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arXiv2020

Dissipation in extreme-mass ratio binaries with a spinning secondary

We present the gravitational-wave flux balance law in an extreme mass-ratio binary with a spinning secondary. This law relates the flux of energy (angular momentum) radiated to null infinity and through the event horizon to the local change in the secondary's orbital energy (angular momentum) for generic (non-resonant) bound orbits in Kerr spacetime. As an explicit example we compute these quantities for a spin-aligned body moving on a circular orbit around a Schwarzschild black hole. We perform this calculation both analytically, via a high-order post-Newtonian expansion, and numerically in two different gauges. Using these results we demonstrate explicitly that our new balance law holds.

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arXiv2018

Nonlinear-in-spin effects in effective-one-body waveform models of spin-aligned, inspiralling, neutron star binaries

Spinning neutron stars acquire a quadrupole moment due to their own rotation. This quadratic-in-spin, self-spin effect depends on the equation of state (EOS) and affects the orbital motion and rate of inspiral of neutron star binaries. We incorporate the EOS-dependent self-spin (or monopole-quadrupole) terms in the spin-aligned effective-one-body (EOB) waveform model TEOBResumS at next-to-next-to-leading (NNLO) order, together with other (bilinear, cubic and quartic) nonlinear-in-spin effects (at leading order, LO). The structure of the Hamiltonian of TEOBResumS is such that it already incorporates, in the binary black hole case, the recently computed quartic-in-spin LO term. Using the gauge-invariant characterization of the phasing provided by the function $Q_ω=ω^2/\dotω$ of $ω=2πf$ , where $f$ is the gravitational wave frequency, we study the EOS dependence of the self-spin effects and show that: (i) the next-to-leading order (NLO) and NNLO monopole-quadrupole corrections yield increasingly phase-accelerating effects compared to the corresponding LO contribution; (ii) the standard TaylorF2 post-Newtonian (PN) treatment of NLO (3PN) EOS-dependent self-spin effects makes their action stronger than the corresponding EOB description; (iii) the addition to the standard 3PN TaylorF2 post-Newtonian phasing description of self-spin tail effects at LO allows one to reconcile the self-spin part of the TaylorF2 PN phasing with the corresponding TEOBResumS one up to dimensionless frequencies $Mω\simeq 0.04-0.06$. By generating the inspiral dynamics using the post-adiabatic approximation, incorporated in a new implementation of TEOBResumS, one finds that the computational time needed to obtain a typical waveform (including all multipoles up to $\ell=8$) from 10 Hz is of the order of 0.4 sec.

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arXiv2018

A crystal ball for kilonovae

The discovery of gravitational waves from merging compact objects has opened up a new window to the Universe. Planned third-generation gravitational-wave detectors such as Einstein Telescope will potentially deliver hundreds of such events at redshifts below $z\sim0.1$. Finding electromagnetic counterparts to these events will be a major observational challenge. We demonstrate how Einstein Telescope will provide advance warning of such events on a timescale of hours, based on the low frequency emission from the pre-merger system. In addition, we suggest how this early warning enables prompt identification of any electromagnetic counterpart using the Large Synoptic Survey Telescope.

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arXiv2018

Forecasting Gamma-Ray Bursts using Gravitational Waves

We explore the intriguing possibility of employing future ground-based gravitational-wave interferometers to detect the inspiral of binary neutron stars sufficiently early to alert electromagnetic observatories so that a gamma-ray burst (GRB) can be observed in its entirety from its very beginning. We quantify the ability to predict a GRB by computing the time a binary neutron star (BNS) system takes to inspiral from its moment of detection to its final merger. We define the moment of detection to be the instant at which the interferometer network accumulates a signal-to-noise ratio of 15. For our computations, we specifically consider BNS systems at luminosity distances of (i) $D\le200\,$Mpc for the three-interferometer Advanced-LIGO-Virgo network of 2020, and (ii) $D\le 1000\,$Mpc for Einstein Telescope's B and C configurations. In the case of Advanced LIGO-Virgo we find that we may at best get a few minutes of warning time, thus we expect no forecast of GRBs in the 2020s. On the other hand, Einstein Telescope will provide us with advance warning times of more than five hours for $D\le 100$Mpc. Taking one hour as a benchmark advance warning time, we obtain a corresponding range of roughly 600 Mpc for the Einstein Telescope C configuration. Using current BNS merger event rates within this volume, we show that Einstein C will forecast $\gtrsim\mathcal{O}(10^2)$ GRBs in the 2030s. We reapply our warning-time computation to black hole - neutron star inspirals and find one to three tidal disruption events to be forecast by the same detector. This is a pedagogical introduction to gravitational-wave astronomy written at a level accessible to PhD students, advanced undergraduates, and colleagues in astronomy/astrophysics who wish to learn more about the underlying physics. Though many of our results may be known to the experts, they might nonetheless find this article motivating and exciting.

astro-ph.HE↗

arXiv2018

Effective-one-body multipolar waveform for tidally interacting binary neutron stars up to merger

Gravitational-wave astronomy with coalescing binary neutron star sources requires the availability of gravitational waveforms with tidal effects accurate up to merger. This article presents an improved version of \TEOBResum, a nonspinning effective-one-body (EOB) waveform model with enhanced analytical information in its tidal sector. The tidal potential governing the conservative dynamics employs resummed expressions based on post-Newtonian (PN) and gravitational self-force (GSF) information. In particular, we compute a GSF-resummed expression for the leading-order octupolar gravitoelectric term and incorporate the leading-order gravitomagnetic term (either in PN-expanded or GSF-resummed form). The multipolar waveform and fluxes are augmented with gravitoelectric and magnetic terms recently obtained in PN. The new analytical information enhances tidal effects toward merger accelerating the coalescence. We quantify the impact on the gravitational-wave phasing of each physical effect. The most important contribution is given by the resummed gravitoelectric octupolar term entering the EOB interaction potential, that can yield up to 1~rad of dephasing (depending on the NS model) with respect to its nonresummed version. The model's energetics and the gravitational wave phasing are validated with eccentricity-reduced and multi-resolution numerical relativity simulations with different equations of state and mass ratios. We also present EOB-NR waveform comparisons for higher multipolar modes beyond the dominant quadrupole one.

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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.

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arXiv2017

Self-force correction to geodetic spin precession in Kerr spacetime

We present an expression for the gravitational self-force correction to the geodetic spin precession of a spinning compact object with small, but non-negligible mass in a bound, equatorial orbit around a Kerr black hole. We consider only conservative back-reaction effects due to the mass of the compact object ($m_1$) thus neglecting the effects of its spin $s_1$ on its motion, i.e, we impose $s_1 \ll G m_1^2/c$ and $m_1 \ll m_2$, where $m_2$ is the mass parameter of the background Kerr spacetime. We encapsulate the correction to the spin precession in $ψ$, the ratio of the accumulated spin-precession angle to the total azimuthal angle over one radial orbit in the equatorial plane. Our formulation considers the gauge-invariant $\ord(m_1)$ part of the correction to $ψ$, denoted by $Δψ$, and is a generalization of the results of [Class. Quan. Grav., 34, 084001 (2017)] to Kerr spacetime. Additionally, we compute the zero-eccentricity limit of $Δψ$ and show that this quantity differs from the circular-orbit $Δψ^\text{circ}$ by a gauge-invariant quantity containing the gravitational self-force correction to general relativistic periapsis advance in Kerr spacetime. Our result for $Δψ$ is expressed in a manner that readily accommodates numerical/analytical self-force computations, e.g., in radiation gauge, and paves the way for the computation of a new eccentric-orbit Kerr gauge invariant beyond the generalized redshift.

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arXiv2017

Spin-orbit precession for eccentric black hole binaries at first order in the mass ratio

We consider spin-orbit ("geodetic") precession for a compact binary in strong-field gravity. Specifically, we compute $ψ$, the ratio of the accumulated spin-precession and orbital angles over one radial period, for a spinning compact body of mass $m_1$ and spin $s_1$, with $s_1 \ll G m_1^2/c$, orbiting a non-rotating black hole. We show that $ψ$ can be computed for eccentric orbits in both the gravitational self-force and post-Newtonian frameworks, and that the results appear to be consistent. We present a post-Newtonian expansion for $ψ$ at next-to-next-to-leading order, and a Lorenz-gauge gravitational self-force calculation for $ψ$ at first order in the mass ratio. The latter provides new numerical data in the strong-field regime to inform the Effective One-Body model of the gravitational two-body problem. We conclude that $ψ$ complements the Detweiler redshift $z$ as a key invariant quantity characterizing eccentric orbits in the gravitational two-body problem.

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arXiv2016

Numerical computation of the EOB potential q using self-force results

The effective-one-body theory (EOB) describes the conservative dynamics of compact binary systems in terms of an effective Hamiltonian approach. The Hamiltonian for moderately eccentric motion of two non-spinning compact objects in the extreme mass-ratio limit is given in terms of three potentials: $a(v), \bar{d}(v), q(v)$. By generalizing the first law of mechanics for (non-spinning) black hole binaries to eccentric orbits, [\prd{\bf92}, 084021 (2015)] recently obtained new expressions for $\bar{d}(v)$ and $q(v)$ in terms of quantities that can be readily computed using the gravitational self-force approach. Using these expressions we present a new computation of the EOB potential $q(v)$ by combining results from two independent numerical self-force codes. We determine $q(v)$ for inverse binary separations in the range $1/1200 \le v \lesssim 1/6$. Our computation thus provides the first-ever strong-field results for $q(v)$. We also obtain $\bar{d}(v)$ in our entire domain to a fractional accuracy of $\gtrsim 10^{-8}$. We find to our results are compatible with the known post-Newtonian expansions for $\bar{d}(v)$ and $q(v)$ in the weak field, and agree with previous (less accurate) numerical results for $\bar{d}(v)$ in the strong field.

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arXiv2015

Comparison Between Self-Force and Post-Newtonian Dynamics: Beyond Circular Orbits

The gravitational self-force (GSF) and post-Newtonian (PN) schemes are complementary approximation methods for modelling the dynamics of compact binary systems. Comparison of their results in an overlapping domain of validity provides a crucial test for both methods, and can be used to enhance their accuracy, e.g. via the determination of previously unknown PN parameters. Here, for the first time, we extend such comparisons to noncircular orbits---specifically, to a system of two nonspinning objects in a bound (eccentric) orbit. To enable the comparison we use a certain orbital-averaged quantity $\langle U \rangle$ that generalizes Detweiler's redshift invariant. The functional relationship $\langle U \rangle(Ω_r,Ω_ϕ)$, where $Ω_r$ and $Ω_ϕ$ are the frequencies of the radial and azimuthal motions, is an invariant characteristic of the conservative dynamics. We compute $\langle U \rangle(Ω_r,Ω_ϕ)$ numerically through linear order in the mass ratio $q$, using a GSF code which is based on a frequency-domain treatment of the linearized Einstein equations in the Lorenz gauge. We also derive $\langle U \rangle(Ω_r,Ω_ϕ)$ analytically through 3PN order, for an arbitrary $q$, using the known near-zone 3PN metric and the generalized quasi-Keplerian representation of the motion. We demonstrate that the $\mathcal{O}(q)$ piece of the analytical PN prediction is perfectly consistent with the numerical GSF results, and we use the latter to estimate yet unknown pieces of the 4PN expression at $\mathcal{O}(q)$.

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arXiv2013

Evolution of inspiral orbits around a Schwarzschild black hole

We present results from calculations of the orbital evolution in eccentric binaries of nonrotating black holes with extreme mass-ratios. Our inspiral model is based on the method of osculating geodesics, and is the first to incorporate the full gravitational self-force (GSF) effect, including conservative corrections. The GSF information is encapsulated in an analytic interpolation formula based on numerical GSF data for over a thousand sample geodesic orbits. We assess the importance of including conservative GSF corrections in waveform models for gravitational-wave searches.

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arXiv2013

Frequency-domain algorithm for the Lorenz-gauge gravitational self-force

State-of-the-art computations of the gravitational self-force (GSF) on massive particles in black hole spacetimes involve numerical evolution of the metric perturbation equations in the time-domain, which is computationally very costly. We present here a new strategy, based on a frequency-domain treatment of the perturbation equations, which offers considerable computational saving. The essential ingredients of our method are (i) a Fourier-harmonic decomposition of the Lorenz-gauge metric perturbation equations and a numerical solution of the resulting coupled set of ordinary equations with suitable boundary conditions; (ii) a generalized version of the method of extended homogeneous solutions [Phys. Rev. D {\bf 78}, 084021 (2008)] used to circumvent the Gibbs phenomenon that would otherwise hamper the convergence of the Fourier mode-sum at the particle's location; and (iii) standard mode-sum regularization, which finally yields the physical GSF as a sum over regularized modal contributions. We present a working code that implements this strategy to calculate the Lorenz-gauge GSF along eccentric geodesic orbits around a Schwarzschild black hole. The code is far more efficient than existing time-domain methods; the gain in computation speed (at a given precision) is about an order of magnitude at an eccentricity of 0.2, and up to three orders of magnitude for circular or nearly circular orbits. This increased efficiency was crucial in enabling the recently reported calculation of the long-term orbital evolution of an extreme mass ratio inspiral [Phys. Rev. D {\bf 85}, 061501(R) (2012)]. Here we provide full technical details of our method to complement the above report.

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arXiv2012

Critical phenomena at the threshold of immediate merger in binary black hole systems: the extreme mass ratio case

In numerical simulations of black hole binaries, Pretorius and Khurana [Class. Quant. Grav. {\bf 24}, S83 (2007)] have observed critical behaviour at the threshold between scattering and immediate merger. The number of orbits scales as $n\simeq -γ\ln|p-p_*|$ along any one-parameter family of initial data such that the threshold is at $p=p_*$. Hence they conjecture that in ultrarelavistic collisions almost all the kinetic energy can be converted into gravitational waves if the impact parameter is fine-tuned to the threshold. As a toy model for the binary, they consider the geodesic motion of a test particle in a Kerr black hole spacetime, where the unstable circular geodesics play the role of critical solutions, and calculate the critical exponent $γ$. Here, we incorporate radiation reaction into this model using the self-force approximation. The critical solution now evolves adiabatically along a sequence of unstable circular geodesic orbits under the effect of the self-force. We confirm that almost all the initial energy and angular momentum are radiated on the critical solution. Our calculation suggests that, even for infinite initial energy, this happens over a finite number of orbits given by $n_\infty\simeq 0.41/η$, where $η$ is the (small) mass ratio. We derive expressions for the time spent on the critical solution, number of orbits and radiated energy as functions of the initial energy and impact parameter.

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arXiv2012

Gravitational self-force and the effective-one-body formalism between the innermost stable circular orbit and the light ring

We compute the conservative piece of the gravitational self-force (GSF) acting on a particle of mass m_1 as it moves along an (unstable) circular geodesic orbit between the innermost stable circular orbit (ISCO) and the light ring of a Schwarzschild black hole of mass m_2>> m_1. More precisely, we construct the function h_{uu}(x) = h_{μν} u^μ u^ν (related to Detweiler's gauge-invariant "redshift" variable), where h_{μν} is the regularized metric perturbation in the Lorenz gauge, u^μ is the four-velocity of m_1, and x= [Gc^{-3}(m_1+m_2)Ω]^{2/3} is an invariant coordinate constructed from the orbital frequency Ω. In particular, we explore the behavior of h_{uu} just outside the "light ring" at x=1/3, where the circular orbit becomes null. Using the recently discovered link between h_{uu} and the piece a(u), linear in the symmetric mass ratio ν, of the main radial potential A(u,ν) of the Effective One Body (EOB) formalism, we compute a(u) over the entire domain 0<u<1/3. We find that a(u) diverges at the light-ring as ~0.25 (1-3u)^{-1/2}, explain the physical origin of this divergence, and discuss its consequences for the EOB formalism. We construct accurate global analytic fits for a(u), valid on the entire domain 0<u<1/3 (and possibly beyond), and give accurate numerical estimates of the values of a(u) and its first 3 derivatives at the ISCO, as well as the O(ν) shift in the ISCO frequency. In previous work we used GSF data on slightly eccentric orbits to compute a certain linear combination of a(u) and its first two derivatives, involving also the O(ν) piece \bar d(u) of a second EOB radial potential {\bar D}(u,ν). Combining these results with our present global analytic representation of a(u), we numerically compute {\bar d}(u)$ on the interval 0<u\leq 1/6.

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arXiv2012

A Fast Frequency-Domain Algorithm for Gravitational Self-Force: I, Circular Orbits in Schwarzschild Spacetime

Fast, reliable orbital evolutions of compact objects around massive black holes will be needed as input for gravitational wave search algorithms in the data stream generated by the planned Laser Interferometer Space Antenna (LISA). Currently, the state of the art is a time-domain code by [Phys. Rev. D{\bf 81}, 084021, (2010)] that computes the gravitational self-force on a point-particle in an eccentric orbit around a Schwarzschild black hole. Currently, time-domain codes take up to a few days to compute just one point in parameter space. In a series of articles, we advocate the use of a frequency-domain approach to the problem of gravitational self-force (GSF) with the ultimate goal of orbital evolution in mind. Here, we compute the GSF for a particle in a circular orbit in Schwarzschild spacetime. We solve the linearized Einstein equations for the metric perturbation in Lorenz gauge. Our frequency-domain code reproduces the time-domain results for the GSF up to $\sim 1000$ times faster for small orbital radii. In forthcoming companion papers, we will generalize our frequency-domain methods to include bound (eccentric) orbits in Schwarzschild and (eventually) Kerr spacetimes for computing the GSF, where we will employ the method of extended homogeneous solutions [Phys. Rev. D {\bf 78}, 084021 (2008)].

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