arXiv subjects

Santiago Jaraba

Publications and source records attributed to Santiago Jaraba.

11 recordsLinked to original sources

arXiv2026

Astrometric constraints on stochastic gravitational wave background with neural networks

Astrometric measurements provide a unique avenue for constraining the stochastic gravitational wave background (SGWB). In this work, we investigate the application of two neural network architectures, a fully connected network and a graph neural network, for analyzing astrometric data to detect the SGWB. Specifically, we generate mock Gaia astrometric measurements of the proper motions of sources and train two networks to predict the energy density of the SGWB, $Ω_\text{GW}$. We evaluate the performance of both models under varying input datasets to assess their robustness across different configurations. We also perform a direct comparison with a likelihood-based approach using Markov chain Monte Carlo (MCMC) methods, finding out that the neural-network-based approach is significantly faster, taking on the order of minutes, compared to MCMC's order of days, while still capturing the same features in the data. Our results demonstrate that neural networks can effectively constrain the SGWB, showing promise as tools for addressing systematic uncertainties and modeling limitations that pose challenges for traditional likelihood-based methods.

astro-ph.CO↗

arXiv2026

Effects of dynamical capture on two equal-mass nonspinning black holes

Dynamical captures of black holes are unique events that provide an exceptional opportunity to probe the strong-field regime of gravitational physics. In this article, we perform numerical relativity simulations to study the events of dynamical capture of two equal-mass nonspinning black holes. We consider a suite of scenarios within a range of initial linear momenta ($p/M=0.095-0.75$) and incidence angles ($θ=6.36^\circ-2.83^\circ$), and study the emitted Weyl scalar ($Ψ_4$) of each case, as well as the spins and masses of the black holes before and after they merge. We provide a simple analytical model which accurately fits the gravitational-wave emission. We study the dependence of the time interval between the capture and the merger emissions with respect to the incidence angle, which can be well parametrized by a first-order divergent behavior, allowing us to find the angle that separates a scattering event from a dynamical capture. We also find that, in general, the parameters that model the first emission can be well described by linear or exponentially decaying functions in terms of the incidence angle, while others display more complex behaviors that offer valuable insights into the nature of these events.

gr-qc↗

arXiv2026

Numerical simulations of oscillating and differentially rotating neutron stars

The remnants of binary neutron star mergers are expected to be massive, rapidly rotating stars whose oscillations produce gravitational waves in the kilohertz band. The degree of differential rotation and the rotation profiles strongly influence their structure, stability and oscillation spectrum, and must therefore be taken into account when modeling their dynamics. We extend the pseudospectral code ROXAS (Relativistic Oscillations of non-aXisymmetric neutron stArS) to enable the dynamical evolution of oscillating, differentially rotating neutron stars. Using the updated code, we aim to study the star's oscillation frequencies. We extend the previous formalism, based on primitive variables and the conformal flatness approximation, to differential rotation. Within this framework, we run a series of axisymmetric and non-axisymmetric simulations of perturbed, differentially rotating neutron stars with different rotation rates, and extract their oscillation frequencies. Axisymmetric modes, as well as those under the Cowling approximation, show excellent agreement with published results. We show that the secondary fundamental mode in the Cowling approximation is an artifact that does not appear in dynamical spacetimes. In addition, we provide, for the first time, frequency values for non-axisymmetric modes in differentially rotating configurations evolved in conformal flatness. This extension broadens the range of physical scenarios that can be studied with ROXAS, and represents a step toward more realistic modeling of post-merger remnants and their gravitational-wave emission.

gr-qc↗

arXiv2025

First astrometric constraints on parity-violation in the gravitational wave background

Astrometry, the precise measurement of stellar positions and velocities, offers a promising approach to probing the low-frequency stochastic gravitational wave background (SGWB). Notably, astrometric vector sky maps are sensitive to parity-violating SGWB signals, which cannot be distinguished using pulsar timing array observations in an isotropic SGWB. We present the first astrometric constraints on parity-violating SGWB using quasar catalogs from Gaia DR3 and VLBA data. By analyzing the $EB$ correlation in the two-point correlation function of the proper motions of the quasars, we find 2$σ$ constraints on the parity-violating SGWB amplitude $h_{70}^2Ω_{V} = -0.020 \pm 0.025$ from Gaia DR3 and $h_{70}^2Ω_{V} = -0.004 \pm 0.010$ from VLBA. These constraints are valid in the frequency range $4.2 \times 10^{-18}\,{\rm Hz} < f < 1.1 \times 10^{-8}\,{\rm Hz}$. Although not currently a tight constraint on theoretical models, this first attempt lays the groundwork for future investigations using more precise astrometric data.

astro-ph.CO↗

arXiv2024

Spin induction from scattering of two spinning black holes in dense clusters

In this paper, we use numerical relativity to study the spin induction effect within close hyperbolic encounters of initially spinning black holes. We review the initially non-spinning case and explore the cases of initially aligned, anti-aligned and orthogonal spins with respect to the orbital angular momentum $\vec L$. We find that, for a given initial effective spin, the black hole with a smaller initial spin acquires a greater spin-up than the other black hole after the interaction. We study three different scenarios regarding initial effective spin ($χ_{\rm eff}=-0.1, \ 0.0, \ 0.1$), using three different scattering angles in order to obtain maximally spin-inducing scenarios. We also find that the final effective spin-ups with respect to the initial spins are well-fitted by a parabola. For spins orthogonal to $\vec L$, we observe that the black hole spins precess, and that the induced spin in the $z$-direction depends quadratically on the value of the initial spins. These phenomena suggest that dense black hole clusters present a rich spin dynamics, where black hole spins may acquire non-trivial distributions.

gr-qc↗

arXiv2024

A numerical-relativity surrogate model for hyperbolic encounters of black holes: challenges in parameter estimation

We present a surrogate numerical-relativity model for close hyperbolic black-hole encounters with equal masses and spins aligned with the orbital momentum. Our model, generated in terms of the Newman-Penrose scalar $ψ_4$, spans impact parameters $b/M\in [11, 15]$ and spin components $χ_{i} \in [-0.5,0.5]$, modeling the $(\ell,m)=(2,0)$, $(2, \pm 2)$, $(3,\pm 2)$ and $(4,\pm 4)$ emission multipoles. The model is faithful to numerical relativity simulations, yielding mismatches lower than $10^{-3}$. We test the ability of our model to recover the parameters of numerically simulated signals. We find that, despite the high accuracy of the model, parameter inference struggles to correctly capture the parameters of the source even for SNRs as large as 50 due to the strong degeneracies present in the parameter space. This indicates that correctly identifying these systems will require of extremely large signal loudness, only typical of third generation detectors. Nevertheless, we also find that, if one attempts to infer certain combinations of such degenerated parameters, there might be a chance to prove the existence of this type of events, even with the current ground-based detectors, as long as these combinations make sense astrophysically and cosmologically.

gr-qc↗

arXiv2024

Constraints on Non-Gaussian primordial curvature perturbation from the LIGO-Virgo-KAGRA third observing run

The scalar-induced gravitational waves (SIGW), arising from large amplitude primordial density fluctuations, provide a unique observational test for directly probing the epoch of inflation. In this work, we provide constraints on the SIGW background by taking into account the non-Gaussianity in the primordial density fluctuations, using the third observing run (O3) data of the LIGO-Virgo-KAGRA collaboration. We find that the non-Gaussianity gives a non-negligible effect on the GW energy density spectrum and starts to affect the analysis of the O3 data when the non-Gaussianity parameter is $F_{\rm NL} > 3.55$. Furthermore, the constraints exhibit asymptotic behavior given by $F_{\rm NL} A_g = \rm{const.}$ at large $F_{\rm NL}$ limit, where $A_g$ denotes the amplitude of the curvature perturbations. In the limit of large $F_{\rm NL}$, we placed a 95% confidence level upper limit $F_{\rm NL} A_g \leq 0.13, 0.09, 0.10$ at fixed scales of $10^{16}, 10^{16.5}, 10^{17}~{\rm Mpc}^{-1}$, respectively.

astro-ph.CO↗

arXiv2023

Stochastic gravitational wave background constraints from Gaia DR3 astrometry

Astrometric surveys can be used to constrain the stochastic gravitational wave background (SGWB) at very low frequencies. We use proper motion data provided by Gaia DR3 to fit a generic dipole+quadrupole field. We analyse several quasar-based datasets and discuss their purity and idoneity to set constraints on gravitational waves. For the cleanest dataset, we derive an upper bound on the (frequency-integrated) energy density of the SGWB $h_{70}^2Ω_{\rm GW}\lesssim 0.087$ for $4.2\times 10^{-18}~\mathrm{Hz}\lesssim f\lesssim 1.1\times 10^{-8}~\mathrm{Hz}$. We also reanalyse previous VLBI-based data to set the constraint $h_{70}^2Ω_{\rm GW}\lesssim 0.024$ for $5.8\times 10^{-18}~\mathrm{Hz}\lesssim f\lesssim 1.4\times 10^{-9}~\mathrm{Hz}$ under the same formalism, standing as the best astrometric constraint on GWs. Based on our results, we discuss the potential of future Gaia data releases to impose tighter constraints.

astro-ph.CO↗

arXiv2023

Bayesian parameter estimation for targeted anisotropic gravitational-wave background

Extended sources of the stochastic gravitational backgrounds have been conventionally searched on the spherical harmonics bases. The analysis during the previous observing runs by the ground-based gravitational wave detectors, such LIGO and Virgo, have yielded the constraints on the angular power spectrum $C_\ell$, yet it lacks the capability of estimating model parameters. In this paper, we introduce an alternative Bayesian formalism to search for such stochastic signals with a particular distribution of anisotropies on the sky. This approach provides a Bayesian posterior of model parameters and also enables selection tests among different signal models. While the conventional analysis fixes the highest angular scale \textit{a priori}, here we show a more systematic and quantitative way to determine the cut-off scale based on a Bayes factor, which depends on the amplitude and the angular scale of observed signals. Also, we analyze the third observing runs of LIGO and Virgo for the population of milli-second pulsars and obtain the 95 % constrains of the signal amplitude, $ε< 2.7\times 10^{-8}$.

astro-ph.IM↗

arXiv2022

The stochastic gravitational wave background from close hyperbolic encounters of primordial black holes in dense clusters

The inner part of dense clusters of primordial black holes is an active environment where multiple scattering processes take place. Some of them give rise from time to time to bounded pairs, and the rest ends up with a single scattering event. The former eventually evolves to a binary black hole (BBH) emitting periodic gravitational waves (GWs), while the latter with a short distance, called close hyperbolic encounters (CHE), emits a strong GW burst. We make the first calculation of the stochastic GW background originating from unresolved CHE sources. Unlike the case for BBH, the low-frequency tail of the SGWB from CHE is sensitive to the redshift dependence of the event rate, which could help distinguish the astrophysical from the primordial black hole contributions. We find that there is a chance that CHE can be tested by third-generation ground-based GW detectors such as Einstein Telescope and Cosmic Explorer.

gr-qc↗

arXiv2021

Black hole induced spins from hyperbolic encounters in dense clusters

The black holes that have been detected via gravitational waves (GW) can have either astrophysical or primordial origin. Some GW events show significant spin for one of the components and have been assumed to be astrophysical, since primordial black holes are generated with very low spins. However, it is worth studying if they can increase their spin throughout the evolution of the universe. Possible mechanisms that have already been explored are multiple black hole mergers and gas accretion. We propose here a new mechanism that can occur in dense clusters of black holes: the spin-up of primordial black holes when they are involved in close hyperbolic encounters. We explore this effect numerically with the Einstein Toolkit for different initial conditions, including variable mass ratios. For equal masses, there is a maximum spin that can be induced on the black holes, $χ= a/m \leq 0.2$. We find however that for large mass ratios one can attain spins up to $χ\simeq 0.8$, where the highest spin is induced on the most massive black hole. For small induced spins we provide simple analytical expressions that depend on the relative velocity and impact parameter.

gr-qc↗