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Alejandro Cárdenas-Avendaño

Publications and source records attributed to Alejandro Cárdenas-Avendaño.

At least 19 recordsLinked to original sources
arXiv2026

Breakdown of hydrodynamic universality in causal first-order viscous neutron star oscillations

Hydrodynamic modeling of viscous neutron stars requires a choice of theory of relativistic hydrodynamics. That distinct formulations contain the same physical content in the long-wavelength limit is referred to as ``hydrodynamic universality.'' It has been recently shown that hydrodynamic universality can break down in radial neutron star oscillations: causality and finite-size effects restrict access to the long-wavelength universal hydrodynamic regime in sufficiently viscous stars. We show that this breakdown occurs for a broad range of causal hydrodynamic frames in Bemfica--Disconzi--Noronha--Kovtun (BDNK) theory. Increasing viscosity collectively pushes this class of causal frames away from the BDNK regime of validity, eventually forcing all to lie outside it at bulk viscosities in the range $ζ\sim10^{30}$--$10^{31}\mathrm{g}/\mathrm{cm}/\mathrm{s}$. This breakdown of universality is accompanied by material changes in the oscillation mode structure: non-hydrodynamic modes become damped on timescales comparable to those of hydrodynamic modes, the first overtone can become longer lived than the fundamental mode, and unphysical instabilities with sub-millisecond timescales can emerge. Even in regimes where universality has broken down, we find that the inviscid threshold of gravitational collapse remains unchanged in BDNK theory. These results provide evidence that first-order theories of relativistic hydrodynamics may be insufficient for modeling sufficiently viscous neutron stars.

gr-qc↗
arXiv2026

The Structure in the Structure Function of Black Hole Light Curves

Of the two black holes, M87$^\ast$ and Sgr A$^\ast$, for which event-horizon-scale imaging is currently achievable, only Sgr A$^\ast$ at our Galactic Center exhibits sub-hour variability, making light curve observations of the inner accretion flow especially valuable for time-domain studies of the spacetime and the surrounding plasma. Observed light curves are often sampled with variable cadence, so a structure function is often used to quantify their variability properties. In this work, we assemble a heterogeneous Sgr A$^\ast$ light curve data set from discrete observations acquired over $\sim12$ years {at 230 GHz}, with different instruments, durations, and cadences, and study its structure function in three frameworks. First, we consider analytic predictions for the features of structure functions of stationary random processes with Fourier spectra dictated by the Matérn covariance. We then study the structure functions resulting from lensed movies of synthetic optically thin accretion flows approximated with Matérn covariance in both space and time, and identify which features in the structure function can be attributed to the black-hole geometry. Lastly, we examine the structure functions of a library of $72$ general relativistic magnetohydrodynamic simulations. We find that, in general, magnetically arrested disk (MAD) models produce smoother structure functions with steeper slopes than standard and normal evolution (SANE) counterparts. On time scales of $5-15$ minutes, the observed structure function slope of Sgr A$^\ast$ more closely aligns with the MAD trend, while both simulation types are generally more variable than the data on all sub-hour timescales.

astro-ph.HE↗
arXiv2026

A Systematic Study of Resonance-Driven Flux Modifications in Extreme-Mass-Ratio Inspirals

Transient orbital resonances can introduce phase-dependent corrections to the evolution of extreme-mass-ratio inspirals (EMRIs), potentially altering their long-term dynamics and emitted gravitational-wave signals. In this work, we quantify the resonance-induced modifications to the energy, axial angular momentum, and Carter constant fluxes and compute the corresponding resonance coefficients across a broad region of the orbital parameter space. Using the publicly available $\texttt{pybhpt}$ code, we solve the Teukolsky equation in the frequency domain to coherently combine the degenerate radial and polar harmonics that arise at resonance. We analytically derive a selection rule governing the relative radial-polar phase dependence of the resonant flux modifications. We argue that the relative strength of the resonant flux modifications reflects a balance between symmetry-induced cancellations and the degree to which the resonant orbit samples the underlying two-dimensional orbital phase space. For the dynamically important $3{:}2$ and $2{:}1$ resonances, we also characterize how the resonance coefficients vary with the primary black-hole spin, orbital eccentricity and inclination. Our results constitute the largest set of Teukolsky-based resonance coefficients calculated to date and provide essential input for future studies of transient orbital resonances in EMRIs.

gr-qc↗
arXiv2026

A Four-Dimensional Gaussian Random Field Generator for Modeling Spatiotemporal Variability in Astrophysical Sources

Semi-analytic models of black-hole movies require both an emitting flow prescription and a time-dependent source variability. Existing prescriptions are often limited to either equatorial emission or time-independent sources. In this work we present a unified model for these two ingredients. First, we prescribe an off-equatorial, nongeodesic Kerr fluid rotation law by lifting an equatorial specific-angular-momentum profile to cylindrical surfaces, setting the polar component of the four-velocity to zero, normalizing the flow with the full Kerr metric at the spacetime point, and retaining the option to recover a geodesic-like plunging prescription when needed. Second, we use this velocity as the disk advection field in a four-dimensional inhomogeneous, anisotropic Matérn-like Gaussian random field. We provide a parametrized model for a torus-like disk and a central jet through a single composite correlation tensor. The resulting effective model is an implementation-ready prescription for time-dependent thick-disk and disk-jet emission for relativistic studies.

astro-ph.HE↗
arXiv2026

Parameter-estimation bias induced by transient orbital resonances in extreme-mass-ratio inspirals

Given the multifrequency nature of relativistic orbits, transient orbital resonances are expected to be ubiquitous during an extreme-mass-ratio inspiral (EMRI). At a resonance, the orbital dynamics is modified in a nontrivial way, imprinting an overall dephasing in the emitted gravitational waves and potentially impacting both the detection and parameter estimation of these sources. In this work, using a Fisher-matrix approach, we investigate the bias induced by transient orbital resonances in EMRI parameter estimation. We focus on the most dynamically significant low-order resonances, 3 : 2 and 2 : 1, as well as on the high-order, subdominant resonances 3 : 1 and 4 : 3. We find that, for most of the orbits considered, neglecting the effect of a resonance crossing leads to significant losses in signal-to-noise ratio and induces bias in parameter recovery. Furthermore, both the sign and the amplitude of the resonance-induced modifications to the integrals of motion play a crucial role and must be modeled accurately. Our results provide further evidence that failing to model transient orbital resonances accurately can hinder EMRI detection and parameter estimation, thereby limiting their scientific potential.

gr-qc↗
arXiv2026

Light Propagation Prescriptions for Black Hole Movies

The spatiotemporal content of a black-hole movie is set jointly by source variability and by the distribution of light-travel times across the image. In the slow-light prescription, an image evaluated at fixed observer time contains photons emitted at different source times, whereas in fast light all rays sample a single source emission time. In this work we compare these light-propagation prescriptions through the lensing-band structure of Kerr geodesic delays in a controlled semi-analytic setting. For a given emitting geometry, black-hole spin, and observer inclination, we show how the coordinate-time delay distributions of Kerr null geodesics, decomposed by image order across lensing bands, can be compared with the source correlation time to quantify differences between light-propagation prescriptions. We find that when the intrinsic variability timescale is comparable to, or shorter than, the relevant delay spread, the high-inclination mismatch between fast- and slow-light curves can reach several tens of percent. Motivated by this geometric structure, we introduce brisk light, an intermediate prescription that compresses each lensing-band delay map to its dominant temporal interval rather than collapsing the full image to a single source time. The proposed methodology provides both a practical criterion for when slow light matters and an efficient route to black-hole movies that retain the leading temporal imprint of strong lensing, a regime of direct relevance for future space-based VLBI targeting photon-ring observables.

astro-ph.HE↗
arXiv2026

Sensitivities of Black Hole Images from GRMHD Simulations

The advent of high-fidelity imaging of supermassive black holes calls for efficient and robust data-analysis methods. In this work, we use $\texttt{Jipole}$, a differentiable, $\texttt{ipole}$-based radiative transfer code, to enable gradient-based analyses of images generated from state-of-the-art general relativistic magnetohydrodynamic (GRMHD) simulations. We compute image sensitivities, i.e., pixel-wise derivatives of the intensity with respect to model parameters, which form the Jacobian of the forward model and define a local map from parameter space to image space. Using these sensitivities in a mock data analysis, we find that GRMHD-based images generate a structured error landscape for parameter fitting, with anisotropies and local minima, making parameter exploration nontrivial but still tractable when guided by gradient information. We characterize this landscape through the Jacobian and assess the feasibility of gradient-based recovery under idealized, blurred, and noisy conditions. Our results show that automatic differentiation-computed image gradients can guide parameter exploration effectively even in the presence of noise. These findings establish a basis for efficient, high-precision model--data comparisons in black hole imaging and motivate the integration of these sensitivities into advanced inference frameworks.

astro-ph.HE↗
arXiv2026

Polarization Signatures of Inspiraling Hotspots around Kerr Black Holes

Polarimetric interferometry is a powerful tool for probing both black hole accretion physics and the background spacetime. Current models aimed at explaining the observed multiwavelength flares in Sgr A* often assume hotspots moving on geodesic, Keplerian orbits. In many scenarios, though, a hotspot may instead follow an inspiraling trajectory, potentially transitioning into a plunge toward the black hole. In this work, we present a general framework to simulate the polarized emission from generic equatorial inspiraling hotspots in Kerr spacetime using a parametric four-velocity profile. This parametrization defines a continuous family of flows, ranging from Cunningham's disk model (fixed radius orbits outside the innermost stable circular orbit and plunging motion within the innermost stable circular orbit) to purely radial motion, thereby extending the standard assumptions. Within this framework, we show that inspiral motion produces a distinctive observational signature: a precessing, unwinding evolution of the polarimetric Stokes Q-U looping pattern, in sharp contrast with the closed Q-U loops associated with stable orbits at a fixed radius. We then explore how the morphology of these signatures depends on black hole spin, observer inclination, and magnetic-field configuration. The presented model can be applied to current and near-future interferometric observations of linear polarization, offering a new avenue to probe the physics of matter spiraling inward and the relativistic velocities of plunging plasma.

astro-ph.HE↗
arXiv2026

Waveform Modelling for the Laser Interferometer Space Antenna

LISA, the Laser Interferometer Space Antenna, will usher in a new era in gravitational-wave astronomy. As the first anticipated space-based gravitational-wave detector, it will expand our view to the millihertz gravitational-wave sky, where a spectacular variety of interesting new sources abound: from millions of ultra-compact binaries in our Galaxy, to mergers of massive black holes at cosmological distances; from the beginnings of inspirals that will venture into the ground-based detectors' view to the death spiral of compact objects into massive black holes, and many sources in between. Central to realising LISA's discovery potential are waveform models, the theoretical and phenomenological predictions of the pattern of gravitational waves that these sources emit. This white paper is presented on behalf of the Waveform Working Group for the LISA Consortium. It provides a review of the current state of waveform models for LISA sources, and describes the significant challenges that must yet be overcome.

gr-qc↗
arXiv2026

$\texttt{Jipole}$: A Differentiable $\texttt{ipole}$-based Code for Radiative Transfer in Curved Spacetimes

Recent imaging of supermassive black holes by the Event Horizon Telescope (EHT) has relied on exhaustive parameter-space searches, matching observations to large, precomputed libraries of theoretical models. As observational data become increasingly precise, the limitations of this computationally expensive approach grow more acute, creating a pressing need for more efficient methods. In this work, we present $\texttt{Jipole}$, an automatically differentiable (AD), $\texttt{ipole}$-based code for radiative transfer in curved spacetimes, designed to compute image gradients with respect to underlying model parameters. These gradients quantify how parameter changes-such as the black hole's spin or the observer's inclination-affect the image, enabling more efficient parameter estimation and reducing the number of required images. We validate $\texttt{Jipole}$ against $\texttt{ipole}$ in two analytical tests and then compare pixel-wise intensity derivatives from AD with those from finite-difference methods. We then demonstrate the utility of these gradients by performing parameter recovery for an analytical model using a conjugate gradient optimizer in three increasingly complex cases for the injected image: ideal, blurred, and blurred with added noise. In most cases, high-accuracy fits are obtained in only a few optimization steps, failing only in cases with extremely low signal-to-noise ratios and large blurring size kernels. These results highlight the potential of AD-based methods to accelerate robust, high-fidelity model-data comparisons in current and future black hole imaging efforts.

astro-ph.HE↗
arXiv2025

Estimating High-Order Time Derivatives of Kerr Orbital Functionals

Functions of bound Kerr geodesic motion play a central role in many calculations in relativistic astrophysics, ranging from gravitational-wave generation to self-force and radiation-reaction modeling. Although these functions can be expressed as a Fourier series using the geodesic fundamental frequencies, reconstructing them in coordinate time is challenging due to the coupling of the radial and polar motions. In this paper, we compare two strategies for performing such reconstructions and their ability to estimate high-order coordinate-time derivatives of the orbital functional. The first method maps Fourier coefficients from Mino to coordinate time; the second method fits a sampled time series of the function to a truncated coordinate-time Fourier series. While the latter method is prone to overfitting, it yields more accurate reconstructions and derivatives than the mapping, but completely misrepresents the harmonic content of the orbital functional. For the purpose of accurate coordinate-time derivative estimation, we propose a hybrid method: fit for the Mino-time coefficients, differentiate with respect to Mino time, then convert to coordinate time. Applied to the mass quadrupole of a generic Kerr geodesic, this hybrid method recovers the sixth derivative with a fractional residual $\sim10^{-6}$ using only two harmonics. For orbital functionals that depend explicitly on the geodesic orbit expressed in Boyer--Lindquist coordinates, we also provide a recursive procedure for computing coordinate-time derivatives using exact analytic expressions. These results offer a general framework for accurately evaluating high-order time derivatives along Kerr geodesic worldlines, with direct relevance to applications such as extreme-mass-ratio inspiral kludge waveform modeling, where such derivatives are key ingredients for precise gravitational-wave predictions.

gr-qc↗
arXiv2025

Perturbative and non-linear analyses of gravitational turbulence in spacetimes with stable light rings

Some black hole mimickers, as well as black strings and other higher-dimensional spacetimes, exhibit stable light rings-regions where light or high-frequency gravitational waves can be trapped. In these regions, linear perturbations decay slowly, raising the possibility of nonlinear instability mechanisms. In this work, we study the cubic nonlinear wave equation as a proxy for Einstein's equations, using a four-dimensional model geometry that allows stable trapping. By employing a perturbative approach, we show that the nonlinear wave equation on the sphere with dissipative terms captures several features of the full nonlinear problem. This framework allows us to confirm a previous conjecture: all higher-order energy norms grow for arbitrarily small initial fluctuation amplitudes. Additionally, we analyze the system's mode spectrum at late times, revealing an inertial range dominated by a direct energy cascade. These findings further support the notion that spacetimes with stable light rings develop weak, high-frequency radiation hair, which will not generically lead to instabilities.

gr-qc↗
arXiv2025

Morphology of Relativistically Broadened Line Emission from Axisymmetric Equatorial Accretion Disks

Single-frequency emission from an accretion disk around a black hole is broadened into a line profile due to gravitational redshift and the motion of the disk's particles relative to the observer. The ensemble of relativistically broadened emission frequencies from the disk elements forms the spectrum viewed by an observer. Over the past decades, the broadened spectra of accreting systems have been used to constrain the spin of the black hole, the observer's inclination, and the astrophysical model parameters of the system. These inferences are usually made under the assumption that the accretion disk consists of particles orbiting around the black hole on stable circular orbits in the equatorial plane. Under this Standard disk model, in this work, we revisit line profile morphology, i.e., its extent, kinks, and fall-off. We provide a unified analytical explanation for these line profile morphological features, which encode the black hole spin, viewing inclination, and locations of the disk's inner and outer edges. We then show that these features, however, are model-dependent, by parametrically relaxing some of the astrophysical assumptions. In particular, we explore how allowing the disk particles to deviate from stable circular orbits rapidly degenerates the characteristic features of the line profile under the Standard disk model. Our results further demonstrate how sensitive our understanding of black hole and system properties can be to assumptions we make when interpreting these types of measurements.

astro-ph.HE↗
arXiv2025

Inferring Black Hole Spin from Interferometric Measurements of the First Photon Ring: A Geometric Approach

Accurately inferring black hole spin is crucial for understanding black hole dynamics and their astrophysical environments. In this work, we outline a geometric method for spin estimation by using the interferometric shape of the first photon ring ($n=1$) as an approximation to the critical curve, which, given an assumed value of the black hole inclination, is then mapped to a spin value. While future space-based missions will capture a wealth of data on the first photon ring--including the full angle-dependent diameter, angular brightness profile, and astrometric offset from the $n=0$ ring--our analysis is restricted to using only two angle-dependent diameters to compute its shape asymmetry and infer spin. Focusing on low inclinations and moderate-to-high spins, we test the method across various emission models, baselines, and noise sources, including a mock space-based observation. Although the size of the $n=1$ ring depends on the emission model, its interferometric shape remains a robust spin probe at low inclinations. We find that the inferred asymmetry of the $n=1$ image may be biased by the critical curve morphology, and it can be heavily skewed by the presence of noise, whether astrophysical or instrumental. In low-noise limits at low viewing inclination, significant contributions from the $n=0$ image at short baselines may lead to a downward bias in asymmetry estimates. While our method can estimate high spins in noise-free time-averaged images, increasing the noise and astrophysical variability degrades the resulting constraints, providing only lower bounds on the spin when applied to synthetic observed data. Remarkably, even using only the ring's asymmetry, we can establish lower bounds on the spin, underscoring the promise of photon ring-based spin inference in future space-based very long baseline interferometry missions, such as the proposed Black Hole Explorer.

astro-ph.HE↗
arXiv2025

On turbulence for spacetimes with stable trapping

Motivated by understanding the nonlinear gravitational dynamics of spacetimes admitting stably trapped null geodesics, such as ultracompact objects and black string solutions to general relativity, we explore the dynamics of nonlinear scalar waves on a simple (fixed) model geometry with stable trapping. More specifically, we consider the time evolution of solutions to the cubic (defocusing) wave equation on a four-dimensional static, spherically symmetric, and asymptotically flat (horizonless) spacetime admitting a stable photon sphere. Our study shows fundamental differences between linear and nonlinear scalar dynamics. The local energy, as well as all local higher-order energies, of solutions to the linear wave equation on our model spacetime can be rigorously proven to remain uniformly bounded and to decay uniformly in time. However, due to the presence of stable trapping, the uniform decay rate is slow. To help elucidate how the slow linear decay affects solutions to the nonlinear wave equation considered, we examine numerical solutions of the latter, restricting to axisymmetric initial data in this work. In contrast to the linear dynamics, we exhibit a family of nonlinear solutions with turbulent behaviour: Within the region of stable trapping, the slow linear decay allows local higher-order energies of the nonlinear solution to grow over the time interval that we numerically evolve, with the growth being induced by a direct energy cascade. As a complement to the numerical analysis, we provide a heuristic argument suggesting that, if a similar behaviour occurs for gravitational wave perturbations of the motivating spacetimes in general relativity, it would likely not generically lead to black hole or singularity formation.

gr-qc↗
arXiv2025

Cumulative effect of orbital resonances in extreme-mass-ratio inspirals

Orbital resonances in extreme-mass-ratio inspirals (EMRIs) have been proven to be a key feature for accurate gravitational-wave template modeling. Decades of research have led to schemes that can not only model the adiabatic inspiral of such a binary system, but also account for the effects of resonances on their evolution. In this work, we use an effective resonance model that includes analytically derived corrections to the radiation reaction fluxes, to study the combined effects of both dominant (low-order) and subdominant (high-order) orbital resonances in EMRIs. We show that using single, universal shifts for all fluxes overestimates the resonance impact, and therefore individualized shifts for each resonance crossing are needed for accurate modeling. Our analysis reveals that the cumulative effects from multiple resonance crossings can significantly impact the orbital evolution of EMRIs, especially for highly eccentric orbits. Our results provide further evidence that resonance effects have to be included in template production to extract detailed astrophysical parameters from EMRI signals.

gr-qc↗
arXiv2025

Multidisciplinary Science in the Multimessenger Era

Astrophysical observations of the cosmos allow us to probe extreme physics and answer foundational questions on our universe. Modern astronomy is increasingly operating under a holistic approach, probing the same question with multiple diagnostics including how sources vary over time, how they appear across the electromagnetic spectrum, and through their other signatures, including gravitational waves, neutrinos, cosmic rays, and dust on Earth. Astrophysical observations are now reaching the point where approximate physics models are insufficient. Key sources of interest are explosive transients, whose understanding requires multidisciplinary studies at the intersection of astrophysics, gravity, nuclear science, plasma physics, fluid dynamics and turbulence, computation, particle physics, atomic, molecular, and optical science, condensed matter and materials science, radiation transport, and high energy density physics. This white paper provides an overview of the major scientific advances that lay at the intersection of physics and astronomy and are best probed through time-domain and multimessenger astrophysics, an exploration of how multidisciplinary science can be fostered, and introductory descriptions of the relevant scientific disciplines and key astrophysical sources of interest.

astro-ph.HE↗
arXiv2025

Multi-Frequency Models of Black Hole Photon Rings from Low-Luminosity Accretion Disks

Images of black holes encode both astrophysical and gravitational properties. Detecting highly-lensed features in images can differentiate between these two effects. We present an accretion disk emission model coupled to the Adaptive Analytical Ray Tracing (AART) code that allows a fast parameter space exploration of black hole photon ring images produced from synchrotron emission from 10 to 670 GHz. As an application, we systematically study several disk models and compute their total flux density, average radii, and optical depth. The model parameters are chosen around fiducial values calibrated to general relativistic magnetohydrodynamic (GRMHD) simulations and observations of M87*. For the parameter space studied, we characterize the transition between optically thin and thick regimes and the frequency at which the first photon ring is observable. Our results highlight the need for careful definitions of photon ring radius in the image domain, as in certain models, the highly lensed photon ring is dimmer than the direct emission at certain angles. We find that at low frequencies, the ring radii are set by the electron temperature, while at higher frequencies, the magnetic field strength plays a more significant role, demonstrating how multi-frequency analysis can also be used to infer plasma parameters. Lastly, we show how our implementation can qualitatively reproduce multifrequency black hole images from GRMHD simulations when adding time-variability to our disk model through Gaussian random fields. This approach provides a new method for simulating observations from the Event Horizon Telescope (EHT) and the proposed Black Hole Explorer (BHEX) space mission.

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