arXiv subjects

Carlos Palenzuela

Publications and source records attributed to Carlos Palenzuela.

At least 19 recordsLinked to original sources

arXiv2026

Signatures of Compact Object Mergers Inside Stars in AGN Disks

Disks of gas accreting onto supermassive black holes, powering active galactic nuclei (AGN), can capture stars from nuclear star clusters or form stars in situ via gravitational instability. The dense, hot disk environment can drive rapid accretion onto embedded stars, dramatically altering their evolution. Models predict that, for sufficiently rapid accretion, fresh gas replenishes hydrogen in stellar cores as quickly as it is burned, and the stars reach a quasi-steady state. Here we study encounters of such massive, long-lived (''immortal'') stars with compact objects in AGN disks. We estimate the encounter rate and the timescale for a single compact remnant to spiral into an AGN star; depending on how strongly feedback regulates the ensuing accretion, the star is either consumed in a collapsar-like, engine-driven transient or converted into a long-lived, quasi-star-like object hosting a central black hole. We then study the merger of a binary black hole (BBH) inside the AGN star, and show that gas drag hardens the binary to merger far faster than gravitational-wave emission alone. The resulting merger is a loud LIGO-Virgo-KAGRA (LVK) source, but the characteristic imprint of the dense environment -a strong suppression and dephasing of the inspiral relative to vacuum- falls in the deci-Hz band rather than the milli-Hz LISA band, and is best resolved by next-generation detectors such as DECIGO. We estimate that this channel could contribute a BBH merger rate of up to $\sim8\,{\rm Gpc^{-3}\,yr^{-1}}$ in favorable scenarios, and discuss the model uncertainties and directions for future work.

astro-ph.HE↗

arXiv2026

Neutron star evolution with the Bemfica-Disconzi-Noronha-Kovtun viscous hydrodynamics framework

The recently proposed first-order viscous relativistic hydrodynamics formulation by Bemfica, Disconzi, Noronha, and Kovtun (commonly known as the BDNK formulation) has been shown to be causal, stable, strongly hyperbolic, and thus locally well-posed. It is now a viable new option for modelling out-of-equilibrium effects in fluids, and has attracted wide attention in its potential applications to astrophysical systems. In this work, we present the first non-linear numerical simulation of spherically symmetric neutron stars using the BDNK formulation under the Cowling approximation. Using a simplified equation of state, we show that stable evolutions can be constructed within a restricted parameter space up to the simulation time we explored. From these simulations, we analyse the frequency content of the quasi-normal modes and the decay rate of the fundamental mode. This analysis serves as a first step towards constructing a fully consistent model of neutron stars using the BDNK formulation.

gr-qc↗

arXiv2026

Nonlinear Stability of Rotating Hairy Black Holes

Rotating hairy black holes (RHBHs) are axisymmetric equilibrium solutions of the Einstein--Klein--Gordon equations, consisting of a spinning black hole surrounded by a toroidal distribution of complex scalar field. Despite their potential astrophysical relevance, the stability of these configurations -- naturally expected to form through superradiant growth of light bosonic fields -- remains uncertain. In this work, we investigate the stability of RHBHs by performing fully non-linear numerical evolutions of several configurations that differ in the relative mass contribution of the scalar-field torus. We find that configurations in which the scalar field mass is subdominant compared to the black hole mass remain stable throughout the evolution, at least on timescales of order $μt > 1600$, where $μ$ is the scalar field mass. These configurations might therefore be stable, with possible superradiant instabilities developing only on much longer timescales $μt \sim 10^{11}$, according to previous linear stability analyses. In contrast, when the scalar-field mass dominates, the system develops an instability on a much shorter timescale around $μt \sim \mathcal{O}(100)$, similar to the non-axisymmetric instability observed in rotating boson stars. Given the expected upper limits on scalar-field mass growth achievable through superradiance, our results suggest that rotating hairy black holes formed predominantly via this mechanism are likely to remain stable, at least up to the onset of the superradiant instability.

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arXiv2025

MHDuet : a high-order General Relativistic Radiation MHD code for CPU and GPU architectures

We present MHDuet, an open source evolution code for general relativistic magnetohydrodynamics with neutrino transport. The code solves the full set of Einstein equations coupled to a relativistic, magnetized fluid with an M1 neutrino radiation scheme using advanced techniques, including adaptive mesh and large eddy simulation techniques, to achieve high accuracy. The Simflowny platform generates the code from a high-level specification of the computational system, producing code that runs with either the SAMRAI or AMReX infrastructure. The choice of AMReX enables compilation and execution on GPUs, running an order of magnitude faster than on CPUs at the node level. We validate the code against benchmark tests, reproducing previous results obtained with the SAMRAI infrastructure, and demonstrate its capabilities with simulations of neutron stars employing realistic tabulated equations of state. Resolution studies clearly demonstrate convergence faster than second order in the grid spacing. Scaling tests reveal excellent strong and weak scaling performance when running on GPUs. The goal of the code is to provide a powerful tool for studying the dynamics of compact objects within multi-messenger astrophysics.

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arXiv2025

Robustness of Magnetic Field Amplification in Neutron Star Mergers

The dynamics of a binary neutron stars merger is governed by physics under the most extreme conditions, including strong spacetime curvature, ultra-high matter densities, luminous neutrino emission and the rapid amplification of the initial neutron star magnetic fields. Here we systematically explore how sensitive the magnetic field evolution is to the total mass of the merging binary, to the mass ratio of its components, the stellar spins and to the equation of state. For this purpose, we analyze 16 state-of-the-art GRMHD simulations that employ a subgrid-scale model to account for the unresolved small-scale turbulence. We find that strong and rapid amplification of the magnetic field to volume-averaged values of $\sim 10^{16}$~G in the high-density regions is a very robust outcome of a neutron star merger and this result is only marginally impacted by either mass, mass ratio, spin or equation of state.

astro-ph.HE↗

arXiv2025

Implications of Magnetic Flux-Disk Mass Correlation in Black Hole-Neutron Star Mergers for GRB sub-populations

We perform numerical relativity simulations of black hole-neutron star (BH-NS) mergers with a fixed mass ratio of $q = 3$, varying the BH spin to produce a wide range of post-merger accretion disk masses. Our high-order numerical scheme, fine resolution, and Large Eddy Simulation techniques enable us to achieve likely the most resolved BH-NS merger simulations to date, capturing the post-merger magnetic field amplification driven by turbulent dynamo processes. Following tidal disruption and during disk formation, the Kelvin-Helmholtz instability in the spiral arm drives a turbulent state in which the magnetic field, initialized to a realistic average value of $10^{11}\, \rm{G}$, grows to an average of approximately $10^{14}\, \rm{G}$ in the first $\approx 20\, \mathrm{ms}$ post-merger. Notably, the dimensionless magnetic flux on the BH, $ ϕ$, evolves similarly across nearly two orders of magnitude in disk mass. This similarity, along with estimates from longer numerical simulations of the decay of the mass accretion rate, suggests a universal timescale at which the dimensionless flux saturates at a magnetically arrested state (MAD) such that $ ϕ\approx 50 $ at $t_{\rm MAD} \gtrsim 10\,{\rm s}$. The unified framework of Gottlieb et al. (2023) established that the MAD timescale sets the duration of the resulting compact binary gamma-ray burst (cbGRB), implying that all BH-NS mergers contribute to the recently detected new class of long-duration cbGRBs.

astro-ph.HE↗

arXiv2024

Large Eddy Simulations of Magnetized Mergers of Black Holes and Neutron Stars

The LIGO-Virgo-Kagra collaboration has observed gravitational waves consistent with the mergers of a black hole and a neutron star, namely GW200105 and GW200115, providing evidence for such cataclysmic events. Although no electromagnetic counterpart was reported for either of these two events, under certain conditions black hole--neutron star mergers are expected to form a significant accretion disk and to produce both a short gamma ray burst and a kilonova, much as observed in the binary neutron star merger GW170817. Here, we extend our publicly available code $\texttt{MHDuet}$ to study numerically the merger of a magnetized neutron star with a black hole. $\texttt{MHDuet}$ employs Large Eddy Simulation (LES) techniques to help capture the magnetic field amplification resulting from turbulence and other sub-grid scale dynamics in the post-merger stage. In particular, we simulate a merger with parameters favorable to producing an accretion disk, focusing on the formation and dynamics of the turbulent disk and the resulting magnetic field amplification. Following the tidal disruption and during the formation of the accretion disk, the magnetic field undergoes significant amplification driven by the Kelvin-Helmholtz instability, reaching strengths of more than $10^{14}\,\rm{G}$ from a realistic initial strength of $10^{11}\,\rm{G}$ in short timescales of approximately $20\,\rm{ms}$. Despite employing LES techniques with a finest resolution of $120\,\text{m}$ that is among the highest in black hole-neutron star mergers, it is still insufficient to demonstrate convergence of the magnetic field growth. Although the effects of the LES are here rather modest, we expect them to be more significant at higher resolution, as observed in binary neutron star merger simulations.

astro-ph.HE↗

arXiv2024

Delayed jet launching in binary neutron star mergers with realistic initial magnetic fields

We analyze a long-lived hyper-massive neutron star merger remnant (post-merger lifetime $>250$ ms) that has been obtained via large eddy simulations with a gradient subgrid-scale model. We find a clear helicoidal magnetic field structure that is governed by the toroidal component of the magnetic field. Although no jet emerges during the simulation time, we observe at late times a significant increase of the poloidal component of the magnetic field at all scales. We also compare with the results of several binary neutron star simulations with moderate resolution of $120$~m, that are evolved up to $50$~ms after the merger, which differ in terms of the initial topology and strength of the magnetic field. We find that the best choice is an isotropic small-scale magnetic field distribution that mimics the turbulent state that generically develops during the merger. This initial configuration reaches a closer agreement with our high-resolution simulation results than the purely dipolar large-scale fields that are commonly employed in these type of simulations. This provides a recipe to perform such simulations avoiding the computationally expensive grids required to faithfully capture the amplification of the magnetic field by Kelvin-Helmholtz instabilities.

astro-ph.HE↗

arXiv2024

The Guided Moments formalism: a new efficient full-neutrino treatment for astrophysical simulations

We present the new Guided Moments ($\texttt{GM}$) formalism for neutrino modeling in astrophysical scenarios like core-collapse supernovae and neutron star mergers. The truncated moments approximation ($\texttt{M1}$) and Monte-Carlo ($\texttt{MC}$) schemes have been proven to be robust and accurate in solving the Boltzmann's equation for neutrino transport. However, it is well-known that each method exhibits specific strengths and weaknesses in various physical scenarios. The $\texttt{GM}$ formalism effectively solves these problems, providing a comprehensive scheme capable of accurately capturing the optically thick limit through the exact $\texttt{M1}$ closure and the optically thin limit through a $\texttt{MC}$ based approach. In addition, the $\texttt{GM}$ method also approximates the neutrino distribution function with a reasonable computational cost, which is crucial for the correct estimation of the different neutrino-fluid interactions. Our work provides a comprehensive discussion of the formulation and application of the $\texttt{GM}$ method, concluding with a thorough comparison across several test problems involving the three schemes ($\texttt{M1}$, $\texttt{MC}$, $\texttt{GM}$) under consideration.

astro-ph.HE↗

arXiv2023

Magnetic winding and turbulence in ultra-hot Jupiters

While magnetism in exoplanets remains largely unknown, Hot Jupiters have been considered as natural candidates to harbour intense magnetic fields, both due to their large masses and their high energy budgets coming from irradiation as a consequence of their vicinity to their host stars. In this work we perform MHD simulations of a narrow day-side atmospheric column of ultra-hot Jupiters, suitable for very high local temperatures (T > 3000 K). Since the conductivity in this regime is very high, the dominant effect is winding due to the intense zonal winds. By including a forcing that mimics the wind profiles obtained in global circulation models, the shear layer induces a strong toroidal magnetic field (locally reaching hundreds of gauss), supported by meridional currents. Such fields and the sustaining currents don$'$t depend on the internally generated field, but are all confined in the thin (less than a scale-height) shear layer around 1 bar. Additionally, we add random perturbations that induce turbulent motions, which lead to further (but much smaller) magnetic field generation to a broader range of depths. These results allow an evaluation of the currents induced by the atmospheric dynamo. Although here we use ideal MHD and the only resistivity comes from the numerical scheme, we estimate a-posteriori the amount of Ohmic heat deposited in the outer layers, which could be employed in evolutionary models for Hot Jupiters' inflated radii.

astro-ph.EP↗

arXiv2023

The role of turbulence and winding in the development of large-scale, strong magnetic fields in long-lived remnants of binary neutron star mergers

We perform a long and accurate Large-Eddy Simulation of a binary neutron star merger, following the newly formed remnant up to 110 milliseconds. The combination of high-order schemes, high-resolution and the gradient subgrid-scale model allow us to have among the highest effective resolutions ever achieved. Our results show that, although the magnetic fields are strongly amplified by the Kelvin-Helmholtz instability, they are coherent only over very short spatial scales until t \gtrsim 30 ms. Around that time, magnetic winding becomes more efficient leading to a linear growth of the toroidal component and slowly ordering the field to more axisymmetric, large scales. The poloidal component only starts to grow at small scales at much later times t \gtrsim 90 ms, in a way compatible with the magneto-rotational instability. No strong large-scale poloidal field or jet is produced in the timescales spanned by our simulation, although there is an helicoidal structure gradually developing at late times. We highlight that soon after the merger the topology is always strongly dominated by toroidal structures, with a complex distribution in the meridional plane and highly turbulent perturbations. Thus, starting with strong purely dipolar fields before the merger is largely inconsistent with the outcomes of a realistic evolution. Finally, we confirm the universality of the evolved topology, even when starting with very different magnetic fields confined to the outermost layers of each neutron star.

astro-ph.HE↗

arXiv2023

Global high-order numerical schemes for the time evolution of the general relativistic radiation magneto-hydrodynamics equations

Modeling correctly the transport of neutrinos is crucial in some astrophysical scenarios such as core-collapse supernovae and binary neutron star mergers. In this paper, we focus on the truncated-moment formalism, considering only the first two moments (M1 scheme) within the grey approximation, which reduces Boltzmann seven-dimensional equation to a system of $3+1$ equations closely resembling the hydrodynamic ones. Solving the M1 scheme is still mathematically challenging, since it is necessary to model the radiation-matter interaction in regimes where the evolution equations become stiff and behave as an advection-diffusion problem. Here, we present different global, high-order time integration schemes based on Implicit-Explicit Runge-Kutta (IMEX) methods designed to overcome the time-step restriction caused by such behavior while allowing us to use the explicit RK commonly employed for the MHD and Einstein equations. Finally, we analyze their performance in several numerical tests.

gr-qc↗

arXiv2023

Dynamical Boson Stars

The idea of stable, localized bundles of energy has strong appeal as a model for particles. In the 1950s John Wheeler envisioned such bundles as smooth configurations of electromagnetic energy that he called {\em geons}, but none were found. Instead, particle-like solutions were found in the late 1960s with the addition of a scalar field, and these were given the name {\em boson stars}. Since then, boson stars find use in a wide variety of models as sources of dark matter, as black hole mimickers, in simple models of binary systems, and as a tool in finding black holes in higher dimensions with only a single killing vector. We discuss important varieties of boson stars, their dynamic properties, and some of their uses, concentrating on recent efforts.

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arXiv2022

Relativistic force-free models of the thermal X-ray emission in millisecond pulsars observed by NICER

Several important properties of rotation-powered millisecond pulsars (MSPs), such as their mass-radius ratio, equation of state and magnetic field topology, can be inferred from precise observations and modelling of their X-ray light curves. In the present study, we model the thermal X-ray signals originated in MSPs, all the way from numerically solving the surrounding magnetospheres up to the ray tracing of the emitted photons and the final computation of their light curves and spectra. Our modelled X-ray signals are then compared against a set of very accurate NICER observations of four target pulsars: PSR J0437-4715, PSR J1231-1411, PSR J2124-3358 and PSR J0030+0451. We find very good simultaneous fits for the light curve and spectral distribution in all these pulsars. The magnetosphere is solved by performing general relativistic force-free simulations of a rotating neutron star (NS) endowed with a simple centered dipolar magnetic field, for many different stellar compactness and pulsar misalignments. From these solutions, we derive an emissivity map over the surface of the star, which is based on the electric currents in the magnetosphere. In particular, the emission regions (ERs) are determined in this model by spacelike four-currents that reach the NS. We show that this assumption, together with the inclusion of the gravitational curvature on the force-free simulations, lead to non-standard ERs facing the closed-zone of the pulsar, in addition to other ERs within the polar caps. The combined X-ray signals from these two kinds of ERs (both antipodal) allow to approximate the non-trivial interpulses found in all the target MSPs light curves.

astro-ph.HE↗

arXiv2022

Gravitational waves and kicks from the merger of unequal mass, highly compact boson stars

Boson stars have attracted much attention in recent decades as simple, self-consistent models of compact objects and also as self-gravitating structures formed in some dark-matter scenarios. Direct detection of these hypothetical objects through electromagnetic signatures would be unlikely because their bosonic constituents are not expected to interact significantly with ordinary matter and radiation. However, binary boson stars might form and coalesce emitting a detectable gravitational wave signal which might distinguish them from ordinary compact object binaries containing black holes and neutron stars. We study the merger of two boson stars by numerically evolving the fully relativistic Einstein-Klein-Gordon equations for a complex scalar field with a solitonic potential that generates very compact boson stars. Owing to the steep mass-radius diagram, we can study the dynamics and gravitational radiation from unequal-mass binary boson stars with mass ratios up to $q\approx23$ without the difficulties encountered when evolving binary black holes with large mass ratios. Similar to the previously-studied equal-mass case, our numerical evolutions of the merger produce either a nonspinning boson star or a spinning black hole, depending on the initial masses and on the binary angular momentum. We do not find any evidence of synchronized scalar clouds forming around either the remnant spinning black hole or around the remnant boson stars. Interestingly, in contrast to the equal-mass case, one of the mechanisms to dissipate angular momentum is now asymmetric, and leads to large kick velocities (up to a few $10^4\,{\rm km/s}$) which could produce wandering remnant boson stars. We also compare the gravitational wave signals predicted from boson star binaries with those from black hole binaries, and comment on the detectability of the differences with ground interferometers.

gr-qc↗

arXiv2022

Universality of the turbulent magnetic field in hypermassive neutron stars produced by binary mergers

The detection of a binary neutron star merger in 2017 through both gravitational waves and electromagnetic emission opened a new era of multimessenger astronomy. The understanding of the magnetic field amplification triggered by the Kelvin-Helmholtz instability during the merger is still a numerically unresolved problem because of the relevant small scales involved. One of the uncertainties comes from the simplifications usually assumed in the initial magnetic topology of merging neutron stars. We perform high-resolution, convergent large-eddy simulations of binary neutron star mergers, following the newly formed remnant for up to $30$ milliseconds. Here we specifically focus on the comparison between simulations with different initial magnetic configurations, going beyond the widespread-used aligned dipole confined within each star. The results obtained show that the initial topology is quickly forgotten, in a timescale of few miliseconds after the merger. Moreover, at the end of the simulations, the average intensity ($B\sim 10^{16}$ G) and the spectral distribution of magnetic energy over spatial scales barely depend on the initial configuration. This is expected due to the small-scale efficient dynamo involved, and thus it holds as long as: (i) the initial large-scale magnetic field is not unrealistically high (as often imposed in mergers studies); (ii) the turbulent instability is numerically (at least partially) resolved, so that the amplified magnetic energy is distributed across a wide range of scales and becomes orders-of-magnitude larger than the initial one.

gr-qc↗

arXiv2022

No evidence of kinetic screening in simulations of merging binary neutron stars beyond general relativity

We have conducted fully relativistic simulations in a class of scalar-tensor theories with derivative self-interactions and screening of local scales. By using high-resolution shock-capturing methods and a non-vanishing shift vector, we have managed to avoid issues plaguing similar attempts in the past. We have first confirmed recent results by ourselves in spherical symmetry, obtained with an approximate approach and pointing at a partial breakdown of the screening in black-hole collapse. Then, we considered the late inspiral and merger of binary neutron stars. We found that screening tends to suppress the (subdominant) dipole scalar emission, but not the (dominant) quadrupole scalar mode. Our results point at quadrupole scalar signals as large as (or even larger than) in Fierz-Jordan-Brans-Dicke theories with the same conformal coupling, for strong-coupling scales in the MeV range that we can simulate.

gr-qc↗

arXiv2021

Kinetic screening in nonlinear stellar oscillations and gravitational collapse

We consider k-essence, a scalar-tensor theory with first-order derivative self-interactions that can screen local scales from scalar fifth forces, while allowing for sizeable deviations from General Relativity on cosmological scales. We construct fully nonlinear static stellar solutions that show the presence of this screening mechanism, and we use them as initial data for simulations of stellar oscillations and gravitational collapse in spherical symmetry. We find that for k-essence theories of relevance for cosmology, the screening mechanism works in the case of stellar oscillation and suppresses the monopole scalar emission to undetectable levels. In collapsing stars, we find that the Cauchy problem, although locally well posed, can lead to diverging characteristic speeds for the scalar field. By introducing a ''fixing equation'' in the spirit of J. Cayuso, N. Ortiz, and L. Lehner [Phys. Rev. D 96, 084043 (2017)], inspired in turn by dissipative relativistic hydrodynamics, we manage to evolve collapsing neutron stars past the divergence of the characteristic speeds. We show that, in these systems, the screening mechanism is less efficient than for oscillating and static stars, because the collapsing star must shed away all of its scalar hair before forming a black hole. For k-essence theories of relevance for cosmology, the characteristic frequency of the resulting scalar monopole signal is too low for terrestrial detectors, but we conjecture that space-borne interferometers such as LISA might detect it if a supernova explodes in the Galaxy.

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