arXiv subjects

Luciano Rezzolla

Publications and source records attributed to Luciano Rezzolla.

At least 37 records · Page 2Linked to original sources

arXiv2025

Electromagnetic Energy Extraction from Kerr Black Holes: Ab-Initio Calculations

The possibility of extracting energy from a rotating black hole via the Blandford-Znajek mechanism represents a cornerstone of relativistic astrophysics. We present general-relativistic collisionless kinetic simulations of Kerr black-hole magnetospheres covering a wide range in the black-hole spin. Considering a classical split-monopole magnetic field, we can reproduce with these ab-initio calculations the force-free electrodynamics of rotating black holes and measure the power of the jet launched as a function of the spin. The Blandford-Znajek luminosity we find is in very good agreement with analytic calculations and compatible with general-relativistic magnetohydrodynamics simulations via a simple rescaling. These results provide strong evidence of the robustness of the Blandford-Znajek mechanism and accurate estimates of the electromagnetic luminosity to be expected in those scenarios involving rotating black holes across the mass scale.

gr-qc↗

arXiv2025

Dense Matter in Neutron Stars with eXTP

In this White Paper, we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission to constrain the equation of state of dense matter in neutron stars, exploring regimes not directly accessible to terrestrial experiments. By observing a diverse population of neutron stars - including isolated objects, X-ray bursters, and accreting systems - eXTP's unique combination of timing, spectroscopy, and polarimetry enables high-precision measurements of compactness, spin, surface temperature, polarimetric signals, and timing irregularity. These multifaceted observations, combined with advances in theoretical modeling, pave the way toward a comprehensive description of the properties and phases of dense matter from the crust to the core of neutron stars. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is planned to be launched in early 2030.

astro-ph.HE↗

arXiv2025

Postmerger: a new and dominant contribution to the gravitational-wave background from binary neutron stars

The stochastic gravitational-wave background (SGWB) generated by the inspiral and merger of binary neutron stars is traditionally modelled assuming that the inspiral is promptly followed by the collapse of the merger remnant to a rotating black hole. While this is reasonable for the most massive binaries, it is not what is expected in general, as the remnant may survive for up to hundreds of milliseconds and radiate an amount of energy that is significantly larger than that lost during the whole inspiral. To account for this additional contribution to the SGWB, we consider a waveform model that includes both the inspiral and the postmerger emission. We show for the first time that for a large set of parameterized equations of state (EOSs) compatible with observational constraints, there is considerable spectral power in the $1-2\,{\rm kHz}$ range, distinct from that associated with the inspiral and leading to a dimensionless GW energy density $Ω_{\rm GW} \simeq 10^{-10}-10^{-9}$. We discuss the enhanced detectability of the SGWB by third-generation detectors such as the Einstein Telescope and Cosmic Explorer, and show how it depends on the signal-to-noise ratio of foreground binaries and on the remnant lifetime. Interestingly, even a non-detection of the high-frequency part of the SGWB could provide valuable constraints on the remnant lifetime, offering novel insights into the postmerger dynamics and the EOS of nuclear matter.

astro-ph.HE↗

arXiv2025

Long-term impact of the magnetic-field strength on the evolution and electromagnetic emission by neutron-star merger remnants

Numerical simulations are essential to understand the complex physics accompanying the merger of binary systems of neutron stars. However, these simulations become computationally challenging when they have to model the merger remnants on timescales over which secular phenomena, such as the launching of magnetically driven outflows, develop. To tackle these challenges, we have recently developed a hybrid approach that combines, via a hand-off transition, a fully general-relativistic code (FIL) with a more efficient code making use of the conformally flat approximation (BHAC+). We here report important additional developments of BHAC+ consisting of the inclusion of gravitational-wave radiation-reaction contributions and of higher-order formulations of the equations of general-relativistic magnetohydrodynamics. Both improvements have allowed us to explore BNS merger remnants with high accuracy and over timescales that would have been computationally prohibitive otherwise. More specifically, we have investigated the impact of the magnetic-field strength on the long-term (i.e., $\sim 200\,{\rm ms}$) and high-resolution (i.e., $150\,{\rm m}$) evolutions of the "magnetar" resulting from the merger of two neutron stars with a realistic equation of state. In this way, and for sufficiently large magnetic fields, we observe the weakening or suppression of differential rotation and the generation of magnetic flares in the outer layers of the remnant. These flares, driven mostly by the Parker instability, are responsible for intense and collimated Poynting flux outbursts and mass ejections. This novel phenomenology offers the possibility of seeking corresponding signatures from the observations of short gamma-ray bursts and hence revealing the existence of a long-lived strongly magnetized remnant.

astro-ph.HE↗

arXiv2025

Towards a Non-singular Paradigm of Black Hole Physics

The study of regular black holes and black hole mimickers as alternatives to standard black holes has recently gained significant attention, driven both by the need to extend general relativity to describe black hole interiors, and by recent advances in observational technologies. Despite considerable progress in this field, significant challenges remain in identifying and characterizing physically well-motivated classes of regular black holes and black hole mimickers. This report provides an overview of these challenges, and outlines some of the promising research directions -- as discussed during a week-long focus programme held at the Institute for Fundamental Physics of the Universe (IFPU) in Trieste from November 11th to 15th, 2024.

gr-qc↗

arXiv2025

Accurate muonic interactions in neutron star mergers and impact on heavy-element nucleosynthesis

The abundances resulting from $r$-process nucleosynthesis as predicted by simulations of binary neutron star (BNS) mergers remain an open question as the current state of the art is still restricted to three-species neutrino transport. We present the first BNS merger simulations employing a moment-based general-relativistic neutrino transport with five neutrino species, thus including (anti)muons and advanced muonic $β$-processes, and contrast them with traditional three-neutrino-species simulations. Our results show that a muonic trapped-neutrino equilibrium is established, forming a different trapped-neutrino hierarchy akin to the electronic equilibrium. The formation of (anti)muons and the muonization via muonic $β$-processes enhance the neutrino luminosity, leading to rapid cooling in the early postmerger phase. Since muonic processes redirect part of the energy otherwise used for protonization by electronic processes, they yield a cooler remnant and disk, together with neutrino-driven winds that are more neutron-rich. Importantly, the unbound ejected mass is smaller than three-species simulations, and, because of its comparatively smaller temperature and proton fraction, it can enhance lanthanide production and reduce the overproduction of light $r$-process elements for softer equations of state. This finding underlines the importance of muonic interactions and five neutrino species in long-lived BNS remnants.

astro-ph.HE↗

arXiv2025

Physics-Informed Priors Improve Gravitational-Wave Constraints on Neutron-Star Matter

Gravitational-wave astronomy shows great promise in determining nuclear physics in a regime not accessible to terrestrial experiments. We introduce physics-informed priors constrained by nuclear theory and perturbative Quantum Chromodynamics calculations, as well as astrophysical measurements of neutron-star masses and radii. When these priors are used in gravitational-wave astrophysical inference, we show a significant improvement on nuclear equation of state constraints. Applying these to the first observed gravitational-wave binary neutron-star merger GW170817, the constraints on the radius of a $1.4\,M_\odot$ neutron star improve from $R_{1.4} ={12.54^{+1.05}_{-1.54}} \, {\rm km}$ to $R_{1.4} = 12.11^{+0.91}_{-1.11} \,{\rm km}$ and those on the tidal deformability from $\tildeΛ_{1.186} < 720$ to $\tildeΛ_{1.186} = 384^{+306}_{-158}$ ($90\%$ confidence intervals) at the events measured chirp mass $\mathcal{M}=1.186\,M_\odot$. We also show these priors can be used to perform model selection between binary neutron star and neutron star-black hole mergers; in the case of GW190425, the results provide only marginal evidence with a Bayes factor $\mathcal{BF}=1.33$ in favour of the binary neutron star merger hypothesis. Given their ability to improve the astrophysical inference of binary mergers involving neutron stars, we advocate for these physics-informed priors to be used as standard in the literature and provide open-source code for reproducibility and adaptation of the method.

astro-ph.HE↗

arXiv2025

Self-consistent multidimensional Penrose process driven by magnetic reconnection

Astronomical observations and numerical simulations are providing increasing evidence that resistive effects in plasmas around black holes play an important role in determining the phenomenology observed from these objects. In this spirit, we present a general approach to the study of a Penrose process driven by plasmoids that are produced at reconnection sites along current sheets. Our formalism is meant to determine the physical conditions that make a plasmoid-driven Penrose process energetically viable and can be applied to scenarios that are matter- or magnetic-field-dominated, that is, in magnetohydrodynamical or force-free descriptions. By exploring reconnection from an axisymmetric but curved surface, our approach can be considered genuinely multidimensional and allows us to explore conditions that are beyond the ones explored so far and that have been restricted to the equatorial plane. Furthermore, it provides a direct contact with numerical simulations of accretion onto black holes, which exhibit an intense reconnection activity outside the equatorial plane. Finally, to describe the kinematics of the plasma self-consistently, we use the well-known configuration of an equilibrium torus with a purely toroidal magnetic field. For such a torus, we discuss the existence of an ``ergobelt'', \ie a nontrivial surface penetrating the ergosphere and acting as a natural a site for the occurrence of reconnection, and from where we estimate the energetics of a plasmoid-driven Penrose process.

gr-qc↗

arXiv2025

Numerical modelling of bulk viscosity in neutron stars

The early post-merger phase of a binary neutron-star coalescence is shaped by characteristic rotational velocities as well as violent density oscillations and offers the possibility to constrain the properties of neutron star matter by observing the gravitational wave emission. One possibility to do so is the investigation of gravitational wave damping through the bulk viscosity which originates from violations of weak chemical equilibrium. Motivated by these prospects, we present a comprehensive report about the implementation of the self-consistent and second-order formulation of the equations of relativistic hydrodynamics for dissipative fluids proposed by Müller, Israel and Stewart. Furthermore, we report on the results of two test problems, namely the viscous damping of linear density oscillations of isolated nonrotating neutron stars and the viscous migration test, both of which confirm our implementation and can be used for future code tests. Finally, we present fully general-relativistic simulations of viscous binary neutron-star mergers. We explore the structural and thermal properties of binary neutron-star mergers with a constant bulk-viscosity prescription and investigate the impact of bulk viscosity on dynamical mass ejection. We find that inverse Reynolds numbers $\sim 1\%$ can be achieved for the highest employed viscosity thereby suppressing the dynamically ejected mass by a factor of $\sim 5$ compared to the inviscid case.

gr-qc↗

arXiv2025

Impact of bulk viscosity on the post-merger gravitational-wave signal from merging neutron stars

In the violent post-merger of binary neutron-star mergers strong oscillations are present that impact the emitted gravitational-wave (GW) signal. The frequencies, temperatures and densities involved in these oscillations allow for violations of the chemical equilibrium promoted by weak-interactions, thus leading to a nonzero bulk viscosity that can impact dynamics and GW signals. We present the first simulations of binary neutron-star mergers employing the self-consistent and second-order formulation of the equations of relativistic hydrodynamics for dissipative fluids proposed by Müller, Israel and Stewart. With the spirit of obtaining a first assessment of the impact of bulk viscosity on the structure and radiative efficiency of the merger remnant we adopt a simplified but realistic approach for the viscosity, which we assume to be determined by direct and modified Urca reactions and hence to vary within the stars. At the same time, to compensate for the lack of a precise knowledge about the strength of bulk viscosity, we explore the possible behaviours by considering three different scenarios of low, medium, and high bulk viscosity. In this way, we find that large values of the bulk viscosities damp the collision-and-bounce oscillations that characterize the dynamics of the stellar cores right after the merger. At the same time, large viscosities tend to preserve the $m=2$ deformations in the remnant, thus leading to a comparatively more efficient GW emission and to changes in the post-merger spectrum that can be up to $100\,{\rm Hz}$ in the case of the most extreme configurations. Overall, our self-consistent results indicate that bulk viscosity increases the energy radiated in GWs soon after the merger by $\lesssim 2\%$ in the (realistic) scenario of small viscosity, and by $\lesssim 30\%$ in the (unrealistic) scenario of large viscosity.

gr-qc↗

arXiv2025

General-Relativistic Lattice-Boltzmann Method for Radiation Transport

We present the first extension of the special-relativistic Lattice-Boltzmann Method for radiative transport developed by Weih et al. (2020), to solve the radiative-transfer equation in curved spacetimes. The novel approach is based on the streaming of carefully selected photons along null geodesics and interpolating their final positions, velocities, and frequency shifts to all photons in a given velocity stencil. Furthermore, by transforming between the laboratory frame, the Eulerian frame, and the fluid frame, we are able to perform the collision step in the fluid frame, thus retaining the collision operator of the special-relativistic case with only minor modifications. As a result, with the new method we can model the evolution of the frequency-independent (grey) radiation field as it interacts with a background fluid via absorption, emission, and scattering in a curved background spacetime. Finally, by introducing a refined adaptive stencil, which is suitably distorted in the direction of propagation of the photon bundle, we can reduce the computational costs of the method while improving its performance in the optically-thin regime. A number of standard and novel tests are presented to validate the approach and exhibit its robustness and accuracy.

gr-qc↗

arXiv2025

Constraining the equation of state in neutron-star cores via the long-ringdown signal

Multimessenger signals from binary neutron star (BNS) mergers are promising tools to infer the largely unknown properties of nuclear matter at densities that are presently inaccessible to laboratory experiments. The gravitational waves (GWs) emitted by BNS merger remnants, in particular, have the potential of setting tight constraints on the neutron-star equation of state (EOS) that would complement those coming from the late inspiral, direct mass-radius measurements, or ab-initio dense-matter calculations. To explore this possibility, we perform a representative series of general-relativistic simulations of BNS systems with EOSs carefully constructed so as to cover comprehensively the high-density regime of the EOS space. From these simulations, we identify a novel and tight correlation between the ratio of the energy and angular-momentum losses in the late-time portion of the post-merger signal, i.e., the ``long ringdown'', and the properties of the EOS at the highest pressures and densities in neutron-star cores. When applying this correlation to post-merger GW signals, we find a significant reduction of the EOS uncertainty at densities several times the nuclear saturation density, where no direct constraints are currently available. Hence, the long ringdown has the potential of providing new and stringent constraints on the state of matter in neutron stars in general and, in particular, in their cores.

astro-ph.HE↗

arXiv2025

Can light-rings self-gravitate?

In a spherically symmetric and static spacetime of a compact object, such as that of a Schwarzschild black hole, the light-ring is a 2-sphere where photons experience the only possible circular orbits. As a "Gedankenexperiment", we imagine an advanced civilisation able to populate the light-ring of a nonrotating black hole of mass $M$ with photons having a fine-tuned impact parameter that allows their orbits to be exactly circular with radius $r=3M$. As the number of photons in the light-ring increases in time, its mass will no longer be negligible and hence it will impact on the background spacetime, that is, it will "self-gravitate". We here consider two different routes to assign a nonzero mass to the light-ring that are either based on a discrete concentration of photons on a specific radial location or on a suitable distribution of photons in a given region. In both cases, and using the Einstein equations, we find that the inclusion of the energy from the accumulated photons leads to the generation of new light-rings. Such new light-rings can either appear at well-defined but discrete locations, or be fused in a well-defined region. In either case, we show that such light-ring configurations are dynamically unstable and a small perturbation, either via the inclusion of an additional photon onto the light-ring or via the absorption of a photon by the black hole, leads to a catastrophic destruction of the light-ring structures.

gr-qc↗

arXiv2024

The impact of resistivity on the variability of black hole accretion flows

Context. The accretion of magnetized plasma onto black holes is a complex and dynamic process, where the magnetic field plays a crucial role. The amount of magnetic flux accumulated near the event horizon significantly impacts the accretion flow behavior. Resistivity, a measure of how easily magnetic fields can dissipate, is thought to be a key factor influencing this process. This work explores the influence of resistivity on accretion flow variability. We investigate simulations reaching the magnetically arrested disk (MAD) limit and those with an initial multi-loop magnetic field configuration. Methods. We employ 3D resistive general relativistic magnetohydrodynamic (GRMHD) simulations to model the accretion process under various regimes, where resistivity has a global uniform value. Results. Our findings reveal distinct flow behaviors depending on resistivity. High resistivity simulations never achieve the MAD state, indicating a disturbed magnetic flux accumulation process. Conversely, low resistivity simulations converge towards the ideal MHD limit. The key results are: i) For the standard MAD model, resistivity plays a minimal role in flow variability, suggesting that flux eruption events dominate the dynamics. ii) High resistivity simulations exhibit strong magnetic field diffusion into the disk, rearranging efficient magnetic flux accumulation from the accretion flow. iii) In multi-loop simulations, resistivity significantly reduces flow variability, which was not expected. However, magnetic flux accumulation becomes more variable due to frequent reconnection events at very low resistivity values. Conclusions. This study shows that resistivity affects how much the flow is distorted due to magnetic field dissipation. Our findings provide new insights into the interplay between magnetic field accumulation, resistivity, variability and the dynamics of black hole accretion.

astro-ph.HE↗

arXiv2024

On the impact of neutrinos on the launching of relativistic jets from "magnetars" produced in neutron-star mergers

A significant interest has emerged recently in assessing whether collimated and ultra-relativistic outflows can be produced by a long-lived remnant from a binary neutron-star (BNS) merger, with different approaches leading to different outcomes. To clarify some of the aspect of this process, we report the results of long-term (\ie $\sim~110\,{\rm ms}$) state-of-the-art general-relativistic magnetohydrodynamics simulations of the inspiral and merger of a BNS system of magnetized stars. We find that after $\sim~50\,{\rm ms}$ from the merger, an $α$-$Ω$~dynamo driven by the magnetorotational instability (MRI) sets-in in the densest regions of the disk and leads to the breakout of the magnetic-field lines from the accretion disk around the remnant. The breakout, which can be associated with the violation of the Parker-stability criterion, is responsible for the generation of a collimated, magnetically-driven outflow with only mildly relativistic velocities that is responsible for a violent eruption of electromagnetic energy. We provide evidence that this outflow is partly collimated via a Blandford-Payne mechanism driven by the open field lines anchored in the inner disk regions. Finally, by including or not the radiative transport via neutrinos, we determine the role they play in the launching of the collimated wind. In this way, we conclude that the mechanism of magnetic-field breakout we observe is robust and takes place even without neutrinos. Contrary to previous expectations, the inclusion of neutrinos absorption and emission leads to a smaller baryon pollution in polar regions, and hence accelerates the occurrence of the breakout, yielding a larger electromagnetic luminosity. Given the mildly relativistic nature of these disk-driven breakout outflows, it is difficult to consider them responsible for the jet phenomenology observed in short gamma-ray bursts.

astro-ph.HE↗

arXiv2024

BHAC-QGP: three-dimensional MHD simulations of relativistic heavy-ion collisions, I. Methods and tests

We present BHAC-QGP, a new numerical code to simulate the evolution of matter created in heavy-ion collisions in the presence of electromagnetic fields. It is derived from the Black Hole Accretion Code (BHAC), which has been designed to model astrophysical processes in a general-relativistic magnetohydrodynamical description. As the original Black Hole Accretion Code, BHAC-QGP benefits from the use of Adaptive Mesh Refinement (AMR), which allows us to dynamically adjust the resolution where necessary, and makes use of time-dependent Milne coordinates and the ultrarelativistic equation of state, $P = e/3$. We demonstrate that BHAC-QGP accurately passes a number of systematic and rigorous tests.

hep-ph↗

arXiv2024

BHAC-QGP: three-dimensional MHD simulations of relativistic heavy-ion collisions, II. Application to Au-Au collisions

We present BHAC-QGP, a new numerical code to simulate the evolution of matter created in heavy-ion collisions. BHAC-QGP is based on the Black Hole Accretion Code (BHAC), which has been designed to model astrophysical processes through the solution of the equations of general-relativistic magnetohydrodynamics. Like the mother code, BHAC-QGP uses Adaptive Mesh Refinement (AMR), which allows for a dynamic adjustment of the resolution in regions of the computational domain where a particularly high accuracy is needed. We here discuss a number of applications of BHAC-QGP to Au-Au collisions at Relativistic Heavy-Ion Collider (RHIC) energies and show that the code is able to reproduce results of other simulations of these scenarios, but with much higher accuracy.

hep-ph↗

arXiv2024

Black hole-neutron star binaries with high spins and large mass asymmetries: I. Properties of quasi-equilibrium sequences

Black hole - neutron star (BHNS) mergers are a promising target of current gravitational-wave (GW) and electromagnetic (EM) searches, being the putative origin of ultra-relativistic jets, gamma-ray emission, and r-process nucleosynthesis. However, the possibility of any EM emission accompanying a GW detection crucially depends on the amount of baryonic mass left after the coalescence, i.e. whether the neutron star (NS) undergoes a `tidal disruption' or `plunges' into the black hole (BH) while remaining essentially intact. As the first of a series of two papers, we here report the most systematic investigation to date of quasi-equilibrium sequences of initial data across a range of stellar compactnesses $\mathcal{C}$, mass ratios $q$, BH spins $χ_{_{\rm BH}}$, and equations of state satisfying all present observational constraints. Using an improved version of the elliptic initial-data solver FUKA, we have computed more than $1000$ individual configurations and estimated the onset of mass-shedding or the crossing of the innermost stable circular orbit in terms of the corresponding characteristic orbital angular velocities $Ω_{_{\rm MS}}$ and $Ω_{_{\rm ISCO}}$ as a function of $\mathcal{C}, q$, and $χ_{_{\rm BH}}$. To the best of our knowledge, this is the first time that the dependence of these frequencies on the BH spin is investigated. In turn, by setting $Ω_{_{\rm MS}} = Ω_{_{\rm ISCO}}$ it is possible to determine the separatrix between the `tidal disruption' or `plunge' scenarios as a function of the fundamental parameters of these systems, namely, $q, \mathcal{C}$, and $χ_{_{\rm BH}}$. Finally, we present a novel analysis of quantities related to the tidal forces in the initial data and discuss their dependence on spin and separation.

gr-qc↗