Relativistic Astrophysics
Relativistic Astrophysics is the field of astrophysics dealing with compact objects such as neutron stars and black holes. The study of its different areas, such as High-Energy Astrophysics or Gravitational-Wave Astronomy, relies on the Theory of Relativity, either Special or General, and requires solving the relativistic magneto-hydrodynamic (MHD) equations for the matter and magnetic fields, Einstein's field equations for a dynamical gravitational field, and transport equations for neutrinos and EM radiation. Progress in Relativistic Astrophysics has traditionally been driven by the analysis of the data collected by high-energy telescopes and satellites. New facilities as gravitational-wave detectors are already boosting this progress as well, and neutrino detectors will also likely contribute in the near future. On the other hand, theoretical studies are needed to explain those observations. Theoretical astrophysics relies on numerical simulations to improve our understanding of the dynamics of astrophysical systems. The continuous improvements in computing and software technologies make possible large scale numerical simulations within a relativistic framework, in which our group has a long and successful track record.
A particular active area of our research is the modelling and interpretation of relativistic jets. The origin of the emission from AGN jets and the effect on the host galaxy and its surroundings is a hot topic of research in extragalactic astrophysics. These relativistic outflows are only directly detectable via their observed radiation, while their true physical properties remain hidden. Currently, the most promising method to study these outflows is by performing MHD simulations in either special or general relativity, and computing the synthetic emission from the fluid, topics in which members of our group have been working for more than twenty-five years.
These are the specific goals of this line of research of the project:
Extragalactic jets at parsec scales
We shall continue our work on the modelling of the most compact regions in extragalactic jets to unveil the nature of the outflows and their constituents (underlying flow, stationary and traveling radio components, magnetic field structure). One of the most important topics here is the study of the propagation of perturbations along the jet, which manifest as superluminal radio components in Very Long Baseline Interferometry (VLBI) observations. Initial steady jet models with the desired properties (internal shocks, radial variations, transversal structure) can be produced with the quasi-1D-RMHD code mentioned above and then injected (suitably perturbed) in the axially symmetric or fully 3D-RMHD codes. Our new 3D-RMHD code will be applied to the study of jet dynamics close to supermassive BH where jets are generated by means of MHD processes. With our new 3D-RMHD code we will also start exploring the stability of magnetized (relativistic) flows against (magnetized) Kelvin-Helmholtz and current-driven perturbations.
In the last years, a fraction of our work has focused on the interpretation of observations of the inner regions of jets in close collaboration with observational teams. Recent works along this line include the theoretical interpretation of (mm and sub-mm) VLBI, VLBA, LOFAR, and space-VLBI (RadioAstron) observations of a number of radio galaxies. This line of work will continue in the present project.
Jets, stars and the host galaxy
The interaction between jet outflows and clouds and/or stellar winds has been proposed as a scenario not only related to the production of very-high-energy radiation, and the load of the flow with baryonic matter, which could be ultimately associated to neutrino production in blazars, but also with jet deceleration between the inner hundred pc and several kpc away from the active nucleus. The deceleration and decollimation of FRI radio galaxies is still under debate, although there are hints that it is produced by small-scale processes affecting the boundaries between the jet and the ambient medium and propagating inwards via turbulent mixing. It has been recently proposed that the stars crossing the jet boundaries as they orbit the galactic nucleus, could trigger perturbations at the boundaries that would ultimately produce mixing and the development of a global mixing layer. We plan to test this theoretical model via numerical simulations.
Our new version of the code including hydrogen ionization/recombination physics allows us to set up more realistic ambient media within the inner kpc, and handle the dramatically different conditions within the hot, relativistic jet flows, composed of electron-positron pairs mainly, and the cold, dense ambient medium. We plan to improve our set-ups to make them as similar as possible to the interstellar medium in the narrow line regions (NLR) of active galaxies.
Extragalactic jets at kiloparsec scales
We will continue our work on jet evolution at large scales, namely, tens to hundreds of kpc, aiming to understand the role played by galactic activity in the evolution of their host galaxies and their environments. our plan is to extend our research to low-power jets, much more numerous in the Universe, and to improve our simulations by introducing realistic environmental conditions (galactic profiles) derived from cosmological simulations (discussed in line of research #2), and different jet injection properties. We plan to run 3D simulations of very large-scale jets with the aim to characterize the effects of these objects on the galaxy cluster environments. Additionally, we will study the effect of jets as a trigger for star formation and diffuse light in clusters. Finally, in connection with the line of research #2, we shall include the effect of relativistic jets in an improved phenomenological model to implement AGN feedback in cosmological simulations.
Relativistic flows in X-ray and Gamma-ray binary stars
Evolved binary stars represent sources of high and very high energy radiation. In the case of X-ray binaries, radiation is known to be emitted by the accretion-disc-jet system, but the details of jet propagation and impact of the jets on the ambient media has not been studied in depth so far. In addition, for gamma-ray binary stars, the source of emission is still under debate. We aim to apply our new RMHD code to the study of these relativistic outflows and winds in binary stars, building on our previous work using purely relativistic hydrodynamic simulations.
Numerical Relativity - Formulations and methods
To understand the physics of compact binary coalescences and make quantitative predictions about their properties (such as equation of state (EOS), post-merger remnants, and gravitational wave and electromagnetic signatures) numerical relativity (NR) is required. Our group has many years expertise in large-scale supercomputing applications in the field of NR and, specifically, in the development of new formulations of Einstein's equations, in particular in constraint formulations. Not only the form of the resulting system of equations is crucial to guarantee good mathematical properties; equally important is to employ high-order methods for solving the Einstein equations (evolution plus constraint equations) and high-resolution shock-capturing schemes for solving the relativistic fluid and MHD equations for non-vacuum dynamical space-times. The specific objectives in this line of work are: (a) Reformulation of Einstein's equations in the Fully Constrained Formulation to guarantee mathematical properties and improve their solution by taking into account the post-Newtonian expansion of the different terms; (b) Exploration of the horizon excision technique in the modelling of spherically symmetric black hole spacetimes in a general way and analysis of its extension to the full 3D case with no symmetries; (c) Development of numerical methods for hyperbolic equations with stiff source terms, specifically designed for the Relativistic Resistive MHD equations and also for the Boltzmann equations for neutrino transport in the M1 approximation.
NR simulations of BNS mergers and post-merger HMNS remnants
NR simulations of BNS mergers show that the outcome depends primarily on the masses of the individual stars and on the EOS. Below a certain threshold mass a long-lived remnant, a hypermassive NS (HMNS), may form. Through the study of its gravitational-wave emission, precious information on the finite-temperature EOS of supranuclear matter can be obtained. In this project we will focus on the evolution of HMNS by conducting NR simulations accounting for the amplification of the magnetic field, finite-temperature effects on the EOS, and neutrino transport. We will exploit, in particular, a new sub-grid model recently developed by our group to probe the effects of the magnetic field ammplification on the dynamical ejection of matter (kilonova) and on the launching of magnetically-driven jets, hence probing possible EM counterparts of gravitational-wave signals in BNS mergers. In addition, in this project we will introduce a generic set of microscopic finite-temperature EOS to quantify their impact in BNS simulations. In particular, we will investigate whether convection-driven inertial modes appear in long-term simulations of HMNS when finite-temperature EOS and neutrino transport are used, to understand differences with previous studies conducted by the group based on piecewise polytropic EOS and probe the effects of magnetic viscosity.
Multimessenger aspects of neutron star crustal fracture and magnetars
Neutron stars have an external crust composed of exotic nuclei, expected to be in a solid, crystalline phase. From a nuclear physics perspective, the modelling of the crust is a significant challenge, with few observational constraints. Reecent work has conjectured that the resonant shattering of this solid crust in the inspiral process of a BNS merger could generate both (a) a strong, detectable signal in the form of a homogeneous EM flare and (b) a modification in the GW signal itself. A similar scenario can be found in isolated magnetars, which exhibit intense X-ray activity in the form of flares and outbursts linked to crustal fractures and magnetospheric instabilities. In this project we will model the excitation of the crust-core i-mode and its coupling to the f-mode, relevant for GW signals, using perturbation analysis. We will also compute the energy required to crack the crust and excite high-energy emission with different crust-core interface models to assess the relevance of nuclear physics input and will estimate the GW emission from crustal fractures developed by tidal interaction, comparing it with the GW emission during isolated magnetar flares.
Accretion disks and shadows of compact objects
Testing the nature of gravity in the strong-field regime is becoming increasingly possible using gravitational-wave and electromagnetic observations. Regarding the latter, the Event Horizon Telescope (EHT) Collaboration has recently resolved the shadows of the supermassive compact objects at the centre of M87 and our own galaxy. The EHT observations also allow to test modified theories of gravity, by studying the shadows from accretion disks around black holes in such theories and comparing them with the black hole solutions of general relativity. These experimental efforts place within observational reach the exploration of additional proposals for compact astrophysical objects, collectively known as Exotic Compact Objects (ECO). In this project we will employ the BHOSS code to compute shadows from different systems of interest in which the source of light is an accretion disk: those include viscous accretions disks around Schwarzschild and Kerr black holes, fully dynamical models corresponding to evolutions of BH-torus systems in F(R) gravity with a Yukawa-like potential, as well as accretion tori around ECO such as boson stars and Proca stars.
