Magnetised Polish doughnuts revisited

We discuss a procedure to build new sequences of magnetised, equilibrium tori around Kerr black holes which combines two approaches previously considered in the literature. For simplicity we assume that the test-fluid approximation holds, and hence we neglect the self-gravity of the fluid. The models are built assuming a particular form of the angular momentum distribution from which the location and morphology of equipotential surfaces can be computed. This ansatz includes, in particular, the constant angular momentum case originally employed in the construction of thick tori - or Polish doughnuts - and it has already been used to build equilibrium sequences of purely hydrodynamical models. We discuss the properties of the new models and their dependence on the initial parameters. These new sequences can be used as initial data for magnetohydrodynamical evolutions in general relativity.Comment: 9 pages, 6 figures. Accepted for publication in Astronomy & Astrophysics July 12, 201

Magnetized accretion disks around compact objects

Los recientes avances observacionales logrados por la colaboración LIGO-Virgo- KAGRA – con las primeras detecciones históricas de ondas gravitacionales de fusiones de sistemas binarios de objetos compactos – junto con la colaboración Event Horizon Telescope - con la imagen del agujero negro supermasivo que reside en el centro de la galaxia M87 - han brindado una oportunidad sin precedentes para investigar la física de la gravedad en el regimen de campo intenso e incluso analizar la Teoría General de la Relatividad de Einstein en esta situación tan extrema. En esta tesis se discute un caso particular de sistema astrofísico relativista que suele considerarse como el ejemplo paradigmático de sistema en el régimen de campo intenso – un agujero negro rodeado de un disco grueso de acreción (o toro). Se presentan nuevas prescripciones para construir datos iniciales de discos de acreción magnetizados alrededor de objetos compactos, extendiendo el estado actual del tema en varias direcciones, a saber, acomodando diferentes configuraciones de campo magnético, distribuciones de momento angular, y tipos de espacio-tiempo (incluyendo agujeros negros de Kerr, agujeros negros con pelo escalar y agujeros negros de Yukawa) sobre los que evoluciona el fluido. Los resultados obtenidos en esta tesis nos proporcionan valiosa información sobre los efectos que tiene añadir diferentes fenómenos físicos en las propiedades morfológicas y físicas del sistema. Además, los resultados de esta investigación también proporcionan una amplia muestra de datos iniciales de dominio público para que puedan ser usados en simulaciones evolutivas del sistema toro de acreción-agujero negro, gobernado por las ecuaciones de la magnetohidrodinámica relativista .The recent observational breakthroughs accomplished by the LIGO-Virgo-KAGRA Collaboration – with the historical first detections of gravitational waves from mergers of compact binaries – and by the Event Horizon Telescope Collaboration – with the image of the supermassive black hole lurking in the centre of the M87 galaxy – have provided an unprecedented opportunity to investigate the physics of strong gravity and to even test Einstein’s General Theory of Relativity in such a extreme situation. This thesis discusses one particular relativistic astrophysical system which is often invoked as a paradigmatic example of such strong-gravity realm – a black hole surrounded by a geometrically thick accretion disk (or torus). New prescriptions to build initial data of magnetized accretion disks around compact objects are presented, extending the current state-of-the-art in several directions, namely accommodating diverse magnetic field configurations, angular momentum distributions, and types of spacetimes (including Kerr black holes, black holes with scalar hair, and Yukawa black holes) where the fluid evolves. The results reported in this thesis provide insight on the eff ects that adding diff erent physics has in the system’s morphological and physical properties. In addition, this research o ers large new samples of open-source initial data to conduct time-dependent general-relativistic, magneto-hydrodynamical simulations of black hole-torus systems

Self-gravitating magnetised tori around black holes in general relativity

We investigate stationary, self-gravitating, magnetised disks (or tori) around black holes. The models are obtained by numerically solving the coupled system of the Einstein equations and the equations of ideal general-relativistic magnetohydrodynamics. The mathematical formulation and numerical aspects of our approach are similar to those reported in previous works modeling stationary self-gravitating perfect-fluid tori, but the inclusion of magnetic fields represents a new ingredient. Following previous studies of purely hydrodynamical configurations, we construct our models assuming Keplerian rotation in the disks and both spinning and spinless black holes. We focus on the case of a toroidal distribution of the magnetic field and build a large set of models corresponding to a wide range of values of the magnetisation parameter, starting with weakly magnetised disks and ending at configurations in which the magnetic pressure dominates over the thermal one. In all our models, the magnetic field affects the equilibrium structure of the torus mainly due to the magnetic pressure. In particular, an increasing contribution of the magnetic field shifts the location of the maximum of the rest-mass density towards inner regions of the disk. The total mass of the system and the angular momentum are affected by the magnetic field in a complex way, that depends on the black hole spin and the location of the inner radius of the disk. The non-linear dynamical stability of the solutions presented in this paper will be reported elsewhere.Comment: 17 pages, 5 figures, 1 tabl

Magnetized accretion disks around Kerr black holes with scalar hair: nonconstant angular momentum disks

We present new equilibrium solutions of stationary models of magnetized thick disks (or tori) around Kerr black holes with synchronised scalar hair. The models reported here largely extend our previous results based on constant radial distributions of the specific angular momentum along the equatorial plane. We introduce a new way to prescribe the distribution of the disk's angular momentum based on a combination of two previous proposals and compute the angular momentum distribution outside the equatorial plane by resorting to the construction of von Zeipel cylinders. We find that the effect of the scalar hair on the black hole spacetime can yield significant differences in the disk morphology and properties compared to what is found if the spacetime is purely Kerr. Some of the tori built within the most extreme, background hairy black hole spacetime of our sample exhibit the appearance of two maxima in the gravitational energy density which impacts the radial profile distributions of the disk's thermodynamical quantities. The models reported in this paper can be used as initial data for numerical evolutions with GRMHD codes to study their stability properties. Moreover, they can be employed as illuminating sources to build shadows of Kerr black holes with scalar hair which might help further constrain the no-hair hypothesis as new observational data is collected.publishe

Magnetized accretion disks around Kerr black holes with scalar hair: Constant angular momentum disks

Testing the true nature of black holes - the no-hair hypothesis - will become increasingly more precise in the next few years as new observational data is collected in both the gravitational-wave channel and the electromagnetic channel. In this paper we consider numerically generated spacetimes of Kerr black holes with synchronized scalar hair and build stationary models of magnetized thick disks (or tori) around them. Our approach assumes that the disks are not self-gravitating, they obey a polytropic equation of state, the distribution of their specific angular momentum is constant, and they are marginally stable, i.e., the disks completely fill their Roche lobe. Moreover, contrary to existing approaches in the literature, our models are thermodynamically relativist, as the specific enthalpy of the fluid can adopt values significantly larger than unity. We study the dependence of the morphology and properties of the accretion tori on the type of black hole considered, from purely Kerr black holes with varying degrees of spin parameter, namely from a Schwarzschild black hole to a nearly extremal Kerr case, to Kerr black holes with scalar hair with different Arnowitt-Deser-Misner mass and horizon angular velocity. Comparisons between the disk properties for both types of black holes are presented. The sequences of magnetized, equilibrium disks around Kerr black holes with scalar hair discussed in this study are morphologically and thermodynamically different than their Kerr black hole counterparts, namely their vertical size is larger, the high-density central region is more extended, and the fluid is more relativistic. Therefore, we expect significant differences to appear when these sequences are used as initial data for numerical relativity codes to investigate their dynamical (nonlinear) stability and used in tandem with ray-tracing codes to obtain synthetic images of black holes (i.e., shadows) in astrophysically relevant situations where the light source is provided by an emitting accretion diskpublishe

Magnetised tori with magnetic polarisation around Kerr black holes: variable angular momentum discs

Analytical models of magnetised, geometrically thick discs are relevant to understand the physical conditions of plasma around compact objects and to explore its emitting properties. This has become increasingly important in recent years in the light of the Event Horizon Telescope observations of Sgr A* and M87. Models of thick discs around black holes usually consider constant angular momentum distributions and do not take into account the magnetic response of the fluid to applied magnetic fields. We present a generalisation of our previous work on stationary models of magnetised accretion discs with magnetic polarisation (Pimentel et al. 2018). This extension is achieved by accounting for non-constant specific angular momentum profiles, done through a two-parameter ansatz for those distributions. We build a large number of new equilibrium solutions of thick discs with magnetic polarisation around Kerr black holes, selecting suitable parameter values within the intrinsically substantial parameter space of the models. We study the morphology and the physical properties of those solutions, finding qualitative changes with respect to the constant angular momentum tori of (Pimentel et al. 2018). However, the dependences found on the angular momentum distribution or on the black hole spin do not seem to be strong. Some of the new solutions, however, exhibit a local maximum of the magnetisation function, absent in standard magnetised tori. Due to the enhanced development of the magneto-rotational instability as a result of magnetic susceptibility, those models might be particularly well-suited to investigate jet formation through general-relativistic MHD simulations. The new equilibrium solutions reported here can be used as initial data in numerical codes to assess the impact of magnetic susceptibility in the dynamics and observational properties of thick disc-black hole systems.Comment: 15 pages, 8 figures. Comments welcom

Multipactor radiation analysis within a waveguide region based on a frequency-domain representation of the dynamics of charged particles

A technique for the accurate computation of the electromagnetic fields radiated by a charged particle moving within a parallel-plate waveguide is presented. Based on a transformation of the time-varying current density of the particle into a time-harmonic current density, this technique allows the evaluation of the radiated electromagnetic fields both in the frequency and time domains, as well as in the near- and far-field regions. For this purpose, several accelerated versions of the parallel-plate Green’s function in the frequency domain have been considered. The theory has been successfully applied to the multipactor discharge occurring within a two metal-plates region. The proposed formulation has been tested with a particle-in-cell code based on the finite- difference time-domain method, obtaining good agreement.The authors would like to thank ESA/ESTEC for having funded this research activity through the Contract “RF Breakdown in Multicarrier Systems” ͑Contract No. 19918/06/NL/GLC͒

An analytical model to evaluate the radiated power spectrum of a multipactor discharge in a parallel-plate region

This paper is aimed at studying the electromagnetic radiation pattern of a multipactor discharge occurring in a parallel-plate waveguide. The proposed method is based on the Fourier expansion of the multipactor current in terms of timeharmonic currents radiating in the parallel-plate region. Classical radiation theory combined with the frequency domain Green’s function of the problem allows the calculation of both the electric and the magnetic radiated fields. A novel analytical formula for the total radiated power of each multipactor harmonic has been derived. This formula is suitable for predicting multipactor with the third-harmonic technique. The proposed formulation has been successfully tested with a particle-in-cell code