19 research outputs found
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 colaboracioÌn LIGO-Virgo- KAGRA â con las primeras detecciones histoÌricas de ondas gravitacionales de fusiones de sistemas binarios de objetos compactos â junto con la colaboracioÌn 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 fiÌsica de la gravedad en el regimen de campo intenso e incluso analizar la TeoriÌa General de la Relatividad de Einstein en esta situacioÌn tan extrema. En esta tesis se discute un caso particular de sistema astrofiÌsico relativista que suele considerarse como el ejemplo paradigmaÌtico de sistema en el reÌgimen de campo intenso â un agujero negro rodeado de un disco grueso de acrecioÌn (o toro). Se presentan nuevas prescripciones para construir datos iniciales de discos de acrecioÌn magnetizados alrededor de objetos compactos, extendiendo el estado actual del tema en varias direcciones, a saber, acomodando diferentes configuraciones de campo magneÌtico, 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 informacioÌn sobre los efectos que tiene anÌadir diferentes fenoÌmenos fiÌsicos en las propiedades morfoloÌgicas y fiÌsicas del sistema. AdemaÌs, los resultados de esta investigacioÌn tambieÌn proporcionan una amplia muestra de datos iniciales de dominio puÌblico para que puedan ser usados en simulaciones evolutivas del sistema toro de acrecioÌn-agujero negro, gobernado por las ecuaciones de la magnetohidrodinaÌmica relativista .The recent observational breakthroughs accomplished by the LIGO-Virgo-KAGRA Collaboration â with the historical first detections of gravitational waves from mergers of compact binaries â and by the Event Horizon Telescope Collaboration â with the image of the supermassive black hole lurking in the centre of the M87 galaxy â have provided an unprecedented opportunity to investigate the physics of strong gravity and to even test Einsteinâs 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 â 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 systemâs 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 ïŹelds 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 ïŹelds both in the frequency and time domains, as well as in the near- and far-ïŹeld regions. For
this purpose, several accelerated versions of the parallel-plate Greenâs 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 ïŹnite-
difference time-domain method, obtaining good agreement.The authors would like to thank ESA/ESTEC for having funded this research activity through the Contract âRF
Breakdown in Multicarrier Systemsâ ÍContract No. 19918/06/NL/GLCÍ
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 Greenâs
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