Search results for "astronomía"
showing 10 items of 247 documents
The force-free twisted magnetosphere of a neutron star
2016
We present a detailed analysis of the properties of twisted, force-free magnetospheres of non-rotating neutron stars, which are of interest in the modelling of magnetar properties and evolution. In our models the magnetic field smoothly matches to a current-free (vacuum) solution at some large external radius, and they are specifically built to avoid pathological surface currents at any of the interfaces. By exploring a large range of parameters, we find a few remarkable general trends. We find that the total dipolar moment can be increased by up to $40\%$ with respect to a vacuum model with the same surface magnetic field, due to the contribution of magnetospheric currents to the global ma…
Instability of relativistic sheared jets and distinction between FRI and FRII sources
2002
We investigate the shear-driven instability of nonmagnetic relativistic jets with the bulk velocity, V , dependent on the cylindric radius, r. It is shown that instability can arise for any dependence of the velocity (or the Lorentz factor that is the same) on r. The shear-driven instability can e ectively operate in the whole volume of a jet. The growth time can be shorter than that of the Kelvin-Helmholtz instability. The considered instability leads to a turbulization of jets and can account for a distiction between the jets in the FRI and FRII sources. Urpin, V., Vadim.Urpin@uv.es
Deceleration of arbitrarily magnetized GRB ejecta: the complete evolution
2008
(Abridged) We aim to quantitatively understand the dynamical effect and observational signatures of magnetization of the GRB ejecta on the onset of the afterglow. We perform ultrahigh-resolution one-dimensional relativistic MHD simulations of the interaction of a radially expanding, magnetized ejecta with the interstellar medium. The need of ultrahigh numerical resolution derives from the extreme jump conditions in the region of interaction between the ejecta and the circumburst medium. We study the evolution of an ultrarelativistic shell all the way to a the self-similar asymptotic phase. Our simulations show that the complete evolution can be characterized in terms of two parameters, name…
A new general relativistic magnetohydrodynamics code for dynamical spacetimes
2008
We present a new numerical code which solves the general relativistic magneto-hydrodynamics (GRMHD) equations coupled to the Einstein equations for the evolution of a dynamical spacetime within the conformally-flat approximation. This code has been developed with the main objective of studying astrophysical scenarios in which both, high magnetic fields and strong gravitational fields appear, such as the magneto-rotational collapse of stellar cores, the collapsar model of GRBs, and the evolution of neutron stars. The code is based on an existing and thoroughly tested purely hydrodynamics code and on its extension to accommodate weakly magnetized fluids (passive magnetic field approximation).…
Local simulations of the magnetized Kelvin-Helmholtz instability in neutron-star mergers
2010
Context. Global MHD simulations show Kelvin-Helmholtz (KH) instabilities at the contact surface of two merging neutron stars. That region has been identified as the site of efficient amplification of magnetic fields. However, these global simulations, due to numerical limitations, were unable to determine the saturation level of the field strength, and thus the possible back-reaction of the magnetic field onto the flow. Aims. We investigate the amplification of initially weak fields in KH unstable shear flows, and the back-reaction of the field onto the flow. Methods. We use a high-resolution ideal MHD code to perform 2D and 3D local simulations of shear flows. Results. In 2D, the magnetic …
General relativistic simulations of pasive-magneto-rotational core collapse with microphysics
2007
This paper presents results from axisymmetric simulations of magneto-rotational stellar core collapse to neutron stars in general relativity using the passive field approximation for the magnetic field. These simulations are performed using a new general relativistic numerical code specifically designed to study this astrophysical scenario. The code is based on the conformally-flat approximation of Einstein's field equations and conservative formulations of the magneto-hydrodynamics equations. The code has been recently upgraded to incorporate a tabulated, microphysical equation of state and an approximate deleptonization scheme. This allows us to perform the most realistic simulations of m…
Internal shocks in relativistic outflows: collisions of magnetized shells
2007
(Abridged): We study the collision of magnetized irregularities (shells) in relativistic outflows in order to explain the origin of the generic phenomenology observed in the non-thermal emission of both blazars and gamma-ray bursts. We focus on the influence of the magnetic field on the collision dynamics, and we further investigate how the properties of the observed radiation depend on the strength of the initial magnetic field and on the initial internal energy density of the flow. The collisions of magnetized shells and the radiation resulting from these collisions are calculated using the 1D relativistic magnetohydrodynamics code MRGENESIS. The interaction of the shells with the externa…
Entanglement-Based dc magnetometry with separated ions
2017
We demonstrate sensing of inhomogeneous dc magnetic fields by employing entangled trapped ions, which are shuttled in a segmented Paul trap. As sensor states, we use Bell states of the type j↑↓i þ eiφj↓↑i encoded in two 40Caþ ions stored at different locations. The linear Zeeman effect leads to the accumulation of a relative phase φ, which serves for measuring the magnetic-field difference between the constituent locations. Common-mode magnetic-field fluctuations are rejected by the entangled sensor state, which gives rise to excellent sensitivity without employing dynamical decoupling and therefore enables accurate dc sensing. Consecutive measurements on sensor states encoded in the S1=2 g…
Indirect search for dark matter in the Sun and the Galactic Centre with the ANTARES neutrino telescope
2017
La búsqueda de materia oscura es actualmente uno de los temas de investigación más candentes en física fundamental. Sabemos de su posible existencia únicamente por los efectos gravitatorios que causa, pero desconocemos su naturaleza real. Una de las hipótesis más estudiadas es la de que pueda estar compuesta por partículas elementales aún no descubiertas. Diversas consideraciones sobre su abundancia actual a partir de su producción térmica en el Universo primitivo conducen a la posibilidad de que dichas partículas sean muy masivas e interaccionen muy débilmente con la materia ordinaria, lo que se ha dado en llamar WIMPs (Weakly Interacting Massive Particles). El trabajo de investigación de …
Current-induced fingering instability in magnetic domain walls
2015
The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain wall motion is driven by the spin transfer torque mechanism. A current density gradient is found either to stabilize domains with walls perpendicular to current lines or to produce finger-like patterns, depending on the domain wall motion direction. The instability mechanism is shown to result from the non-adiabatic contribution of the spin transfer torque mechanism.