913 research outputs found
GRMHD in axisymmetric dynamical spacetimes: the X-ECHO code
We present a new numerical code, X-ECHO, for general relativistic
magnetohydrodynamics (GRMHD) in dynamical spacetimes. This is aimed at studying
astrophysical situations where strong gravity and magnetic fields are both
supposed to play an important role, such as for the evolution of magnetized
neutron stars or for the gravitational collapse of the magnetized rotating
cores of massive stars, which is the astrophysical scenario believed to
eventually lead to (long) GRB events. The code is based on the extension of the
Eulerian conservative high-order (ECHO) scheme [Del Zanna et al., A&A 473, 11
(2007)] for GRMHD, here coupled to a novel solver for the Einstein equations in
the extended conformally flat condition (XCFC). We fully exploit the 3+1
Eulerian formalism, so that all the equations are written in terms of familiar
3D vectors and tensors alone, we adopt spherical coordinates for the conformal
background metric, and we consider axisymmetric spacetimes and fluid
configurations. The GRMHD conservation laws are solved by means of
shock-capturing methods within a finite-difference discretization, whereas, on
the same numerical grid, the Einstein elliptic equations are treated by
resorting to spherical harmonics decomposition and solved, for each harmonic,
by inverting band diagonal matrices. As a side product, we build and make
available to the community a code to produce GRMHD axisymmetric equilibria for
polytropic relativistic stars in the presence of differential rotation and a
purely toroidal magnetic field. This uses the same XCFC metric solver of the
main code and has been named XNS. Both XNS and the full X-ECHO codes are
validated through several tests of astrophysical interest.Comment: 18 pages, 9 figures, accepted for publication in A&
Dynamo action in thick disks around Kerr black holes: high-order resistive GRMHD simulations
We present the first kinematic study of an -dynamo in the
General Relativistic Magneto-HydroDynamics (GRMHD) regime, applied to thick
disks orbiting around Kerr black holes and using a fully covariant mean field
dynamo closure for the Ohm law. We show that the -dynamo
mechanism leads to a continuous exponential growth of the magnetic field within
the disk and to the formation of dynamo waves drifting away or toward the
equatorial plane. Since the evolution of the magnetic field occurs
qualitatively in the same fashion as in the Sun, we present also butterfly
diagrams that characterize our models and show the establishment of an
additional timescale, which depends on the microscopic properties of the
turbulent motions, possibly providing an alternative explanation to
periodicities observed in many high-energy astrophysical sources where
accretion onto a rotating black hole is believed to operate.Comment: 5 pages, 4 figures. Accepted for publication in MNRA
Axisymmetric equilibrium models for magnetised neutron stars in Scalar-Tensor Theories
Among the possible extensions of General Relativity that have been put
forward in order to address some long standing issues in our understanding of
the Universe, Scalar-Tensor Theories have received a lot of attention for their
simplicity. Interestingly, some of these predict a potentially observable
non-linear phenomenon, known as \textit{spontaneous scalarisation}, in the
presence of highly compact matter distributions, like the case of neutron
stars. Neutron stars are ideal laboratories to investigate the properties of
matter under extreme conditions, and in particular they are known to harbour
the strongest magnetic fields in the Universe. Here, for the first time, we
present a detailed study of magnetised neutron stars in Scalar-Tensor Theories.
First, we show that the formalism developed for the study of magnetised neutron
stars in General Relativity, based on the \textit{eXtended Conformally Flat
Condition}, can easily be extended in the presence of a non-minimally coupled
scalar field, retaining many of its numerical advantages. We then carry out a
study of the parameter space considering the two extreme geometries of purely
toroidal and purely poloidal magnetic fields, varying both the strength of the
magnetic field and the intensity of scalarisation. We compare our results with
magnetised general-relativistic solutions and un-magnetised scalarised
solutions, showing how the mutual interplay between magnetic and scalar fields
affect the magnetic and the scalarisation properties of neutron stars. In
particular, we focus our discussion on magnetic deformability, maximum mass and
range of scalarisation.Comment: accepted for publication by A&A; minor language corrections; minor
typos correctio
General relativistic models for rotating magnetized neutron stars in conformally flat spacetime
The extraordinary energetic activity of magnetars is usually explained in
terms of dissipation of a huge internal magnetic field of the order of
G. How such a strong magnetic field can originate during the
formation of a neutron star is still subject of active research. An important
role can be played by fast rotation: if magnetars are born as millisecond
rotators dynamo mechanisms may efficiently amplify the magnetic field inherited
from the progenitor star during the collapse. In this case, the combination of
rapid rotation and strong magnetic field determine the right physical condition
not only for the development of a powerful jet driven explosion, manifesting as
a gamma ray burst, but also for a copious gravitational waves emission. Strong
magnetic fields are indeed able to induce substantial quadrupolar deformations
in the star. In this paper we analyze the joint effect of rotation and
magnetization on the structure of a polytropic and axisymmetric neutron star,
within the ideal magneto-hydrodynamic regime. We will consider either purely
toroidal or purely poloidal magnetic field geometries. Through the sampling of
a large parameter space, we generalize previous results in literature,
inferring new quantitative relations that allow for a parametrization of the
induced deformation, that takes into account also the effects due to the
stellar compactness and the current distribution. Finally, in the case of
purely poloidal field, we also discuss how different prescriptions on the
surface charge distribution (a gauge freedom) modify the properties of the
surrounding electrosphere and its physical implications.Comment: 25 pages, 17 figures, 6 tables, accepted for publication in MNRA
Recommended from our members
Surface constraints on the depth of the Atlantic meridional overturning circulation: Southern Ocean versus North Atlantic
Paleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean
Modeling the structure of magnetic fields in Neutron Stars: from the interior to the magnetosphere
The phenomenology of the emission of pulsars and magnetars depends
dramatically on the structure and properties of their magnetic field. In
particular it is believed that the outbursting and flaring activity observed in
AXPs and SRGs is strongly related to their internal magnetic field. Recent
observations have moreover shown that charges are present in their
magnetospheres supporting the idea that their magnetic field is tightly twisted
in the vicinity of the star. In principle these objects offer a unique
opportunity to investigate physics in a regime beyond what can be obtained in
the laboratory. We will discuss the properties of equilibrium models of
magnetized neutron stars, and we will show how internal and external currents
can be related. These magnetic field configurations will be discussed
considering also their stability, relevant for their origin and possibly
connected to events like SNe and GRBs. We will also show what kind of
deformations they induce in the star, that could lead to emission of
gravitational waves. In the case of a twisted magnetosphere we will show how
the amount of twist regulates their general topology. A general formalism based
on the simultaneous numerical solution of the general relativistic
Grad-Shafranov equation and Einstein equations will be presented.Comment: 9 pages, 3 figures, Proceedings of the 10th International Conference
on Numerical Modeling of Space Plasma Flows, 8-12 June 2015, Avignon, Franc
The role of currents distribution in general relativistic equilibria of magnetized neutron stars
Magnetic fields play a critical role in the phenomenology of neutron stars.
There is virtually no observable aspect which is not governed by them. Despite
this, only recently efforts have been done to model magnetic fields in the
correct general relativistic regime, characteristic of these compact objects.
In this work we present, for the first time a comprehensive and detailed
parameter study, in general relativity, of the role that the current
distribution, and the related magnetic field structure, have in determining the
precise structure of neutron stars. In particular, we show how the presence of
localized currents can modify the field strength at the stellar surface, and we
look for general trends, both in terms of energetic properties, and magnetic
field configurations. Here we verify that, among other things, for a large
class of different current distributions the resulting magnetic configurations
are always dominated by the poloidal component of the current.Comment: 14 pages, 13 figures, accepted for publication in MNRA
- …