203 research outputs found
CCOs and the hidden magnetic field scenario
CCOs are X-ray sources lying close the center of supernova remnants, with
inferred values of the surface magnetic fields significantly lower (less than
about 1e11 G) than those of standard pulsars. In this paper, we revise the
hidden magnetic field scenario, presenting the first 2D simulations of the
submergence and reemergence of the magnetic field in the crust of a neutron
star. A post-supernova accretion stage of about 1e-4-1e-3 solar masses over a
vast region of the surface is required to bury the magnetic field into the
inner crust. When accretion stops, the field reemerges on a typical timescale
of 1-100 kyr, depending on the submergence conditions. After this stage, the
surface magnetic field is restored close to its birth values. A possible
observable consequence of the hidden magnetic field is the anisotropy of the
surface temperature distribution, in agreement with observations of several of
these sources. We conclude that the hidden magnetic field model is viable as
alternative to the anti-magnetar scenario, and it could provide the missing
link between CCOs and the other classes of isolated neutron stars.Comment: 7 pages, 7 figures, MNRA
The exact solution of the Riemann problem with non-zero tangential velocities in relativistic hydrodynamics
We have generalised the exact solution of the Riemann problem in special
relativistic hydrodynamics for arbitrary tangential flow velocities. The
solution is obtained by solving the jump conditions across shocks plus an
ordinary differential equation arising from the self-similarity condition along
rarefaction waves, in a similar way as in purely normal flow. The dependence of
the solution on the tangential velocities is analysed, and the impact of this
result on the development of multidimensional relativistic hydrodynamic codes
(of Godunov type) is discussed.Comment: 26 pages, 4 figures. Accepted for publication in Journal of Fluid
Mechanic
Hyperbolic character of the angular moment equations of radiative transfer and numerical methods
We study the mathematical character of the angular moment equations of
radiative transfer in spherical symmetry and conclude that the system is
hyperbolic for general forms of the closure relation found in the literature.
Hyperbolicity and causality preservation lead to mathematical conditions
allowing to establish a useful characterization of the closure relations. We
apply numerical methods specifically designed to solve hyperbolic systems of
conservation laws (the so-called Godunov-type methods), to calculate numerical
solutions of the radiation transport equations in a static background. The
feasibility of the method in any kind of regime, from diffusion to
free-streaming, is demonstrated by a number of numerical tests and the effect
of the choice of the closure relation on the results is discussed.Comment: 37 pags, 12 figures, accepted for publication in MNRA
Triggering magnetar outbursts in 3D force-free simulations
In this letter, we present the first 3D force-free general relativity
simulations of the magnetosphere dynamics related to the magnetar
outburst/flare phenomenology. Starting from an initial dipole configuration, we
adiabatically increase the helicity by twisting the footprints of a spot on the
stellar surface and follow the succession of quasi-equilibrium states until a
critical twist is reached. Twisting beyond that point triggers instabilities
that results in the rapid expansion of magnetic field lines, followed by
reconnection, as observed in previous axi-symmetric simulations. If the
injection of magnetic helicity goes on, the process is recurrent, periodically
releasing a similar amount of energy, of the order of a few % of the total
magnetic energy. From our current distribution, we estimate the local
temperature assuming that dissipation occurs mainly in the highly resistive
outermost layer of the neutron star. We find that the temperature smoothly
increases with injected twist, being larger for spots located in the tropical
regions than in polar regions, and rather independent of their sizes. After the
injection of helicity ceases, the magnetosphere relaxes to a new stable state,
in which the persistent currents maintain the footprints area slightly hotter
than before the onset of the instability.Comment: 6 pages, 5 figure
Evolution of Proto-Neutron Stars with Quarks
Neutrino fluxes from proto-neutron stars with and without quarks are studied.
Observable differences become apparent after 10--20 s of evolution.
Sufficiently massive stars containing negatively-charged, strongly interacting,
particles collapse to black holes during the first minute of evolution. Since
the neutrino flux vanishes when a black hole forms, this is the most obvious
signal that quarks (or other types of strange matter) have appeared. The
metastability timescales for stars with quarks are intermediate between those
containing hyperons and kaon condensates.Comment: 4 pages including 4 figures. Version with minor revisions. To be
published in Physical Review Letter
Numerical 3+1 general relativistic magnetohydrodynamics: a local characteristic approach
We present a general procedure to solve numerically the general relativistic
magnetohydrodynamics (GRMHD) equations within the framework of the 3+1
formalism. The work reported here extends our previous investigation in general
relativistic hydrodynamics (Banyuls et al. 1997) where magnetic fields were not
considered. The GRMHD equations are written in conservative form to exploit
their hyperbolic character in the solution procedure. All theoretical
ingredients necessary to build up high-resolution shock-capturing schemes based
on the solution of local Riemann problems (i.e. Godunov-type schemes) are
described. In particular, we use a renormalized set of regular eigenvectors of
the flux Jacobians of the relativistic magnetohydrodynamics equations. In
addition, the paper describes a procedure based on the equivalence principle of
general relativity that allows the use of Riemann solvers designed for special
relativistic magnetohydrodynamics in GRMHD. Our formulation and numerical
methodology are assessed by performing various test simulations recently
considered by different authors. These include magnetized shock tubes,
spherical accretion onto a Schwarzschild black hole, equatorial accretion onto
a Kerr black hole, and magnetized thick accretion disks around a black hole
prone to the magnetorotational instability.Comment: 18 pages, 8 figures, submitted to Ap
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