468 research outputs found
Directionality effects in the transfer of X-rays from a magnetized atmosphere: Beam pulse shape
A formalism is presented for radiation transfer in two normal polarization modes in finite and semiinfinite plane parallel uniform atmospheres with a magnetic field perpendicular to the surface and arbitrary propagation angles. This method is based on the coupled integral equations of transfer, including emission, absorption, and scattering. Calculations are performed for atmosphere parameters typical of X-ray pulsars. The directionality of the escaping radiation is investigated for several cases, varying the input distributions. Theoretical pencil beam profiles and X-ray pulse shapes are obtained assuming the radiation is emitted from the polar caps of spinning neutron stars. Implications for realistic models of accreting magnetized X-ray sources are briefly discussed
Numerical approach for high precision 3-D relativistic star models
A multi-domain spectral method for computing very high precision 3-D stellar
models is presented. The boundary of each domain is chosen in order to coincide
with a physical discontinuity (e.g. the star's surface). In addition, a
regularization procedure is introduced to deal with the infinite derivatives on
the boundary that may appear in the density field when stiff equations of state
are used. Consequently all the physical fields are smooth functions on each
domain and the spectral method is absolutely free of any Gibbs phenomenon,
which yields to a very high precision. The power of this method is demonstrated
by direct comparison with analytical solutions such as MacLaurin spheroids and
Roche ellipsoids. The relative numerical error reveals to be of the order of
. This approach has been developed for the study of relativistic
inspiralling binaries. It may be applied to a wider class of astrophysical
problems such as the study of relativistic rotating stars too.Comment: Minor changes, Phys. Rev. D in pres
Inertial modes in stratified rotating neutron stars : An evolutionary description
With (non-barotropic) equations of state valid even when the neutron, proton
and electron content of neutron star cores is not in beta equilibrium, we study
inertial and composition gravity modes of relativistic rotating neutron stars.
We solve the relativistic Euler equations in the time domain with a three
dimensional numerical code based on spectral methods, in the slow rotation,
relativistic Cowling and anelastic approximations. Principally, after a short
description of the gravity modes due to smooth composition gradients, we focus
our analysis on the question of how the inertial modes are affected by
non-barotropicity of the nuclear matter. In our study, the deviation with
respect to barotropicity results from the frozen composition of non-superfluid
matter composed of neutrons, protons and electrons, when beta equilibrium is
broken by millisecond oscillations. We show that already for moderatly fast
rotating stars the increasing coupling between polar and axial modes makes
those two cases less different than for very slowly rotating stars. In
addition, as we directly solve the Euler equations, without coupling only a few
number of spherical harmonics, we always found, for the models that we use, a
discrete spectrum for the inertial mode. Finally, we find that, for
non-barotropic stars, the frequency of this mode, which is our main focus,
decreases in a non-negligible way, whereas the time dependence of the energy
transfer between polar and axial modes is substantially different due to the
existence of low-frequencies gravity modes.Comment: 34 pages, 24 figures, published versio
Darwin-Riemann problems in general relativity
A review is given of recent results about the computation of irrotational
Darwin-Riemann configurations in general relativity. Such configurations are
expected to represent fairly well the late stages of inspiralling binary
neutron stars.Comment: 20 pages, 11 PostScript figures, uses PTPTeX, to appear in the
Proceedings of Yukawa International Seminar 99 "Black Holes and Gravitational
Waves", edited by T. Nakamura & H. Kodama, Prog. Theor. Phys. Supp
Analytical representations of unified equations of state of neutron-star matter
Analytical representations are derived for two equations of state (EOSs) of
neutron-star matter: FPS and SLy. Each of these EOSs is unified, that is, it
describes the crust and the core of a neutron star using the same physical
model. Two versions of the EOS parametrization are considered. In the first
one, pressure and mass density are given as functions of the baryon density. In
the second version, pressure, mass density, and baryon density are given as
functions of the pseudo-enthalpy, which makes this representation particularly
useful for 2-D calculations of stationary rotating configurations of neutron
stars.Comment: 7 pages, 5 figures, 3 tables, accepted by A&A. In v.2, auxiliary fits
(15) and (16) are correcte
Jacobi-like bar mode instability of relativistic rotating bodies
We perform some numerical study of the secular triaxial instability of
rigidly rotating homogeneous fluid bodies in general relativity. In the
Newtonian limit, this instability arises at the bifurcation point between the
Maclaurin and Jacobi sequences. It can be driven in astrophysical systems by
viscous dissipation. We locate the onset of instability along several constant
baryon mass sequences of uniformly rotating axisymmetric bodies for compaction
parameter . We find that general relativity weakens the Jacobi
like bar mode instability, but the stabilizing effect is not very strong.
According to our analysis the critical value of the ratio of the kinetic energy
to the absolute value of the gravitational potential energy for compaction parameter as high as 0.275 is only 30% higher than the
Newtonian value. The critical value of the eccentricity depends very weakly on
the degree of relativity and for is only 2% larger than the
Newtonian value at the onset for the secular bar mode instability. We compare
our numerical results with recent analytical investigations based on the
post-Newtonian expansion.Comment: 15 pages, 8 figures, submitted to Phys. Rev.
Rotating star initial data for a constrained scheme in numerical relativity
A new numerical code for computing stationary axisymmetric rapidly rotating
stars in general relativity is presented. The formulation is based on a fully
constrained-evolution scheme for 3+1 numerical relativity using the Dirac gauge
and maximal slicing. We use both the polytropic and MIT bag model equations of
state to demonstrate that the code can construct rapidly rotating neutron star
and strange star models. We compare numerical models obtained by our code and a
well-established code, which uses a different gauge condition, and show that
the two codes agree to high accuracy.Comment: Minor changes and one figure added. Version accepted for publication
in Class. Quant. Gra
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