402 research outputs found
Physical properties of Tolman-Bayin solutions: some cases of static charged fluid spheres in general relativity
In this article, Einstein-Maxwell space-time has been considered in
connection to some of the astrophysical solutions as previously obtained by
Tolman (1939) and Bayin (1978). The effect of inclusion of charge into these
solutions has been investigated thoroughly and also the nature of fluid
pressure and mass density throughout the sphere have been discussed.
Mass-radius and mass-charge relations have been derived for various cases of
the charged matter distribution. Two cases are obtained where perfect fluid
with positive pressures give rise to electromagnetic mass models such that
gravitational mass is of purely electromagnetic origin.Comment: 15 pages, 12 figure
Trans-sonic propeller stage
We follow the approach used by Davies and Pringle (1981) and discuss the
trans-sonic substage of the propeller regime. This substage is intermediate
between the supersonic and subsonic propeller substages. In the trans-sonic
regime an envelope around a magnetosphere of a neutron star passes through a
kind of a reorganization process. The envelope in this regime consists of two
parts. In the bottom one turbulent motions are subsonic. Then at some distance
the turbulent velocity becomes equal to the sound velocity.
During this substage the boundary propagates outwards till it
reaches the outer boundary, and so the subsonic regime starts.
We found that the trans-sonic substage is unstable, so the transition between
supersonic and subsonic substages proceeds on the dynamical time scale. For
realistic parameters this time is in the range from weeks to years.Comment: 8 pages with figures, submitted to Astron. Astroph. Transaction
Three-Dimensional Magnetohydrodynamic Simulations of Spherical Accretion
We present three-dimensional numerical magnetohydrodynamic simulations of
radiatively inefficient spherical accretion onto a black hole. The simulations
are initialized with a Bondi flow, and with a weak, dynamically unimportant,
large-scale magnetic field. The magnetic field is amplified as the gas flows
in. When the magnetic pressure approaches equipartition with the gas pressure,
the field begins to reconnect and the gas is heated up. The heated gas is
buoyant and moves outward, causing line stretching of the frozen-in magnetic
field. This leads to further reconnection, and more heating and
buoyancy-induced motions, so that the flow makes a transition to a state of
self-sustained convection. The radial structure of the flow changes
dramatically from its initial Bondi profile, and the mass accretion rate onto
the black hole decreases significantly. Motivated by the numerical results, we
develop a simplified analytical model of a radiatively inefficient spherical
flow in which convective transport of energy to large radii plays an important
role. In this "convection-dominated Bondi flow" the accretion velocity is
highly subsonic and the density varies with radius as ~R^{-1/2} rather than the
standard Bondi scaling ~R^{-3/2}. We estimate that the mass accretion rate onto
the black hole is significantly less than the Bondi accretion rate.
Convection-dominated Bondi flows may be relevant for understanding many
astrophysical phenomena, e.g. post-supernova fallback and radiatively
inefficient accretion onto supermassive black holes, stellar-mass black holes
and neutron stars.Comment: 23 pages, 6 figures, submitted to Ap
Where Are All The Fallback Disks? Constraints on Propeller Systems
Fallback disks are expected to form around new-born neutron stars following a
supernova explosion. In almost all cases, the disk will pass through a
propeller stage. If the neutron star is spinning rapidly (initial period ms) and has an ordinary magnetic moment ( G cm), the
rotational power transferred to the disk by the magnetic field of the neutron
star will exceed the Eddington limit by many orders of magnitude, and the disk
will be rapidly disrupted. Fallback disks can thus survive only around
slow-born neutron stars and around black holes, assuming the latter do not
torque their surrounding disks as strongly as do neutron stars. This might
explain the apparent rarity of fallback disks around young compact objects.Comment: Submitted to Astrophysical Journal Letter
Sequential piezoresponse force microscopy and the 'small-data' problem
The term big-data in the context of materials science not only stands for the volume, but also for the heterogeneous nature of the characterization data-sets. This is a common problem in combinatorial searches in materials science, as well as chemistry. However, these data-sets may well be 'small' in terms of limited step-size of the measurement variables. Due to this limitation, application of higher-order statistics is not effective, and the choice of a suitable unsupervised learning method is restricted to those utilizing lower-order statistics. As an interesting case study, we present here variable magnetic-field Piezoresponse Force Microscopy (PFM) study of composite multiferroics, where due to experimental limitations the magnetic field dependence of piezoresponse is registered with a coarse step-size. An efficient extraction of this dependence, which corresponds to the local magnetoelectric effect, forms the central problem of this work. We evaluate the performance of Principal Component Analysis (PCA) as a simple unsupervised learning technique, by pre-labeling possible patterns in the data using Density Based Clustering (DBSCAN). Based on this combinational analysis, we highlight how PCA using non-central second-moment can be useful in such cases for extracting information about the local material response and the corresponding spatial distribution
Three-dimensional MHD Simulations of Radiatively Inefficient Accretion Flows
We present three-dimensional MHD simulations of rotating radiatively
inefficient accretion flows onto black holes. In the simulations, we
continuously inject magnetized matter into the computational domain near the
outer boundary, and we run the calculations long enough for the resulting
accretion flow to reach a quasi-steady state. We have studied two limiting
cases for the geometry of the injected magnetic field: pure toroidal field and
pure poloidal field. In the case of toroidal field injection, the accreting
matter forms a nearly axisymmetric, geometrically-thick, turbulent accretion
disk. The disk resembles in many respects the convection-dominated accretion
flows found in previous numerical and analytical investigations of viscous
hydrodynamic flows. Models with poloidal field injection evolve through two
distinct phases. In an initial transient phase, the flow forms a relatively
flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow.
However, when the flow later achieves steady state, it changes in character
completely. The magnetized accreting gas becomes two-phase, with most of the
volume being dominated by a strong dipolar magnetic field from which a thermal
low-density wind flows out. Accretion occurs mainly via narrow slowly-rotating
radial streams which `diffuse' through the magnetic field with the help of
magnetic reconnection events.Comment: 35 pages including 3 built-in plots and 14 separate jpg-plots;
version accepted by Ap
Magnetic Field Limitations on Advection Dominated Flows
Recent papers discussing advection dominated accretion flows (ADAF) as a
solution for astrophysical accretion problems should be treated with some
caution because of their uncertain physical basis. The suggestions underlying
ADAF involve ignoring the magnetic field reconnection in heating of the plasma
flow, assuming electron heating due only to binary Coulomb collisions with
ions. Here, we analyze the physical processes in optically thin accretion flows
at low accretion rates including the influence of an equipartition turbulent
magnetic field. For these conditions there is continuous destruction of
magnetic flux by reconnection.
The reconnection is expected to significantly heat the electrons which can
efficiently emit magnetobremstrahlung radiation. Because of this electron
emission, the radiative efficiency of the ADAF is not small. We suggest that
the small luminosities of nearby galactic black holes is due to outflows rather
than ADAF accretion.Comment: 7 pages, 3 figures, Submitted to Ap
Relativistic Structure, Stability and Gravitational Collapse of Charged Neutron Stars
Charged stars have the potential of becoming charged black holes or even
naked singularities. It is presented a set of numerical solutions of the
Tolman-Oppenheimer-Volkov equations that represents spherical charged compact
stars in hydrostatic equilibrium. The stellar models obtained are evolved
forward in time integrating the Einstein-Maxwell field equations. It is assumed
an equation of state of a neutron gas at zero temperature. The charge
distribution is taken as been proportional to the rest mass density
distribution. The set of solutions present an unstable branch, even with charge
to mass ratios arbitrarily close to the extremum case. It is performed a direct
check of the stability of the solutions under strong perturbations, and for
different values of the charge to mass ratio. The stars that are in the stable
branch oscillates and do not collapse, while models in the unstable branch
collapse directly to form black holes. Stars with a charge greater or equal
than the extreme value explode. When a charged star is suddenly discharged, it
don't necessarily collapse to form a black hole. A non-linear effect that gives
rise to the formation of an external shell of matter (see Ghezzi and Letelier
2005), is negligible in the present simulations. The results are in agreement
with the third law of black hole thermodynamics and with the cosmic censorship
conjecture.Comment: 27 pages, 14 figures, 4 tables, paper accepte
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