256 research outputs found
Vacuum Breakdown near a Black Hole Charged by Hypercritical Accretion
We consider a black hole accreting spherically from the surrounding medium.
If accretion produces a luminosity close to the Eddington limit the hole
acquires a net charge so that electrons and ions can fall with the same
velocity. The condition for the electrostatic field to be large enough to break
the vacuum near the hole horizon translates into an upper limit for the hole
mass, The astrophysical conditions under
which this phaenomenon can take place are rather extreme, but in principle they
could be met by a mini black hole residing at the center of a star.Comment: 6 pages, accepted for publication in the Astrophysical Journa
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
Effect of Distributed Photovoltaic Generation on the Voltage Magnitude in a Self-Contained Power Supply System
A promising way to increase the technical and economic characteristics of standalone power supply systems is to incorporate renewable energy installations in their structure. This saves fuel and extends the operational life of diesel power stations. The most common option is a hybrid system with photovoltaic power stations incorporated into the local network of the diesel power station. This paper deals with the dependence of the deflection voltage and power losses in the electric power transmission line on the graphs of electrical loads, the parameters of elements of the power supply system, connection points and the capacity of distributed photovoltaic power stations. Research has been carried out on the common low-voltage power supply systems of the radial type (0.4 kV) with an installed capacity of up to 100 kW. The studies have been conducted by simulating the operating modes of hybrid power systems of various configurations. As a result of these studies recommendations to reduce losses and voltage variations in the network by selecting the power and photovoltaic power connection points have been put forward
Prospect for relic neutrino searches
Unlike the relic photons, relic neutrinos have not so far been observed. The
Cosmic Neutrino Background (CB) is the oldest relic from the Big Bang,
produced a few seconds after the Bang itself. Due to their impact in cosmology,
relic neutrinos may be revealed indireclty in the near future through
cosmological observations. In this talk we concentrate on other proposals, made
in the last 30 years, to try to detect the CB directly, either in
laboratory searches (through tiny accelerations they produce on macroscopic
targets) or through astrophysical observations (looking for absorption dips in
the flux of Ultra-High Energy neutrinos, due to the annihilation of these
neutrinos with relic neutrinos at the Z-resonance). We concentrate mainly on
the first of these two possibilities.Comment: Talk given at the Nobel Symposium on Neutrino Physics, Enkoping,
Sweden, Augus 19-24, 2004; 16 page
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
Cosmology of Axions and Moduli: A Dynamical Systems Approach
This paper is concerned with string cosmology and the dynamics of multiple
scalar fields in potentials that can become negative, and their features as
(Early) Dark Energy models. Our point of departure is the "String Axiverse", a
scenario that motivates the existence of cosmologically light axion fields as a
generic consequence of string theory. We couple such an axion to its
corresponding modulus. We give a detailed presentation of the rich cosmology of
such a model, ranging from the setting of initial conditions on the fields
during inflation, to the asymptotic future. We present some simplifying
assumptions based on the fixing of the axion decay constant , and on the
effective field theory when the modulus trajectory is adiabatic, and find the
conditions under which these assumptions break down. As a by-product of our
analysis, we find that relaxing the assumption of fixed leads to the
appearance of a new meta-stable de-Sitter region for the modulus without the
need for uplifting by an additional constant. A dynamical systems analysis
reveals the existence of many fixed point attractors, repellers and saddle
points, which we analyse in detail. We also provide geometric interpretations
of the phase space. The fixed points can be used to bound the couplings in the
model. A systematic scan of certain regions of parameter space reveals that the
future evolution of the universe in this model can be rich, containing multiple
epochs of accelerated expansion.Comment: 27 pages, 12 figures, comments welcome, v2 minor correction
Ultra-light Axions: Degeneracies with Massive Neutrinos and Forecasts for Future Cosmological Observations
A generic prediction of string theory is the existence of many axion fields.
It has recently been argued that many of these fields should be light and, like
the well known QCD axion, lead to observable cosmological consequences. In this
paper we study in detail the effect of the so-called string axiverse on large
scale structure, focusing on the morphology and evolution of density
perturbations, anisotropies in the cosmic microwave background and weak
gravitational lensing of distant galaxies. We quantify specific effects that
will arise from the presence of the axionic fields and highlight possible
degeneracies that may arise in the presence of massive neutrinos. We take
particular care understanding the different physical effects and scales that
come into play. We then forecast how the string axiverse may be constrained and
show that with a combination of different observations, it should be possible
to detect a fraction of ultralight axions to dark matter of a few percent.Comment: 24 pages, 16 figures, this version: corrected typos, some comments
added, matches published versio
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
Searching for stellar mass black holes in the solar neighborhood
We propose a strategy for searching for isolated stellar mass black holes in
the solar neighborhood with the Sloan Digital Sky Survey. Due to spherical
accretion of the inter-stellar medium and the ambient magnetic field, an
isolated black hole is expected to emit a blended, thermal synchrotron spectrum
with a roughly flat peak from the optical down to the far infra-red. We find
that the Sloan Survey will be able to detect isolated black holes, in the
considered mass range of 1--100, out to a few hundred parsecs,
depending on the local conditions of the ISM. We also find that the black holes
are photmetrically distinguishable from field stars and they have a photometry
similar to QSOs. They can be further singled out from QSO searches because they
have a featureless spectrum with no emission lines. The Sloan Survey will
likely find hundreds of objects that meet these criteria, and to further reduce
the number of candidates, we suggest other selection criteria such as infra-red
searches and proper motion measurements. Estimates indicate that dozens of
black holes may exist out to a few hundred parsecs. If no black hole candidates
are found in this survey, important limits can be placed on the local density
of black holes and the halo fraction in black holes, especially for masses
greater than about .Comment: Latex, 7 pages, 3 postscript figures, submitted to ApJ Letters. Also
available at http://fnas08.fnal.gov
Stellar-Mass Black Holes in the Solar Neighborhood
We search for nearby, isolated, accreting, ``stellar-mass'' (3 to
) black holes. Models suggest a synchrotron spectrum in visible
wavelengths and some emission in X-ray wavelengths. Of 3.7 million objects in
the Sloan Digital Sky Survey Early Data Release, about 150,000 objects have
colors and properties consistent with such a spectrum, and 87 of these objects
are X-ray sources from the ROSAT All Sky Survey. Thirty-two of these have been
confirmed not to be black-holes using optical spectra. We give the positions
and colors of these 55 black-hole candidates, and quantitatively rank them on
their likelihood to be black holes. We discuss uncertainties the expected
number of sources, and the contribution of blackholes to local dark matter.Comment: Replaced with version accepted by ApJ. 40 pages, 8 figure
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