1,123 research outputs found
The Effect of Sources on the Inner Horizon of Black Holes
Single pulse of null dust and colliding null dusts both transform a regular
horizon into a space-like singularity in the space of colliding waves. The
local isometry between such space-times and black holes extrapolates these
results to the realm of black holes. However, inclusion of particular scalar
fields instead of null dusts creates null singularities rather than space-like
ones on the inner horizons of black holes.Comment: Final version to appear in PR
Electrodynamics in Friedmann-Robertson-Walker Universe: Maxwell and Dirac fields in Newman-Penrose formalism
Maxwell and Dirac fields in Friedmann-Robertson-Walker spacetime is
investigated using the Newman-Penrose method. The variables are all separable,
with the angular dependence given by the spin-weighted spherical harmonics. All
the radial parts reduce to the barrier penetration problem, with mostly
repulsive potentials representing the centrifugal energies. Both the helicity
states of the photon field see the same potential, but that of the Dirac field
see different ones; one component even sees attractive potential in the open
universe. The massless fields have the usual exponential time dependencies;
that of the massive Dirac field is coupled to the evolution of the cosmic scale
factor . The case of the radiation filled flat universe is solved in terms
of the Whittaker function. A formal series solution, valid in any FRW universe,
is also presented. The energy density of the Maxwell field is explicitly shown
to scale as . The co-moving particle number density of the massless
Dirac field is found to be conserved, but that of the massive one is not.
Particles flow out of certain regions, and into others, creating regions that
are depleted of certain linear and angular momenta states, and others with
excess. Such current of charged particles would constitute an electric current
that could generate a cosmic magnetic field. In contrast, the energy density of
these massive particles still scales as .Comment: 18 pages including 9 figure
On aberration in gravitational lensing
It is known that a relative translational motion between the deflector and
the observer affects gravitational lensing. In this paper, a lens equation is
obtained to describe such effects on actual lensing observables. Results can be
easily interpreted in terms of aberration of light-rays. Both radial and
transverse motions with relativistic velocities are considered. The lens
equation is derived by first considering geodesic motion of photons in the
rest-frame Schwarzschild spacetime of the lens, and, then, light-ray detection
in the moving observer's frame. Due to the transverse motion images are
displaced and distorted in the observer's celestial sphere, whereas the radial
velocity along the line of sight causes an effective re-scaling of the lens
mass. The Einstein ring is distorted to an ellipse whereas the caustics in the
source plane are still point-like. Either for null transverse motion or up to
linear order in velocities, the critical curve is still a circle with its
radius corrected by a factor (1+z_d) with respect to the static case, z_d being
the relativistic Doppler shift of the deflector. From the observational point
of view, the orbital motion of the Earth can cause potentially observable
corrections of the order of the microarcsec in lensing towards the
super-massive black hole at the Galactic center. On a cosmological scale,
tangential peculiar velocities of cluster of galaxies bring about a typical
flexion in images of background galaxies in the weak lensing regime but future
measurements seem to be too much challenging.Comment: 8 pages, 2 figures, in press on PR
Use of isotopes for studying reaction mechanisms: 3. Secondary kinetic isotope effect
The effects of isotopic substitution on equilibrium constants and reaction rates in processes which do not directly involve the isotopic atom are described. In particular, the mechanistic details which can be obtained by quantifying the secondary kinetic isotope effect and steric isotope effect are illustrated
Temperature dependent photoluminescence of organic semiconductors with varying backbone conformation
We present photoluminescence studies as a function of temperature from a
series of conjugated polymers and a conjugated molecule with distinctly
different backbone conformations. The organic materials investigated here are:
planar methylated ladder type poly para-phenylene, semi-planar polyfluorene,
and non-planar para hexaphenyl. In the longer-chain polymers the
photoluminescence transition energies blue shift with increasing temperatures.
The conjugated molecules, on the other hand, red shift their transition
energies with increasing temperatures. Empirical models that explain the
temperature dependence of the band gap energies in inorganic semiconductors can
be extended to explain the temperature dependence of the transition energies in
conjugated molecules.Comment: 8 pages, 9 figure
R-modes of neutron stars with the superfluid core
We investigate the modal properties of the -modes of rotating neutron
stars with the core filled with neutron and proton superfluids, taking account
of entrainment effects between the superfluids. The stability of the -modes
against gravitational radiation reaction is also examined considering viscous
dissipation due to shear and a damping mechanism called mutual friction between
the superfluids in the core. We find the -modes in the superfluid core are
split into ordinary -modes and superfluid -modes, which we call,
respectively, - and -modes. The two superfluids in the core flow
together for the -modes, while they counter-move for the -modes. For
the -modes, the coefficient is equal to , almost independent of
the parameter that parameterizes the entrainment effects between the
superfluids, where is the angular frequency of rotation, the
oscillation frequency observed in the corotating frame of the star, and
and are the indices of the spherical harmonic function
representing the angular dependence of the -modes. For the -modes, on
the other hand, is equal to at
(no entrainment), and it almost linearly increases as is increased from
. The mutual friction in the superfluid core is found ineffective to
stabilize the -mode instability caused by the -mode except in a few
narrow regions of . The -mode instability caused by the -modes,
on the other hand, is extremely weak and easily damped by dissipative processes
in the star.Comment: 22 pages, 22 figures, accepted for publication in the Astrophysical
Journa
On the radiative equilibrium of a stellar atmosphere. IV
In this paper the solution to the problem of line formation given in an earlier paper is obtained in its numerical form in the first three approximations. Tables for computing the residual intensity are given
Chandrasekhar-Kendall functions in astrophysical dynamos
Some of the contributions of Chandrasekhar to the field of
magnetohydrodynamics are highlighted. Particular emphasis is placed on the
Chandrasekhar-Kendall functions that allow a decomposition of a vector field
into right- and left-handed contributions. Magnetic energy spectra of both
contributions are shown for a new set of helically forced simulations at
resolutions higher than what has been available so far. For a forcing function
with positive helicity, these simulations show a forward cascade of the
right-handed contributions to the magnetic field and nonlocal inverse transfer
for the left-handed contributions. The speed of inverse transfer is shown to
decrease with increasing value of the magnetic Reynolds number.Comment: 10 pages, 5 figures, proceedings of the Chandrasekhar Centenary
Conference, to be published in PRAMANA - Journal of Physic
The response of a turbulent accretion disc to an imposed epicyclic shearing motion
We excite an epicyclic motion, whose amplitude depends on the vertical
position, , in a simulation of a turbulent accretion disc. An epicyclic
motion of this kind may be caused by a warping of the disc. By studying how the
epicyclic motion decays we can obtain information about the interaction between
the warp and the disc turbulence. A high amplitude epicyclic motion decays
first by exciting inertial waves through a parametric instability, but its
subsequent exponential damping may be reproduced by a turbulent viscosity. We
estimate the effective viscosity parameter, , pertaining to
such a vertical shear. We also gain new information on the properties of the
disc turbulence in general, and measure the usual viscosity parameter,
, pertaining to a horizontal (Keplerian) shear. We find that,
as is often assumed in theoretical studies, is approximately
equal to and both are much less than unity, for the field
strengths achieved in our local box calculations of turbulence. In view of the
smallness () of and we conclude
that for the timescale for diffusion
or damping of a warp is much shorter than the usual viscous timescale. Finally,
we review the astrophysical implications.Comment: 12 pages, 18 figures, MNRAS accepte
Axial perturbations of general spherically symmetric spacetimes
The aim of this paper is to present a governing equation for first order
axial metric perturbations of general, not necessarily static, spherically
symmetric spacetimes. Under the non-restrictive assumption of axisymmetric
perturbations, the governing equation is shown to be a two-dimensional wave
equation where the wave function serves as a twist potential for the
axisymmetry generating Killing vector. This wave equation can be written in a
form which is formally a very simple generalization of the Regge-Wheeler
equation governing the axial perturbations of a Schwarzschild black hole, but
in general the equation is accompanied by a source term related to matter
perturbations. The case of a viscous fluid is studied in particular detail.Comment: 16 pages, no figures, minor correction
- …