2,227 research outputs found
Magnetically Accreting Isolated Old Neutron Stars
Previous work on the emission from isolated old neutron stars (IONS)
accreting the inter-stellar medium (ISM) focussed on gravitational capture -
Bondi accretion. We propose a new class of sources which accrete via magnetic
interaction with the ISM. While for the Bondi mechanism, the accretion rate
decreases with increasing NS velocity, in magnetic accretors (MAGACs="magics")
the accretion rate increases with increasing NS velocity. MAGACs will be
produced among high velocity (~> 100 km s-1) high magnetic field (B> 1e14 G)
radio pulsars - the ``magnetars'' - after they have evolved first through
magnetic dipole spin-down, followed by a ``propeller'' phase (when the object
sheds angular momentum on a timescale ~< 1e10 yr). The properties of MAGACS may
be summarized thus: dipole magnetic fields of B~>1e14 G; minimum velocities
relative to the ISM of >25-100 km s-1, depending on B, well below the median in
the observed radio-pulsar population; spin-periods of >days to years; accretion
luminosities of 1e28- 1e31 ergs s-1 ; and effective temperatures kT=0.3 - 2.5
keV if they accrete onto the magnetic polar cap. We find no examples of MAGACs
among previously observed source classes (anomalous X-ray pulsars,
soft-gamma-ray repeaters or known IONS). However, MAGACs may be more prevelant
in flux-limited X-ray catalogs than their gravitationally accreting
counterparts.Comment: ApJ, accepte
Laudatores Temporis Acti, or Why Cosmology is Alive and Well - A Reply to Disney
A recent criticism of cosmological methodology and achievements by Disney
(2000) is assessed. Some historical and epistemological fallacies in the said
article have been highlighted. It is shown that---both empirically and
epistemologically---modern cosmology lies on sounder foundations than it is
portrayed. A brief historical account demonstrates that this form of
unsatisfaction with cosmology has had a long tradition, and rather meagre
results in the course of the XX century.Comment: 11 pages, no figures; a criticism of astro-ph/0009020; Gen. Rel.
Grav., accepted for publicatio
Potential implications of practice effects in Alzheimer's disease prevention trials.
IntroductionPractice effects (PEs) present a potential confound in clinical trials with cognitive outcomes. A single-blind placebo run-in design, with repeated cognitive outcome assessments before randomization to treatment, can minimize effects of practice on trial outcome.MethodsWe investigated the potential implications of PEs in Alzheimer's disease prevention trials using placebo arm data from the Alzheimer's Disease Cooperative Study donepezil/vitamin E trial in mild cognitive impairment. Frequent ADAS-Cog measurements early in the trial allowed us to compare two competing trial designs: a 19-month trial with randomization after initial assessment, versus a 15-month trial with a 4-month single-blind placebo run-in and randomization after the second administration of the ADAS-Cog. Standard power calculations assuming a mixed-model repeated-measure analysis plan were used to calculate sample size requirements for a hypothetical future trial designed to detect a 50% slowing of cognitive decline.ResultsOn average, ADAS-Cog 13 scores improved at first follow-up, consistent with a PE and progressively worsened thereafter. The observed change for a 19-month trial (1.18 points) was substantively smaller than that for a 15-month trial with 4-month run-in (1.79 points). To detect a 50% slowing in progression under the standard design (i.e., a 0.59 point slowing), a future trial would require 3.4 times more subjects than would be required to detect the comparable percent slowing (i.e., 0.90 points) with the run-in design.DiscussionAssuming the improvement at first follow-up observed in this trial represents PEs, the rate of change from the second assessment forward is a more accurate representation of symptom progression in this population and is the appropriate reference point for describing treatment effects characterized as percent slowing of symptom progression; failure to accommodate this leads to an oversized clinical trial. We conclude that PEs are an important potential consideration when planning future trials
Solution generating theorems for perfect fluid spheres
The first static spherically symmetric perfect fluid solution with constant
density was found by Schwarzschild in 1918. Generically, perfect fluid spheres
are interesting because they are first approximations to any attempt at
building a realistic model for a general relativistic star. Over the past 90
years a confusing tangle of specific perfect fluid spheres has been discovered,
with most of these examples seemingly independent from each other. To bring
some order to this collection, we develop several new transformation theorems
that map perfect fluid spheres into perfect fluid spheres. These transformation
theorems sometimes lead to unexpected connections between previously known
perfect fluid spheres, sometimes lead to new previously unknown perfect fluid
spheres, and in general can be used to develop a systematic way of classifying
the set of all perfect fluid spheres. In addition, we develop new ``solution
generating'' theorems for the TOV, whereby any given solution can be
``deformed'' to a new solution. Because these TOV-based theorems work directly
in terms of the pressure profile and density profile it is relatively easy to
impose regularity conditions at the centre of the fluid sphere.Comment: 8 pages, no figures, to appear in the proceedings of the NEB XII
Conference (Recent Developments in Gravity), 29 June - 2 July, 2006, Napflio,
Greec
On cosmological observables in a swiss-cheese universe
Photon geodesics are calculated in a swiss-cheese model, where the cheese is
made of the usual Friedmann-Robertson-Walker solution and the holes are
constructed from a Lemaitre-Tolman-Bondi solution of Einstein's equations. The
observables on which we focus are the changes in the redshift, in the
angular-diameter--distance relation, in the luminosity-distance--redshift
relation, and in the corresponding distance modulus. We find that redshift
effects are suppressed when the hole is small because of a compensation effect
acting on the scale of half a hole resulting from the special case of spherical
symmetry. However, we find interesting effects in the calculation of the
angular distance: strong evolution of the inhomogeneities (as in the approach
to caustic formation) causes the photon path to deviate from that of the FRW
case. Therefore, the inhomogeneities are able to partly mimic the effects of a
dark-energy component. Our results also suggest that the nonlinear effects of
caustic formation in cold dark matter models may lead to interesting effects on
photon trajectories.Comment: 25 pages, 21 figures; replaced to fit the version accepted for
publication in Phys. Rev.
B2 1144+35: A Giant Low Power Radio Galaxy with Superluminal Motion
We report on centimeter VLA and VLBI observations of the giant, low power
radio galaxy 1144+35. These observations are sensitive to structures on scales
from less than 1 parsec to greater than 1 megaparsec. Diffuse steep spectrum
lobes on the megaparsec scale are consistent with an age of 10
years. On the parsec scale, a complex jet component is seen to move away from
the center of activity with an apparent velocity 2.7 h c. It shows
a central spine -- shear layer morphology. A faint parsec scale counterjet is
detected and an intrinsic jet velocity of 0.95 c and angle to the line of sight
of 25 are derived, consistent with an intrinsically symmetric ejection.
The central spine in the parsec scale jet is expected to move at a higher
velocity and a Lorentz factor 15 has been estimated near the
core.The age of this inner VLBI structure is 300 years. Assuming a
constant angle to the line-of-sight, the jet velocity is found to decrease from
0.95 c at 20 mas (32 pc on the plane of the sky) to 0.02 c at 15 arcsec (24 kpc
on the plane of the sky). These findings lend credence to the claim that (1)
even the jets of low power radio galaxies start out relativistic; and (2) these
jets are decelerated to subrelativistic velocities by the time they reach
kiloparsec scales.Comment: 21 pages, 16 separated figures. A version with figures and table in
the text is available at: ftp://terra.bo.cnr.it/papers/journals - it is a ps
gzipped file, named giovannini_apr99.gz (792kb) - ApJ in pres
Will Jets Identify the Progenitors of Type Ia Supernovae?
We use the fact that a Type Ia supernova has been serendipitously discovered
near the jet of the active galaxy 3C 78 to examine the question of whether jets
can enhance accretion onto white dwarfs. One interesting outcome of such a
jet-induced accretion process is an enhanced rate of novae in the vicinity of
jets. We present results of observations of the jet in M87 which appear to have
indeed discovered 11 novae in close proximity to the jet. We show that a
confirmation of the relation between jets and novae and Type Ia supernovae can
finally identify the elusive progenitors of Type Ia supernovae.Comment: 10 pages, 3 figure
Gravitational radiation from dynamical black holes
An effective energy tensor for gravitational radiation is identified for
uniformly expanding flows of the Hawking mass-energy. It appears in an energy
conservation law expressing the change in mass due to the energy densities of
matter and gravitational radiation, with respect to a Killing-like vector
encoding a preferred flow of time outside a black hole. In a spin-coefficient
formulation, the components of the effective energy tensor can be understood as
the energy densities of ingoing and outgoing, transverse and longitudinal
gravitational radiation. By anchoring the flow to the trapping horizon of a
black hole in a given sequence of spatial hypersurfaces, there is a locally
unique flow and a measure of gravitational radiation in the strong-field
regime.Comment: 5 revtex4 pages. Additional comment
Light-cone averages in a swiss-cheese universe
We analyze a toy swiss-cheese cosmological model to study the averaging
problem. In our model, the cheese is the EdS model and the holes are
constructed from a LTB solution. We study the propagation of photons in the
swiss-cheese model, and find a phenomenological homogeneous model to describe
observables. Following a fitting procedure based on light-cone averages, we
find that the the expansion scalar is unaffected by the inhomogeneities. This
is because of spherical symmetry. However, the light-cone average of the
density as a function of redshift is affected by inhomogeneities. The effect
arises because, as the universe evolves, a photon spends more and more time in
the (large) voids than in the (thin) high-density structures. The
phenomenological homogeneous model describing the light-cone average of the
density is similar to the concordance model. Although the sole source in the
swiss-cheese model is matter, the phenomenological homogeneous model behaves as
if it has a dark-energy component. Finally, we study how the equation of state
of the phenomenological model depends on the size of the inhomogeneities, and
find that the equation-of-state parameters w_0 and w_a follow a power-law
dependence with a scaling exponent equal to unity. That is, the equation of
state depends linearly on the distance the photon travels through voids. We
conclude that within our toy model, the holes must have a present size of about
250 Mpc to be able to mimic the concordance model.Comment: 20 pages, 14 figures; replaced to fit the version accepted for
publication in Phys. Rev.
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