1,725 research outputs found
The Current Ability to Test Theories of Gravity with Black Hole Shadows
Our Galactic Center, Sagittarius A* (Sgr A*), is believed to harbour a
supermassive black hole (BH), as suggested by observations tracking individual
orbiting stars. Upcoming sub-millimetre very-long-baseline-interferometry
(VLBI) images of Sgr A* carried out by the Event-Horizon-Telescope
Collaboration (EHTC) are expected to provide critical evidence for the
existence of this supermassive BH. We assess our present ability to use EHTC
images to determine if they correspond to a Kerr BH as predicted by Einstein's
theory of general relativity (GR) or to a BH in alternative theories of
gravity. To this end, we perform general-relativistic magnetohydrodynamical
(GRMHD) simulations and use general-relativistic radiative transfer (GRRT)
calculations to generate synthetic shadow images of a magnetised accretion flow
onto a Kerr BH. In addition, and for the first time, we perform GRMHD
simulations and GRRT calculations for a dilaton BH, which we take as a
representative solution of an alternative theory of gravity. Adopting the VLBI
configuration from the 2017 EHTC campaign, we find that it could be extremely
difficult to distinguish between BHs from different theories of gravity, thus
highlighting that great caution is needed when interpreting BH images as tests
of GR.Comment: Published in Nature Astronomy on 16.04.18 (including supplementary
information); simulations at https://blackholecam.org/telling_bhs_apart
How to tell an accreting boson star from a black hole
The capability of the Event Horizon Telescope (EHT) to image the nearest
supermassive black hole candidates at horizon-scale resolutions offers a novel
means to study gravity in its strongest regimes and to test different models
for these objects. Here, we study the observational appearance at 230 GHz of a
surfaceless black hole mimicker, namely a non-rotating boson star, in a
scenario consistent with the properties of the accretion flow onto Sgr A*. To
this end, we perform general relativistic magnetohydrodynamic simulations
followed by general relativistic radiative transfer calculations in the boson
star space-time. Synthetic reconstructed images considering realistic
astronomical observing conditions show that, despite qualitative similarities,
the differences in the appearance of a black hole -- either rotating or not --
and a boson star of the type considered here are large enough to be detectable.
These differences arise from dynamical effects directly related to the absence
of an event horizon, in particular, the accumulation of matter in the form of a
small torus or a spheroidal cloud in the interior of the boson star, and the
absence of an evacuated high-magnetization funnel in the polar regions. The
mechanism behind these effects is general enough to apply to other horizonless
and surfaceless black hole mimickers, strengthening confidence in the ability
of the EHT to identify such objects via radio observations.Comment: 16 pages, 12 figures. Published in MNRAS. Adding more information in
the form of appendices, and a new simulation of a different boson star model.
The conclusions do not chang
Classification of the LHC BLM Ionization Chamber
The LHC beam loss monitoring (BLM) system must prevent the super conducting magnets from quenching and protect the machine components from damage. The main monitor type is an ionization chamber. About 4000 of them will be installed around the ring. The lost beam particles initiate hadronic showers through the magnets and other machine components. These shower particles are measured by the monitors installed on the outside of the accelerator equipment. For the calibration of the BLM system the signal response of the ionization chamber is simulated in GEANT4 for all relevant particle types and energies (keV to TeV range). For validation, the simulations are compared to measurements using protons, neutrons, photons and mixed radiation fields at various energies and intensities. This paper will focus on the signal response of the ionization chamber to various particle types and energies including space charge effects at high ionization densities
Post-Newtonian accurate parametric solution to the dynamics of spinning compact binaries in eccentric orbits: The leading order spin-orbit interaction
We derive Keplerian-type parametrization for the solution of post-Newtonian
(PN) accurate conservative dynamics of spinning compact binaries moving in
eccentric orbits. The PN accurate dynamics that we consider consists of the
third post-Newtonian accurate conservative orbital dynamics influenced by the
leading order spin effects, namely the leading order spin-orbit interactions.
The orbital elements of the representation are explicitly given in terms of the
conserved orbital energy, angular momentum and a quantity that characterizes
the leading order spin-orbit interactions in Arnowitt, Deser, and Misner-type
coordinates. Our parametric solution is applicable in the following two
distinct cases: (i) the binary consists of equal mass compact objects, having
two arbitrary spins, and (ii) the binary consists of compact objects of
arbitrary mass, where only one of them is spinning with an arbitrary spin. As
an application of our parametrization, we present gravitational wave
polarizations, whose amplitudes are restricted to the leading quadrupolar
order, suitable to describe gravitational radiation from spinning compact
binaries moving in eccentric orbits. The present parametrization will be
required to construct `ready to use' reference templates for gravitational
waves from spinning compact binaries in inspiralling eccentric orbits. Our
parametric solution for the post-Newtonian accurate conservative dynamics of
spinning compact binaries clearly indicates, for the cases considered, the
absence of chaos in these systems. Finally, we note that our parametrization
provides the first step in deriving a fully second post-Newtonian accurate
`timing formula', that may be useful for the radio observations of relativistic
binary pulsars like J0737-3039.Comment: 18 pages, accepted by Phys. Rev.
Creating an "enabling environment" for taking insecticide treated nets to national scale: the Tanzanian experience
INTRODUCTION: Malaria is the largest cause of health services attendance, hospital admissions and child deaths in Tanzania. At the Abuja Summit in April 2000 Tanzania committed itself to protect 60% of its population at high risk of malaria by 2005. The country is, therefore, determined to ensure that sustainable malaria control using insecticide-treated nets is carried out on a national scale. CASE DESCRIPTION: Tanzania has been involved for two decades in the research process for developing insecticide-treated nets as a malaria control tool, from testing insecticides and net types, to assessing their efficacy and effectiveness, and exploring new ways of distribution. Since 2000, the emphasis has changed from a project approach to that of a concerted multi-stakeholder action for taking insecticide-treated nets to national scale (NATNETS). This means creating conditions that make insecticide-treated nets accessible and affordable to all those at risk of malaria in the country. This paper describes Tanzania's experience in (1) creating an enabling environment for insecticide-treated nets scale-up, (2) promoting the development of a commercial sector for insecticide-treated nets, and (3) targeting pregnant women with highly subsidized insecticide-treated nets through a national voucher scheme. As a result, nearly 2 million insecticide-treated nets and 2.2 million re-treatment kits were distributed in 2004. CONCLUSION: National upscaling of insecticide-treated nets is possible when the programme is well designed, coordinated and supported by committed stakeholders; the Abuja target of protecting 60% of those at high risk is feasible, even for large endemic countries
Circular holonomy in the Taub-NUT spacetime
Parallel transport around closed circular orbits in the equatorial plane of
the Taub-NUT spacetime is analyzed to reveal the effect of the gravitomagnetic
monopole parameter on circular holonomy transformations. Investigating the
boost/rotation decomposition of the connection 1-form matrix evaluated along
these orbits, one finds a situation that reflects the behavior of the general
orthogonally transitive stationary axisymmetric case and indeed along Killing
trajectories in general.Comment: 9 pages, LaTeX iopart class, no figure
Ballistic matter waves with angular momentum: Exact solutions and applications
An alternative description of quantum scattering processes rests on
inhomogeneous terms amended to the Schroedinger equation. We detail the
structure of sources that give rise to multipole scattering waves of definite
angular momentum, and introduce pointlike multipole sources as their limiting
case. Partial wave theory is recovered for freely propagating particles. We
obtain novel results for ballistic scattering in an external uniform force
field, where we provide analytical solutions for both the scattering waves and
the integrated particle flux. Our theory directly applies to p-wave
photodetachment in an electric field. Furthermore, illustrating the effects of
extended sources, we predict some properties of vortex-bearing atom laser beams
outcoupled from a rotating Bose-Einstein condensate under the influence of
gravity.Comment: 42 pages, 8 figures, extended version including photodetachment and
semiclassical theor
Propagation of charged particle waves in a uniform magnetic field
This paper considers the probability density and current distributions
generated by a point-like, isotropic source of monoenergetic charges embedded
into a uniform magnetic field environment. Electron sources of this kind have
been realized in recent photodetachment microscopy experiments. Unlike the
total photocurrent cross section, which is largely understood, the spatial
profiles of charge and current emitted by the source display an unexpected
hierarchy of complex patterns, even though the distributions, apart from
scaling, depend only on a single physical parameter. We examine the electron
dynamics both by solving the quantum problem, i. e., finding the energy Green
function, and from a semiclassical perspective based on the simple cyclotron
orbits followed by the electron. Simulations suggest that the semiclassical
method, which involves here interference between an infinite set of paths,
faithfully reproduces the features observed in the quantum solution, even in
extreme circumstances, and lends itself to an interpretation of some (though
not all) of the rich structure exhibited in this simple problem.Comment: 39 pages, 16 figure
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