156 research outputs found
Non-vacuum Solutions of Bianchi Type VI_0 Universe in f(R) Gravity
In this paper, we solve the field equations in metric f(R) gravity for
Bianchi type VI_0 spacetime and discuss evolution of the expanding universe. We
find two types of non-vacuum solutions by taking isotropic and anisotropic
fluids as the source of matter and dark energy. The physical behavior of these
solutions is analyzed and compared in the future evolution with the help of
some physical and geometrical parameters. It is concluded that in the presence
of isotropic fluid, the model has singularity at and represents
continuously expanding shearing universe currently entering into phantom phase.
In anisotropic fluid, the model has no initial singularity and exhibits the
uniform accelerating expansion. However, the spacetime does not achieve
isotropy as in both of these solutions.Comment: 20 pages, 5 figures, accepted for publication in Astrophys. Space Sc
Two-arm haptic force-feedbacked aid for the shoulder and elbow telerehabilitation
In this paper, we present a telerehabilitation system aiming to help the physiotherapists for the shoulder and elbow treatment. It is based on a two-arm haptic force feedback to avoid excessive efforts and discomfort with the spinal column. This system, remotely controlled by smart phone, has been validated by a physiotherapist with the help of muscular effort measurements (EMG)
Measurement of the Nucleon Structure Function F2 in the Nuclear Medium and Evaluation of its Moments
We report on the measurement of inclusive electron scattering off a carbon
target performed with CLAS at Jefferson Laboratory. A combination of three
different beam energies 1.161, 2.261 and 4.461 GeV allowed us to reach an
invariant mass of the final-state hadronic system W~2.4 GeV with four-momentum
transfers Q2 ranging from 0.2 to 5 GeV2. These data, together with previous
measurements of the inclusive electron scattering off proton and deuteron,
which cover a similar continuous two-dimensional region of Q2 and Bjorken
variable x, permit the study of nuclear modifications of the nucleon structure.
By using these, as well as other world data, we evaluated the F2 structure
function and its moments. Using an OPE-based twist expansion, we studied the
Q2-evolution of the moments, obtaining a separation of the leading-twist and
the total higher-twist terms. The carbon-to-deuteron ratio of the leading-twist
contributions to the F2 moments exhibits the well known EMC effect, compatible
with that discovered previously in x-space. The total higher-twist term in the
carbon nucleus appears, although with large systematic uncertainites, to be
smaller with respect to the deuteron case for n<7, suggesting partial parton
deconfinement in nuclear matter. We speculate that the spatial extension of the
nucleon is changed when it is immersed in the nuclear medium.Comment: 37 pages, 15 figure
Precise Measurements of Beam Spin Asymmetries in Semi-Inclusive production
We present studies of single-spin asymmetries for neutral pion
electroproduction in semi-inclusive deep-inelastic scattering of 5.776 GeV
polarized electrons from an unpolarized hydrogen target, using the CEBAF Large
Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator
Facility. A substantial amplitude has been measured in the
distribution of the cross section asymmetry as a function of the azimuthal
angle of the produced neutral pion. The dependence of this amplitude
on Bjorken and on the pion transverse momentum is extracted with
significantly higher precision than previous data and is compared to model
calculations.Comment: to be submitted PL
The disk of FU Orionis viewed with MATISSE/VLTI:First interferometric observations in <i>L</i> and <i>M</i> bands
Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI
We present the first long baseline mid-infrared interferometric observations of the circumstellar disks surrounding Herbig Ae/Be stars. The observations were obtained using the mid-infrared interferometric instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI on Cerro Paranal. The 102 m baseline given by the telescopes UT1 and UT3 was employed, which provides a maximum full spatial resolution of 20 milli-arcsec (mas) at a wavelength of 10 μm. The interferometric signal was spectrally dispersed at a resolution of 30, giving spectrally resolved visibility information from 8 μm to 13.5 μm. We observed seven nearby Herbig Ae/Be stars and resolved all objects. The warm dust disk of HD 100546 could even be resolved in single-telescope imaging. Characteristic dimensions of the emitting regions at 10 μm are found to be from 1 AU to 10 AU. The 10 μm sizes of our sample stars correlate with the slope of the 10–25 μm infrared spectrum in the sense that the reddest objects are the largest ones. Such a correlation would be consistent with a different geometry in terms of flaring or flat (self-shadowed) disks for sources with strong or moderate mid-infrared excess, respectively. We compare the observed spectrally resolved visibilities with predictions based on existing models of passive centrally irradiated hydrostatic disks made to fit the SEDs of the observed stars. We find broad qualitative agreement of the spectral shape of visibilities corresponding to these models with our observations. Quantitatively, there are discrepancies that show the need for a next step in modelling of circumstellar disks, satisfying both the spatial constraints such as are now available from the MIDI observations and the flux constraints from the SEDs in a consistent way
Strong evidences of hadron acceleration in Tycho's Supernova Remnant
Very recent gamma-ray observations of G120.1+1.4 (Tycho's) supernova remnant
(SNR) by Fermi-LAT and VERITAS provided new fundamental pieces of information
for understanding particle acceleration and non-thermal emission in SNRs. We
want to outline a coherent description of Tycho's properties in terms of SNR
evolution, shock hydrodynamics and multi-wavelength emission by accounting for
particle acceleration at the forward shock via first order Fermi mechanism. We
adopt here a quick and reliable semi-analytical approach to non-linear
diffusive shock acceleration which includes magnetic field amplification due to
resonant streaming instability and the dynamical backreaction on the shock of
both cosmic rays (CRs) and self-generated magnetic turbulence. We find that
Tycho's forward shock is accelerating protons up to at least 500 TeV,
channelling into CRs about the 10 per cent of its kinetic energy. Moreover, the
CR-induced streaming instability is consistent with all the observational
evidences indicating a very efficient magnetic field amplification (up to ~300
micro Gauss). In such a strong magnetic field the velocity of the Alfv\'en
waves scattering CRs in the upstream is expected to be enhanced and to make
accelerated particles feel an effective compression factor lower than 4, in
turn leading to an energy spectrum steeper than the standard prediction
{\propto} E^-2. This latter effect is crucial to explain the GeV-to-TeV
gamma-ray spectrum as due to the decay of neutral pions produced in nuclear
collisions between accelerated nuclei and the background gas. The
self-consistency of such an hadronic scenario, along with the fact that the
concurrent leptonic mechanism cannot reproduce both the shape and the
normalization of the detected the gamma-ray emission, represents the first
clear and direct radiative evidence that hadron acceleration occurs efficiently
in young Galactic SNRs.Comment: Minor changes. Accepted for publication in Astronomy & Astrophysic
Particle acceleration mechanisms
We review the possible mechanisms for production of non-thermal electrons
which are responsible for non-thermal radiation in clusters of galaxies. Our
primary focus is on non-thermal Bremsstrahlung and inverse Compton scattering,
that produce hard X-ray emission. We briefly review acceleration mechanisms and
point out that in most astrophysical situations, and in particular for the
intracluster medium, shocks, turbulence and plasma waves play a crucial role.
We consider two scenarios for production of non-thermal radiation. The first is
hard X-ray emission due to non-thermal Bremsstrahlung by nonrelativistic
particles. Non-thermal tails are produced by accelerating electrons from the
background plasma with an initial Maxwellian distribution. However, these tails
are accompanied by significant heating and they are present for a short time of
<10^6 yr, which is also the time that the tail will be thermalised. Such
non-thermal tails, even if possible, can only explain the hard X-ray but not
the radio emission which needs GeV or higher energy electrons. For these and
for production of hard X-rays by the inverse Compton model, we need the second
scenario where there is injection and subsequent acceleration of relativistic
electrons. It is shown that a steady state situation, for example arising from
secondary electrons produced from cosmic ray proton scattering by background
protons, will most likely lead to flatter than required electron spectra or it
requires a short escape time of the electrons from the cluster. An episodic
injection of relativistic electrons, presumably from galaxies or AGN, and/or
episodic generation of turbulence and shocks by mergers can result in an
electron spectrum consistent with observations but for only a short period of
less than one billion years.Comment: 22 pages, 5 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 11; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
The last stand before MAP: cosmological parameters from lensing, CMB and galaxy clustering
Cosmic shear measurements have now improved to the point where they deserve
to be treated on par with CMB and galaxy clustering data for cosmological
parameter analysis, using the full measured aperture mass variance curve rather
than a mere phenomenological parametrization thereof. We perform a detailed
9-parameter analysis of recent lensing (RCS), CMB (up to Archeops) and galaxy
clustering (2dF) data, both separately and jointly. CMB and 2dF data are
consistent with a simple flat adiabatic scale-invariant model with
Omega_Lambda=0.72+/-0.09, omega_cdm=0.115+/- 0.013, omega_b=0.024+/-0.003, and
a hint of reionization around z~8. Lensing helps further tighten these
constraints, but reveals tension regarding the power spectrum normalization:
including the RCS survey results raises sigma8 significantly and forces other
parameters to uncomfortable values. Indeed, sigma8 is emerging as the currently
most controversial cosmological parameter, and we discuss possible resolutions
of this sigma8 problem. We also comment on the disturbing fact that many recent
analyses (including this one) obtain error bars smaller than the Fisher matrix
bound. We produce a CMB power spectrum combining all existing experiments, and
using it for a "MAP versus world" comparison next month will provide a powerful
test of how realistic the error estimates have been in the cosmology community.Comment: Added references and Fisher error discussion. Combined CMB data,
window and covariance matrix for January "MAP vs World" contest at
http://www.hep.upenn.edu/~max/cmblsslens.html or from [email protected]
Prospects for e+e- physics at Frascati between the phi and the psi
We present a detailed study, done in the framework of the INFN 2006 Roadmap,
of the prospects for e+e- physics at the Frascati National Laboratories. The
physics case for an e+e- collider running at high luminosity at the phi
resonance energy and also reaching a maximum center of mass energy of 2.5 GeV
is discussed, together with the specific aspects of a very high luminosity
tau-charm factory. Subjects connected to Kaon decay physics are not discussed
here, being part of another INFN Roadmap working group. The significance of the
project and the impact on INFN are also discussed. All the documentation
related to the activities of the working group can be found in
http://www.roma1.infn.it/people/bini/roadmap.html.Comment: INFN Roadmap Report: 86 pages, 25 figures, 9 table
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