1,036 research outputs found
Spatial distribution of Cherenkov radiation in periodic dielectric media
The nontrivial dispersion relation of a periodic medium affects both the
spectral and the spatial distribution of Cherenkov radiation. We present a
theory of the spatial distribution of Cherenkov radiation in the far-field zone
inside arbitrary three- and two-dimensional dielectric media. Simple analytical
expressions for the far-field are obtained in terms of the Bloch mode
expansion. Numerical examples of the Cherenkov radiation in a two-dimensional
photonic crystal is presented. The developed analytical theory demonstrates
good agreement with numerically rigorous finite-difference time-domain
calculations.Comment: 14 pages, 5 figures, Journal of Optics A (in press
Particle acceleration by slow modes in strong compressible MHD turbulence, with application to solar flares
Energetic particles that undergo strong pitch-angle scattering and diffuse
through a plasma containing strong compressible MHD turbulence undergo
diffusion in momentum space with diffusion coefficient Dp. In this paper, the
contribution of slow modes to Dp is calculated assuming the rms turbulent
velocity is of order the Alfven speed. The energy spectrum of accelerated
particles is derived assuming slow modes make the dominant contribution to Dp,
taking into account Coulomb losses and particle escape from the acceleration
region with an energy-independent escape time. The results are applied to solar
flares.Comment: 19 pages, 3 figures, accepted for publication in Ap
The 511 keV emission from positron annihilation in the Galaxy
The first gamma-ray line originating from outside the solar system that was
ever detected is the 511 keV emission from positron annihilation in the Galaxy.
Despite 30 years of intense theoretical and observational investigation, the
main sources of positrons have not been identified up to now. Observations in
the 1990's with OSSE/CGRO showed that the emission is strongly concentrated
towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's
INTEGRAL gamma-ray observatory allowed scientists to measure that emission
across the entire Galaxy, revealing that the bulge/disk luminosity ratio is
larger than observed in any other wavelength. This mapping prompted a number of
novel explanations, including rather "exotic ones (e.g. dark matter
annihilation). However, conventional astrophysical sources, like type Ia
supernovae, microquasars or X-ray binaries, are still plausible candidates for
a large fraction of the observed total 511 keV emission of the bulge. A closer
study of the subject reveals new layers of complexity, since positrons may
propagate far away from their production sites, making it difficult to infer
the underlying source distribution from the observed map of 511 keV emission.
However, contrary to the rather well understood propagation of high energy
(>GeV) particles of Galactic cosmic rays, understanding the propagation of low
energy (~MeV) positrons in the turbulent, magnetized interstellar medium, still
remains a formidable challenge. We review the spectral and imaging properties
of the observed 511 keV emission and we critically discuss candidate positron
sources and models of positron propagation in the Galaxy.Comment: 62 pages, 35 figures. Review paper to appear in Reviews of Modern
Physic
Coherent radiation from neutral molecules moving above a grating
We predict and study the quantum-electrodynamical effect of parametric
self-induced excitation of a molecule moving above the dielectric or conducting
medium with periodic grating. In this case the radiation reaction force
modulates the molecular transition frequency which results in a parametric
instability of dipole oscillations even from the level of quantum or thermal
fluctuations. The present mechanism of instability of electrically neutral
molecules is different from that of the well-known Smith-Purcell and transition
radiation in which a moving charge and its oscillating image create an
oscillating dipole.
We show that parametrically excited molecular bunches can produce an easily
detectable coherent radiation flux of up to a microwatt.Comment: 4 page
Diffusive Shock Acceleration with Magnetic Amplification by Non-resonant Streaming Instability in SNRs
We investigate the diffusive shock acceleration in the presence of the
non-resonant streaming instability introduced by Bell (2004). The numerical MHD
simulations of the magnetic field amplification combined with the analytical
treatment of cosmic ray acceleration permit us to calculate the maximum energy
of particles accelerated by high-velocity supernova shocks. The estimates for
Cas A, Kepler, SN1006, and Tycho historical supernova remnants are given. We
also found that the amplified magnetic field is preferentially oriented
perpendicular to the shock front downstream of the fast shock. This explains
the origin of the radial magnetic fields observed in young supernova remnants.Comment: 18 pages, 9 figures, accepted to Ap
Studies of h/e Aharonov-Bohm Photovoltaic Oscillations in Mesoscopic Au Rings
We have investigated a mesoscopic photovoltaic (PV) effect in micron-size Au
rings in which a dc voltage Vdc is generated in response to microwave
radiation. The effect is due to the lack of inversion symmetry in a disordered
system. Aharonov-Bohm PV oscillations with flux period h/e have been observed
at low microwave intensities for temperatures ranging from 1.4 to 13 K. For
moderate microwave intensities the h/e PV oscillations are completely quenched
providing evidence that the microwaves act to randomize the phase of the
electrons. Studies of the temperature dependence of Vdc also provide evidence
of the dephasing nature of the microwave field. A complete theoretical
explanation of the observed behavior seems to require a theory for the PV
effect in a ring geometry.Comment: 10 pages (RevTex twocolumn style), 8 figures-2 pages (one postscript
file) To be published in Phys. Rev.
Hard X-ray Emission Clumps in the gamma-Cygni Supernova Remnant: an INTEGRAL-ISGRI View
Spatially resolved images of the galactic supernova remnant G78.2+2.1
(gamma-Cygni) in hard X-ray energy bands from 25 keV to 120 keV are obtained
with the IBIS-ISGRI imager aboard the International Gamma-Ray Astrophysics
Laboratory INTEGRAL. The images are dominated by localized clumps of about ten
arcmin in size. The flux of the most prominent North-Western (NW) clump is (1.7
+/- 0.4) 10^{-11} erg/cm^2/s in the 25-40 keV band. The observed X-ray fluxes
are in agreement with extrapolations of soft X-ray imaging observations of
gamma-Cygni by ASCA GIS and spatially unresolved RXTE PCA data. The positions
of the hard X-ray clumps correlate with bright patches of optical line
emission, possibly indicating the presence of radiative shock waves in a
shocked cloud. The observed spatial structure and spectra are consistent with
model predictions of hard X-ray emission from nonthermal electrons accelerated
by a radiative shock in a supernova interacting with an interstellar cloud, but
the powerful stellar wind of the O9V star HD 193322 is a plausible candidate
for the NW source as well.Comment: 5 pages, 5 figures, Astronomy and Astrophysics Letter
Radiation pattern of a classical dipole in a photonic crystal: photon focusing
The asymptotic analysis of the radiation pattern of a classical dipole in a
photonic crystal possessing an incomplete photonic bandgap is presented. The
far-field radiation pattern demonstrates a strong modification with respect to
the dipole radiation pattern in vacuum. Radiated power is suppressed in the
direction of the spatial stopband and strongly enhanced in the direction of the
group velocity, which is stationary with respect to a small variation of the
wave vector. An effect of radiated power enhancement is explained in terms of
\emph{photon focusing}. Numerical example is given for a square-lattice
two-dimensional photonic crystal. Predictions of asymptotic analysis are
substantiated with finite-difference time-domain calculations, revealing a
reasonable agreement.Comment: Submitted to Phys. Rev.
A simple formula for the L-gap width of a face-centered-cubic photonic crystal
The width of the first Bragg's scattering peak in the (111)
direction of a face-centered-cubic lattice of air spheres can be well
approximated by a simple formula which only involves the volume averaged
and over the lattice unit cell, being the
(position dependent) dielectric constant of the medium, and the effective
dielectric constant in the long-wavelength limit approximated
by Maxwell-Garnett's formula. Apparently, our formula describes the asymptotic
behaviour of the absolute gap width for high dielectric contrast
exactly. The standard deviation steadily decreases well below
1% as increases. For example for the sphere filling
fraction and . On the interval , our
formula still approximates the absolute gap width (the relative
gap width ) with a reasonable precision, namely with a standard
deviation 3% (4.2%) for low filling fractions up to 6.5% (8%) for the
close-packed case. Differences between the case of air spheres in a dielectric
and dielectric spheres in air are briefly discussed.Comment: 13 pages, 4 figs., RevTex, two references added. For more info see
http://www.amolf.nl/external/wwwlab/atoms/theory/index.htm
Neutrino conversions in random magnetic fields and from the Sun
The magnetic field in the convective zone of the Sun has a random small-scale
component with the r.m.s. value substantially exceeding the strength of a
regular large-scale field. For two Majorana neutrino flavors two
helicities in the presence of a neutrino transition magnetic moment and nonzero
neutrino mixing we analyze the displacement of the allowed ()-parameter region reconciled for the SuperKamiokande(SK) and
radiochemical (GALLEX, SAGE, Homestake) experiments in dependence on the r.m.s.
magnetic field value , or more precisely, on a value assuming the
transition magnetic moment . In contrast to RSFP in
regular magnetic fields we find an effective production of electron
antineutrinos in the Sun even for small neutrino mixing through cascade
conversions , in a random magnetic field that would be a
signature of the Majorana nature of neutrino if will be
registered. Basing on the present SK bound on electron antineutrinos we have
also found an excluded area in the same -plane and
revealed a strong sensitivity to the random magnetic field correlation length
.Comment: LaTex 36 pages including 14 PostScript figure
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