9,591 research outputs found
Wide-range optical studies on various single-walled carbon nanotubes: the origin of the low-energy gap
We present wide-range (3 meV - 6 eV) optical studies on freestanding
transparent carbon nanotube films, made from nanotubes with different diameter
distributions. In the far-infrared region, we found a low-energy gap in all
samples investigated. By a detailed analysis we determined the average
diameters of both the semiconducting and metallic species from the near
infrared/visible features of the spectra. Having thus established the
dependence of the gap value on the mean diameter, we find that the frequency of
the low energy gap is increasing with increasing curvature. Our results
strongly support the explanation of the low-frequency feature as arising from a
curvature-induced gap instead of effective medium effects. Comparing our
results with other theoretical and experimental low-energy gap values, we find
that optical measurements yield a systematically lower gap than tunneling
spectroscopy and DFT calculations, the difference increasing with decreasing
diameter. This difference can be assigned to electron-hole interactions.Comment: 9 pages, 8 figures, to be published in Physical Review B,
supplemental material attached v2: Figures 1, 7 and 8 replaced, minor changes
to text; v3: Figures 3, 4 and 5 replaced, minor changes to tex
Gamma-ray Flares and VLBI Outbursts of Blazars
A model is developed for the time dependent electromagnetic - radio to
gamma-ray - emission of active galactic nuclei, specifically, the blazars,
based on the acceleration and creation of leptons at a propagating
discontinuity or {\it front} of a Poynting flux jet. The front corresponds to a
discrete relativistic jet component as observed with
very-long-baseline-interferometry (VLBI). Equations are derived for the number,
momentum, and energy of particles in the front taking into account synchrotron,
synchrotron-self-Compton (SSC), and inverse-Compton processes as well as
photon-photon pair production. The apparent synchrotron, SSC, and
inverse-Compton luminosities as functions of time are determined. Predictions
of the model are compared with observations in the gamma, optical and radio
bands. The delay between the high-energy gamma-ray flare and the onset of the
radio is explained by self-absorption and/or free-free absorption by external
plasma. Two types of gamma-ray flares are predicted depending on pair creation
in the front.Comment: 11 pages, submitted to ApJ. 10 figures can be obtained from R.
Lovelace by sending postal address to [email protected]
CMB component separation by parameter estimation
We propose a solution to the CMB component separation problem based on
standard parameter estimation techniques. We assume a parametric spectral model
for each signal component, and fit the corresponding parameters pixel by pixel
in a two-stage process. First we fit for the full parameter set (e.g.,
component amplitudes and spectral indices) in low-resolution and high
signal-to-noise ratio maps using MCMC, obtaining both best-fit values for each
parameter, and the associated uncertainty. The goodness-of-fit is evaluated by
a chi^2 statistic. Then we fix all non-linear parameters at their
low-resolution best-fit values, and solve analytically for high-resolution
component amplitude maps. This likelihood approach has many advantages: The
fitted model may be chosen freely, and the method is therefore completely
general; all assumptions are transparent; no restrictions on spatial variations
of foreground properties are imposed; the results may be rigorously monitored
by goodness-of-fit tests; and, most importantly, we obtain reliable error
estimates on all estimated quantities. We apply the method to simulated Planck
and six-year WMAP data based on realistic models, and show that separation at
the muK level is indeed possible in these cases. We also outline how the
foreground uncertainties may be rigorously propagated through to the CMB power
spectrum and cosmological parameters using a Gibbs sampling technique.Comment: 20 pages, 10 figures, submitted to ApJ. For a high-resolution
version, see http://www.astro.uio.no/~hke/docs/eriksen_et_al_fgfit.p
TRIS III: the diffuse galactic radio emission at
We present values of temperature and spectral index of the galactic diffuse
radiation measured at 600 and 820 MHz along a 24 hours right ascension circle
at declination . They have been obtained from a subset of
absolute measurements of the sky temperature made with TRIS, an experiment
devoted to the measurement of the Cosmic Microwave Background temperature at
decimetric-wavelengths with an angular resolution of about .
Our analysis confirms the preexisting picture of the galactic diffuse
emission at decimetric wavelength and improves the accuracy of the measurable
quantities. In particular, the signal coming from the halo has a spectral index
in the range above 600 MHz, depending on the sky position. In the
disk, at TRIS angular resolution, the free-free emission accounts for the 11%
of the overall signal at 600 MHz and 21% at 1420 MHz. The polarized component
of the galactic emission, evaluated from the survey by Brouw and Spoelstra,
affects the observations at TRIS angular resolution by less than 3% at 820 MHz
and less than 2% at 600 MHz. Within the uncertainties, our determination of the
galactic spectral index is practically unaffected by the correction for
polarization.
Since the overall error budget of the sky temperatures measured by TRIS at
600 MHz, that is 66 mK(systematic)18 mK (statistical), is definitely smaller
than those reported in previous measurements at the same frequency, our data
have been used to discuss the zero levels of the sky maps at 150, 408, 820 and
1420 MHz in literature. Concerning the 408 MHz survey, limiting our attention
to the patch of sky corresponding to the region observed by TRIS, we suggest a
correction of the base-level of K.Comment: Accepted for publication in the Astrophysical Journa
V2:Performance of the solid deuterium ultra-cold neutron source at the pulsed reactor TRIGA Mainz
The performance of the solid deuterium ultra-cold neutron source at the
pulsed reactor TRIGA Mainz with a maximum peak energy of 10 MJ is described.
The solid deuterium converter with a volume of V=160 cm3 (8 mol), which is
exposed to a thermal neutron fluence of 4.5x10^13 n/cm2, delivers up to 550 000
UCN per pulse outside of the biological shield at the experimental area. UCN
densities of ~ 10/cm3 are obtained in stainless steel bottles of V ~ 10 L
resulting in a storage efficiency of ~20%. The measured UCN yields compare well
with the predictions from a Monte Carlo simulation developed to model the
source and to optimize its performance for the upcoming upgrade of the TRIGA
Mainz into a user facility for UCN physics.Comment: 23 pages, 8 figure
A New System of Parallel Isolated Nonthermal Filaments Near the Galactic Center: Evidence for a Local Magnetic Field Gradient
We report the discovery of a system of isolated nonthermal filaments
approximately 0.5 deg. northwest (75 pc in projection) of Sgr A. Unlike other
isolated nonthermal filaments which show subfilamentation, braiding of
subfilaments, and flaring at their ends, these filaments are simple linear
structures and more closely resemble the parallel bundled filaments in the
Galactic center radio arc. However, the most unusual feature of these filaments
is that the 20/90 cm spectral index uniformly decreases as a function of
length, in contrast to all other nonthermal filaments in the Galactic center.
This spectral gradient may not be due to simple particle aging but could be
explained by a curved electron energy spectrum embedded in a diverging magnetic
field. If so, the scale of the magnetic gradient is not consistent with a large
scale magnetic field centered on Sgr A* suggesting that this filament system is
tracing a local magnetic field.Comment: 10 pages, AASTeX 5.01 LaTeX2e; 7 figures in 9 PostScript files;
scheduled for publication in the 2001 December 10, v. 563 issue of Ap
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