4,451 research outputs found
Incidence of the Tomonaga-Luttinger liquid state on the NMR spin lattice relaxation in Carbon Nanotubes
We report 13C nuclear magnetic resonance measurements on single wall carbon
nanotube (SWCNT) bundles. The temperature dependence of the nuclear
spin-lattice relaxation rate, 1/T1, exhibits a power-law variation, as expected
for a Tomonage-Luttinger liquid (TLL). The observed exponent is smaller than
that expected for the two band TLL model. A departure from the power law is
observed only at low T, where thermal and electronic Zeeman energy merge.
Extrapolation to zero magnetic field indicates gapless spin excitations. The
wide T range on which power-law behavior is observed suggests that SWCNT is so
far the best realization of a one-dimensional quantum metal.Comment: 5 pages, 4 figure
X-ray Spectral Variability and Rapid Variability of the Soft X-ray Spectrum Seyfert 1 Galaxies Ark 564 and Ton S180
The bright, soft X-ray spectrum Seyfert 1 galaxies Ark 564 and Ton S180 were
monitored for 35 days and 12 days with ASCA and RXTE (and EUVE for Ton S180).
The short time scale (hours-days) variability patterns were very similar across
energy bands, with no evidence of lags between any of the energy bands studied.
The fractional variability amplitude was almost independent of energy band. It
is difficult to simultaneously explain soft Seyferts stronger variability,
softer spectra, and weaker energy-dependence of the variability relative to
hard Seyferts. The soft and hard band light curves diverged on the longest time
scales probed, consistent with the fluctuation power density spectra that
showed relatively greater power on long time scales in the softest bands. The
simplest explanation is that a relatively hard, rapidly-variable component
dominates the total X-ray spectrum and a slowly-variable soft excess is present
in the lowest energy channels of ASCA. Although it would be natural to identify
the latter with an accretion disk and the former with a corona surrounding it,
a standard thin disk could not get hot enough to radiate significantly in the
ASCA band, and the observed variability time scales are much too short. The
hard component may have a more complex shape than a pure power-law. The most
rapid factor of 2 flares and dips occurred within ~1000 sec in Ark 564 and a
bit more slowly in Ton S180. The speed of the luminosity changes rules out
viscous or thermal processes and limits the size of the individual emission
regions to <~15 Schwarzschild radii (and probably much less), that is, to
either the inner disk or small regions in a corona
Schwinger-Dyson equation for non-Lagrangian field theory
A method is proposed of constructing quantum correlators for a general gauge
system whose classical equations of motion do not necessarily follow from the
least action principle. The idea of the method is in assigning a certain BRST
operator to any classical equations of motion, Lagrangian or not.
The generating functional of Green's functions is defined by the equation
that is reduced to the standard Schwinger-Dyson equation
whenever the classical field equations are Lagrangian. The corresponding
probability amplitude of a field is defined by the same equation
although in another representation. When the
classical dynamics are Lagrangian, the solution for is reduced to
the Feynman amplitude , while in the non-Lagrangian case
this amplitude can be a more general distribution.Comment: 33 page
A review on radiomics and the future of theranostics for patient selection in precision medicine
Dusty OB stars in the Small Magellanic Cloud - II: Extragalactic Disks or Examples of the Pleiades Phenomenon?
We use mid-infrared Spitzer spectroscopy and far-infrared Herschel photometry
for a sample of twenty main sequence O9--B2 stars in the Small Magellanic Cloud
(SMC) with strong 24 micron excesses to investigate the origin of the mid-IR
emission. Either debris disks around the stars or illuminated patches of dense
interstellar medium (ISM) can cause such mid-IR emission. In a companion paper,
Paper I, we use optical spectroscopy to show that it is unlikely for any of
these sources to be classical Be stars or Herbig Ae/Be stars. We focus our
analysis on debris disks and cirrus hot spots. We find three out of twenty
stars to be significantly extended in the mid-IR, establishing them as cirrus
hot spots. We then fit the IR spectral energy distributions to determine dust
temperatures and masses. We find the dust masses in the SMC stars to be larger
than for any known debris disks, although this evidence against the debris disk
hypothesis is circumstantial. Finally, we created a local comparison sample of
bright mid-IR OB stars in the Milky Way (MW) by cross-matching the WISE and
Hipparcos catalogs. All such local stars in the appropriate luminosity range
that can be unambiguously classified are young stars with optical emission
lines or are spatially resolved by WISE with sizes too large to be plausible
debris disk candidates. We conclude that the very strong mid-IR flux excesses
are most likely explained as cirrus hot spots, although we cannot rigorously
rule out that a small fraction of the sample is made up of debris disks or
transition disks. We present suggestive evidence that bow-shock heating around
runaway stars may be a contributing mechanism to the interstellar emission.
These sources, interpreted as cirrus hot spots, offer a new localised probe of
diffuse interstellar dust in a low metallicity environment. (Abridged)Comment: Accepted for publication in ApJ, 23 pages, 11 figures, 8 table
Electromagnetic form factors of the baryon octet in the perturbative chiral quark model
We apply the perturbative chiral quark model at one loop to analyze the
electromagnetic form factors of the baryon octet. The analytic expressions for
baryon form factors, which are given in terms of fundamental parameters of
low-energy pion-nucleon physics (weak pion decay constant, axial nucleon
coupling, strong pion-nucleon form factor), and the numerical results for
baryon magnetic moments, charge and magnetic radii are presented. Our results
are in good agreement with experimental data.Comment: 15 pages, 6 figures, to be published in Eur. Phys. J.
Physical properties of Southern infrared dark clouds
It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs)
likely represent the birth sites massive stars. Therefore, this class of
objects gets increasing attention. To enlarge the sample of well-characterised
IRDCs in the southern hemisphere, we have set up a program to study the gas and
dust of southern IRDCs. The present paper aims at characterizing the continuuum
properties of this sample of objects. We cross-correlated 1.2 mm continuum data
from SIMBA@SEST with Spitzer/GLIMPSE images to establish the connection between
emission sources at millimeter wavelengths and the IRDCs we see at 8 m in
absorption against the bright PAH background. Analysing the dust emission and
extinction leads to a determination of masses and column densities, which are
important quantities in characterizing the initial conditions of massive star
formation. The total masses of the IRDCs were found to range from 150 to 1150
(emission data) and from 300 to 1750 (extinction
data). We derived peak column densities between 0.9 and 4.6
cm (emission data) and 2.1 and 5.4 cm
(extinction data). We demonstrate that the extinction method fails for very
high extinction values (and column densities) beyond A values of
roughly 75 mag according to the Weingartner & Draine (2001) extinction relation
model B. The derived column densities, taking into account
the spatial resolution effects, are beyond the column density threshold of 3.0
cm required by theoretical considerations for massive
star formation. We conclude that the values for column densities derived for
the selected IRDC sample make these objects excellent candidates for objects in
the earliest stages of massive star formation.Comment: Accepted for publication in Astronomy & Astrophysic
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