1,924 research outputs found
Locating the most energetic electrons in Cassiopeia A
We present deep (2.4 Ms) observations of the Cassiopeia A supernova
remnant with {\it NuSTAR}, which operates in the 3--79 keV bandpass and is the
first instrument capable of spatially resolving the remnant above 15 keV. We
find that the emission is not entirely dominated by the forward shock nor by a
smooth "bright ring" at the reverse shock. Instead we find that the 15 keV
emission is dominated by knots near the center of the remnant and dimmer
filaments near the remnant's outer rim. These regions are fit with unbroken
power-laws in the 15--50 keV bandpass, though the central knots have a steeper
() spectrum than the outer filaments ().
We argue this difference implies that the central knots are located in the 3-D
interior of the remnant rather than at the outer rim of the remnant and seen in
the center due to projection effects. The morphology of 15 keV emission does
not follow that of the radio emission nor that of the low energy (12 keV)
X-rays, leaving the origin of the 15 keV emission as an open mystery. Even
at the forward shock front we find less steepening of the spectrum than
expected from an exponentially cut off electron distribution with a single
cutoff energy. Finally, we find that the GeV emission is not associated with
the bright features in the {\it NuSTAR} band while the TeV emission may be,
suggesting that both hadronic and leptonic emission mechanisms may be at work.Comment: 12 pages, 11 figures, accepted for publication in Ap
Formation of a stable deacagonal quasicrystalline Al-Pd-Mn surface layer
We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn
quasicrystal overlayer on the 5-fold symmetric surface of an icosahedral
Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by
x-ray photoelectron diffraction, low-energy electron diffraction and electron
backscatter diffraction. This overlayer is also characterized by a reduced
density of states near the Fermi edge as expected for quasicrystals. This is
the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn
is obtained, studied and compared to its icosahedral counterpart.Comment: Submitted to Phys. Ref. Lett. (18 July 2001
Conductance of carbon nanotubes with disorder: A numerical study
We study the conductance of carbon nanotube wires in the presence of
disorder, in the limit of phase coherent transport. For this purpose, we have
developed a simple numerical procedure to compute transmission through carbon
nanotubes and related structures. Two models of disorder are considered, weak
uniform disorder and isolated strong scatterers. In the case of weak uniform
disorder, our simulations show that the conductance is not significantly
affected by disorder when the Fermi energy is close to the band center.
Further, the transmission around the band center depends on the diameter of
these zero bandgap wires. We also find that the calculated small bias
conductance as a function of the Fermi energy exhibits a dip when the Fermi
energy is close to the second subband minima. In the presence of strong
isolated disorder, our calculations show a transmission gap at the band center,
and the corresponding conductance is very small
Satellite detection, long-range transport, and air quality impacts of volcanic sulfur dioxide from the 2014–2015 flood lava eruption at Bárðarbunga (Iceland)
The 2014–2015 Bárðarbunga-Veiðivötn fissure eruption at Holuhraun produced about 1.5 km3 of lava, making it the largest eruption in Iceland in more than 200 years. Over the course of the eruption, daily volcanic sulfur dioxide (SO2) emissions exceeded daily SO2 emissions from all anthropogenic sources in Europe in 2010 by at least a factor of 3. We present surface air quality observations from across Northern Europe together with satellite remote sensing data and model simulations of volcanic SO2 for September 2014. We show that volcanic SO2 was transported in the lowermost troposphere over long distances and detected by air quality monitoring stations up to 2750 km away from the source. Using retrievals from the Ozone Monitoring Instrument (OMI) and the Infrared Atmospheric Sounding Interferometer (IASI), we calculate an average daily SO2 mass burden of 99 ± 49 kilotons (kt) of SO2 from OMI and 61 ± 18 kt of SO2 from IASI for September 2014. This volcanic burden is at least a factor of 2 greater than the average SO2 mass burden between 2007 and 2009 due to anthropogenic emissions from the whole of Europe. Combining the observational data with model simulations using the United Kingdom Met Office's Numerical Atmospheric-dispersion Modelling Environment model, we are able to constrain SO2 emission rates to up to 120 kilotons per day (kt/d) during early September 2014, followed by a decrease to 20–60 kt/d between 6 and 22 September 2014, followed by a renewed increase to 60–120 kt/d until the end of September 2014. Based on these fluxes, we estimate that the eruption emitted a total of 2.0 ± 0.6 Tg of SO2 during September 2014, in good agreement with ground-based remote sensing and petrological estimates. Although satellite-derived and model-simulated vertical column densities of SO2 agree well, the model simulations are biased low by up to a factor of 8 when compared to surface observations of volcanic SO2 on 6–7 September 2014 in Ireland. These biases are mainly due to relatively small horizontal and vertical positional errors in the simulations of the volcanic plume occurring over transport distances of thousands of kilometers. Although the volcanic air pollution episodes were transient and lava-dominated volcanic eruptions are sporadic events, the observations suggest that (i) during an eruption, volcanic SO2 measurements should be assimilated for near real-time air quality forecasting and (ii) existing air quality monitoring networks should be retained or extended to monitor SO2 and other volcanic pollutants
Closed Strings with Low Harmonics and Kinks
Low-harmonic formulas for closed relativistic strings are given. General
parametrizations are presented for the addition of second- and third-harmonic
waves to the fundamental wave. The method of determination of the
parametrizations is based upon a product representation found for the finite
Fourier series of string motion in which the constraints are automatically
satisfied. The construction of strings with kinks is discussed, including
examples. A procedure is laid out for the representation of kinks that arise
from self-intersection, and subsequent intercommutation, for harmonically
parametrized cosmic strings.Comment: 39, CWRUTH-93-
Cosmic ray diffusion near the Bohm limit in the Cassiopeia A supernova remnant
Supernova remnants (SNRs) are believed to be the primary location of the
acceleration of Galactic cosmic rays, via diffusive shock (Fermi) acceleration.
Despite considerable theoretical work the precise details are still unknown, in
part because of the difficulty in directly observing nucleons that are
accelerated to TeV energies in, and affect the structure of, the SNR shocks.
However, for the last ten years, X-ray observatories ASCA, and more recently
Chandra, XMM-Newton, and Suzaku have made it possible to image the synchrotron
emission at keV energies produced by cosmic-ray electrons accelerated in the
SNR shocks. In this article, we describe a spatially-resolved spectroscopic
analysis of Chandra observations of the Galactic SNR Cassiopeia A to map the
cutoff frequencies of electrons accelerated in the forward shock. We set upper
limits on the electron diffusion coefficient and find locations where particles
appear to be accelerated nearly as fast as theoretically possible (the Bohm
limit).Comment: 18 pages, 5 figures. Accepted for publication in Nature Physics (DOI
below), final version available week of August 28, 2006 at
http://www.nature.com/nphy
The theory of pulsar winds and nebulae
We review current theoretical ideas on pulsar winds and their surrounding
nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the
striped wind, together with models of magnetic dissipation are discussed. It is
shown that the observational signature of this dissipation is likely to be
point-like, rather than extended, and that pulsed emission may be produced. The
possible pulse shapes and polarisation properties are described. Particle
acceleration at the termination shock of the wind is discussed, and it is
argued that two distinct mechanisms must be operating, with the first-order
Fermi mechanism producing the high-energy electrons (above 1 TeV) and either
magnetic annihilation or resonant absorption of ion cyclotron waves responsible
for the 100 MeV to 1 TeV electrons. Finally, MHD models of the morphology of
the nebula are discussed and compared with observation.Comment: 33 pages, to appear in Springer Lecture Notes on "Neutron stars and
pulsars, 40 years after the discovery", ed W.Becke
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
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