3,399 research outputs found
Axisymmetric pulse recycling and motion in bulk semiconductors
The Kroemer model for the Gunn effect in a circular geometry (Corbino disks)
has been numerically solved. The results have been interpreted by means of
asymptotic calculations. Above a certain onset dc voltage bias, axisymmetric
pulses of the electric field are periodically shed by an inner circular
cathode. These pulses decay as they move towards the outer anode, which they
may not reach. As a pulse advances, the external current increases continuously
until a new pulse is generated. Then the current abruptly decreases, in
agreement with existing experimental results. Depending on the bias, more
complex patterns with multiple pulse shedding are possible.Comment: 8 pages, 15 figure
Investigating Ca II emission in the RS CVn binary ER Vulpeculae using the Broadening Function Formalism
The synchronously rotating G stars in the detached, short-period (0.7 d),
partially eclipsing binary, ER Vul, are the most chromospherically active
solar-type stars known. We have monitored activity in the Ca II H & K reversals
for almost an entire orbit. Rucinski's Broadening Function Formalism allows the
photospheric contribution to be objectively subtracted from the highly blended
spectra. The power of the BF technique is also demonstrated by the good
agreement of radial velocities with those measured by others from less crowded
spectral regions. In addition to strong Ca II emission from the primary and
secondary, there appears to be a high-velocity stream flowing onto the
secondary where it stimulates a large active region on the surface 30 - 40
degrees in advance of the sub-binary longitude. A model light curve with a spot
centered on the same longitude also gives the best fit to the observed light
curve. A flare with approximately 13% more power than at other phases was
detected in one spectrum. We suggest ER Vul may offer a magnified view of the
more subtle chromospheric effects synchronized to planetary revolution seen in
certain `51 Peg'-type systems.Comment: Accepted to AJ; 17 pages and 16 figure
Free boundary problems describing two-dimensional pulse recycling and motion in semiconductors
An asymptotic analysis of the Gunn effect in two-dimensional samples of bulk
n-GaAs with circular contacts is presented. A moving pulse far from contacts is
approximated by a moving free boundary separating regions where the electric
potential solves a Laplace equation with subsidiary boundary conditions. The
dynamical condition for the motion of the free boundary is a Hamilton-Jacobi
equation. We obtain the exact solution of the free boundary problem (FBP) in
simple one-dimensional and axisymmetric geometries. The solution of the FBP is
obtained numerically in the general case and compared with the numerical
solution of the full system of equations. The agreement is excellent so that
the FBP can be adopted as the basis for an asymptotic study of the
multi-dimensional Gunn effect.Comment: 19 pages, 9 figures, Revtex. To appear in Phys. Rev.
Measuring the dark flow with public X-ray cluster data
We present new results on the "dark flow" from a measurement of the dipole in
the distribution of peculiar velocities of galaxy clusters, applying the
methodology proposed and developed by us earlier. Our latest measurement is
conducted using new, low-noise 7-yr WMAP data as well as an all-sky sample of
X-ray selected galaxy clusters compiled exclusively from published catalogs.
Our analysis of the CMB signature of the kinematic Sunyaev-Zeldovich (SZ)
effect finds a statistically significant dipole at the location of galaxy
clusters. The residual dipole outside the cluster regions is small, rendering
our overall measurement 3-4 sigma significant. The amplitude of the dipole
correlates with cluster properties, being larger for the most X-ray luminous
clusters, as required if the signal is produced by the SZ effect. Since it is
measured at zero monopole, the dipole can not be due to the thermal SZ effect.
Our results are consistent with those obtained earlier by us from 5-yr WMAP
data and using a proprietary cluster catalog. In addition, they are robust to
quadrupole removal, demonstrating that quadrupole leakage contributes
negligibly to the signal. The lower noise of the 7-yr WMAP also allows us, for
the first time, to obtain tentative empirical confirmation of our earlier
conjecture that the adopted filtering flips the sign of the KSZ effect for
realistic clusters and thus of the deduced direction of the flow. The latter is
consistent with our earlier measurement in both the amplitude and direction.
Assuming the filtering indeed flips the sign of the KSZ effect from the
clusters, the direction agrees well also with the results of independent work
using galaxies as tracers at lower distances. We make all maps and cluster
templates derived by us from public data available to the scientific community
to allow independent tests of our method and findings.Comment: ApJ, in press. Replaced with accepted version. The data needed for
these results are at http://www.kashlinsky.info/bulkflows/data_publi
New Results for Diffusion in Lorentz Lattice Gas Cellular Automata
New calculations to over ten million time steps have revealed a more complex
diffusive behavior than previously reported, of a point particle on a square
and triangular lattice randomly occupied by mirror or rotator scatterers. For
the square lattice fully occupied by mirrors where extended closed particle
orbits occur, anomalous diffusion was still found. However, for a not fully
occupied lattice the super diffusion, first noticed by Owczarek and Prellberg
for a particular concentration, obtains for all concentrations. For the square
lattice occupied by rotators and the triangular lattice occupied by mirrors or
rotators, an absence of diffusion (trapping) was found for all concentrations,
except on critical lines, where anomalous diffusion (extended closed orbits)
occurs and hyperscaling holds for all closed orbits with {\em universal}
exponents and . Only one point on these critical lines can be related to a
corresponding percolation problem. The questions arise therefore whether the
other critical points can be mapped onto a new percolation-like problem, and of
the dynamical significance of hyperscaling.Comment: 52 pages, including 18 figures on the last 22 pages, email:
[email protected]
Autoinflammatory constrictive pericarditis and chronic myelomonocytic leukaemia: when one speciality is not enough
We present a case of constrictive pericarditis with concomitant blood and bone marrow appearances of chronic myelomonocytic leukaemia (CMML). Despite surgical treatment with pericardiectomy, the patient deteriorated into multiorgan failure. Pericardial histology disclosed a typical inflammatory picture with no evidence of monocytic or malignant infiltrate. Following intensive collaboration between cardiologists, haematologists and rheumatologists via daily email exchanges, a diagnosis was reached of autoinflammatory constrictive pericarditis with a non-infiltrative coexisting CMML. The key to achieving a rapid and sustained response was a trial of high-dose steroids followed by intravenous immunoglobulins. This achieved restoration of cardiac function, resolution of symptoms and near normalisation of inflammatory markers. A diagnosis of concurrent CMML was confirmed at 3 months. The patient remains well, taking colchicine and steroids
Universality of the Gunn effect: self-sustained oscillations mediated by solitary waves
The Gunn effect consists of time-periodic oscillations of the current flowing
through an external purely resistive circuit mediated by solitary wave dynamics
of the electric field on an attached appropriate semiconductor. By means of a
new asymptotic analysis, it is argued that Gunn-like behavior occurs in
specific classes of model equations. As an illustration, an example related to
the constrained Cahn-Allen equation is analyzed.Comment: 4 pages,3 Post-Script figure
Recent X-ray Observations and the Evolution of Hot Gas in Elliptical Galaxies: Evidence for Circumgalactic Gas
X-ray emitting gaseous halos, such as that in elliptical galaxies like NGC
4472, cannot have been produced solely from gas expelled from galactic stars.
In traditional models for the evolution of hot interstellar gas (cooling flows)
in ellipticals, the galaxies are assumed to have been cleared of gas by
SNII-driven winds at some early time then gas is subsequently replenished by
mass loss from an evolving population of old stars. To test this, we accurately
determine the stellar and dark halo mass of NGC 4472 using hydrostatic
equilibrium, then solve the standard time-dependent cooling flow equations to
recover the observed hot gas temperature and density distributions when evolved
to the present time. This procedure fails: the computed gas density gradient is
too steep, the total gas mass is too low, and the gas temperatures are much too
low. All variants on this basic procedure also fail: increasing the SNIa rate,
using the mass dropout assumption, arbitrarily adjusting uncertain
coefficients, etc. However, agreement is achieved if the galaxy is supplied
with additional, spatially-extended hot gas early in its evolution. This old
``circumgalactic'' gas can be retained to the present time and may be related
to cosmological ``secondary infall''.Comment: 15 pages in two-column AASTEX LaTeX including 1 table and 8 figures;
abstract corrected in replacement; accepted by Astrophysical Journa
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