2,175 research outputs found
Soft Spheres Make More Mesophases
We use both mean-field methods and numerical simulation to study the phase
diagram of classical particles interacting with a hard-core and repulsive, soft
shoulder. Despite the purely repulsive interaction, this system displays a
remarkable array of aggregate phases arising from the competition between the
hard-core and shoulder length scales. In the limit of large shoulder width to
core size, we argue that this phase diagram has a number of universal features,
and classify the set of repulsive shoulders that lead to aggregation at high
density. Surprisingly, the phase sequence and aggregate size adjusts so as to
keep almost constant inter-aggregate separation.Comment: 4 pages, 2 included figure
Bjerrum pairing correlations at charged interfaces
Electrostatic correlations play a fundamental role in aqueous solutions. In
this letter, we identify transverse and lateral correlations as two mutually
exclusive regimes. We show that the transverse regime leads to binding by
generalization of Bjerrum pair formation theory, yielding binding constants
from first-principle statistical-mechanical calculations. We compare our
theoretical predictions with experiments on charged membranes and Langmuir
monolayers and find good agreement. We contrast our approach with existing
theories in the strong-coupling limit and on charged modulated interfaces, and
discuss different scenarios that lead to charge reversal and equal-sign
attraction by macro-ions.Comment: 7 pages, 4 figure
Minimal resonances in annular non-Euclidean strips
Differential growth processes play a prominent role in shaping leaves and
biological tissues. Using both analytical and numerical calculations, we
consider the shapes of closed, elastic strips which have been subjected to an
inhomogeneous pattern of swelling. The stretching and bending energies of a
closed strip are frustrated by compatibility constraints between the curvatures
and metric of the strip. To analyze this frustration, we study the class of
"conical" closed strips with a prescribed metric tensor on their center line.
The resulting strip shapes can be classified according to their number of
wrinkles and the prescribed pattern of swelling. We use this class of strips as
a variational ansatz to obtain the minimal energy shapes of closed strips and
find excellent agreement with the results of a numerical bead-spring model.
Within this class of strips, we derive a condition under which a strip can have
vanishing mean curvature along the center line.Comment: 14 pages, 13 figures. Published version. Updated references and added
2 figure
Finite temperature properties of the Dirac operator with bag boundary conditions
We study the finite temperature free energy and fermion number for Dirac
fields in a one-dimensional spatial segment, under local boundary conditions
compatible with the presence of a spectral asymmetry. We discuss in detail the
contribution of this part of the spectrum to the determinant. We evaluate the
finite temperature properties of the theory for arbitrary values of the
chemical potential.Comment: Talk given at the Seventh International Workshop Quantum Field Theory
under the influence of External Conditions, QFEXT'05, Barcelona, Spain. Final
version, to appear in Journal of Physics A: Mathematical and Genera
Cyclotron lines in highly magnetized neutron stars
Cyclotron lines, also called cyclotron resonant scattering features (CRSF)
are spectral features, generally appearing in absorption, in the X-ray spectra
of objects containing highly magnetized neutron stars, allowing the direct
measurement of the magnetic field strength in these objects. Cyclotron features
are thought to be due to resonant scattering of photons by electrons in the
strong magnetic fields. The main content of this contribution focusses on
electron cyclotron lines as found in accreting X-ray binary pulsars (XRBP) with
magnetic fields on the order of several 1012 Gauss. Also, possible proton
cyclotron lines from single neutron stars with even stronger magnetic fields
are briefly discussed.With regard to electron cyclotron lines, we present an
updated list of XRBPs that show evidence of such absorption lines. The first
such line was discovered in a 1976 balloon observation of the accreting binary
pulsar Hercules X-1, it is considered to be the first direct measurement of the
magnetic field of a neutron star. As of today (mid 2018), we list 36 XRBPs
showing evidence of one ore more electron cyclotron absorption line(s). A few
have been measured only once and must be confirmed (several more objects are
listed as candidates). In addition to the Tables of objects, we summarize the
evidence of variability of the cyclotron line as a function of various
parameters (especially pulse phase, luminosity and time), and add a discussion
of the different observed phenomena and associated attempts of theoretical
modeling. We also discuss our understanding of the underlying physics of
accretion onto highly magnetized neutron stars. For proton cyclotron lines, we
present tables with seven neutron stars and discuss their nature and the
physics in these objects.Comment: 32 pages, 15 figures, 8 Tables, accepted by A&A 201
Observing Ultra High Energy Cosmic Particles from Space: SEUSO, the Super Extreme Universe Space Observatory Mission
The experimental search for ultra high energy cosmic messengers, from eV to beyond eV, at the very end of the known energy
spectrum, constitutes an extraordinary opportunity to explore a largely unknown
aspect of our universe. Key scientific goals are the identification of the
sources of ultra high energy particles, the measurement of their spectra and
the study of galactic and local intergalactic magnetic fields. Ultra high
energy particles might, also, carry evidence of unknown physics or of exotic
particles relics of the early universe. To meet this challenge a significant
increase in the integrated exposure is required. This implies a new class of
experiments with larger acceptances and good understanding of the systematic
uncertainties. Space based observatories can reach the instantaneous aperture
and the integrated exposure necessary to systematically explore the ultra high
energy universe. In this paper, after briefly summarising the science case of
the mission, we describe the scientific goals and requirements of the SEUSO
concept. We then introduce the SEUSO observational approach and describe the
main instrument and mission features. We conclude discussing the expected
performance of the mission
INTEGRAL broadband spectroscopy of Vela X-1
The wind-accreting X-ray binary pulsar and cyclotron line source Vela X-1 has
been observed extensively during INTEGRAL Core Program observations of the Vela
region in June-July and November-December 2003. In the latter set of
observations the source showed intense flaring -- see also Staubert et al.
(2004), these proceedings.
We present early results on time averaged and time resolved spectra, of both
epochs of observations. A cyclotron line feature at ~53 keV is clearly detected
in the INTEGRAL spectra and its broad shape is resolved in SPI spectra. The
remaining issues in the calibration of the instruments do not allow to resolve
the question of the disputed line feature at 20-25 keV.
During the first main flare the average luminosity increases by a factor of
\~10, but the spectral shape remains very similar, except for a moderate
softening.Comment: Accepted for proceedings of 5th INTEGRAL Worksho
Dirac fields in the background of a magnetic flux string and spectral boundary conditions
We study the problem of a Dirac field in the background of an Aharonov-Bohm
flux string. We exclude the origin by imposing spectral boundary conditions at
a finite radius then shrinked to zero. Thus, we obtain a behaviour of
eigenfunctions which is compatible with the self-adjointness of the radial
Hamiltonian and the invariance under integer translations of the reduced flux.
After confining the theory to a finite region, we check the consistency with
the index theorem, and evaluate its vacuum fermionic number and Casimir energy.Comment: 9 pages, 1 figure Two references added To be published in
International Journal of Modern Physics
Strong Field Gravity and X-Ray Observations of 4U1820-30
The behavior of quasi-periodic oscillations (QPOs) at frequencies near 1 kHz
in the x-ray emission from the neutron star x-ray binary 4U1820-30 has been
interpreted as evidence for the existence of the marginally stable orbit, a key
prediction of strong-field general relativity. The signature of the marginally
stable orbit is a saturation in QPO frequency, assumed to track inner disk
radius, versus mass accretion rate. Previous studies of 4U1820-30 have used
x-ray count rate as an indicator of mass accretion rate. However, x-ray count
rate is known to not correlate robustly with mass accretion rate or QPO
frequency in other sources. Here, we examine the QPO frequency dependence on
two other indicators of mass accretion rate: energy flux and x-ray spectral
shape. Using either of these indicators, we find that the QPO frequency
saturates at high mass accretion rates. We interpret this as strong evidence
for the existence of the marginally stable orbit.Comment: accepted to the Astrophysical Journal Letters, 7 page
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