1,110 research outputs found
Nonequilibrium Kinetics of One-Dimensional Bose Gases
We study cold dilute gases made of bosonic atoms, showing that in the
mean-field one-dimensional regime they support stable out-of-equilibrium
states. Starting from the 3D Boltzmann-Vlasov equation with contact
interaction, we derive an effective 1D Landau-Vlasov equation under the
condition of a strong transverse harmonic confinement. We investigate the
existence of out-of-equilibrium states, obtaining stability criteria similar to
those of classical plasmas.Comment: 16 pages, 6 figures, accepted for publication in Journal of
Statistical Mechanics: Theory and Experimen
What is the length of a knot in a polymer?
We give statistical definitions of the length, l, of a loose prime knot tied
into a long, fluctuating ring macromolecule. Monte Carlo results for the
equilibrium, good solvent regime show that ~ N^t, where N is the ring
length and t ~ 0.75 is independent of the knot topology. In the collapsed
regime below the theta temperature, length determinations based on the entropic
competition of different knots within the same ring show delocalization (t~1).Comment: 9 pages, 5 Postscript figure
The broad band spectral properties of galactic X-ray binary pulsars
BeppoSAX observed several galactic binary X-ray pulsars during the Science
Verification Phase and in the first year of the regular program. The complex
emission spectra of these sources are an ideal target for the BeppoSAX
instrumentation, that can measure the emission spectra in an unprecedented
broad energy band. Using this capability of BeppoSAX a detailed observational
work can be done on the galactic X-ray pulsars. In particular the 0.1-200 keV
energy band allows the shape of the continuum emission to be tightly
constrained. A better determination of the underlying continuum allows an
easier detection of features superimposed onto it, both at low energy (Fe K and
L, Ne lines) and at high energies (cyclotron features). We report on the
spectral properties of a sample of X-ray pulsars observed with BeppoSAX
comparing the obtained results. Some ideas of common properties are also
discussed and compared with our present understanding of the emission
mechanisms and processes.Comment: 6 pages, 2 figures. Uses espcrc2.sty (included).To appear in
Proceedings of "The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE
Bistable defect structures in blue phase devices
Blue phases (BPs) are liquid crystals made up by networks of defects, or
disclination lines. While existing phase diagrams show a striking variety of
competing metastable topologies for these networks, very little is known as to
how to kinetically reach a target structure, or how to switch from one to the
other, which is of paramount importance for devices. We theoretically identify
two confined blue phase I systems in which by applying an appropriate series of
electric field it is possible to select one of two bistable defect patterns.
Our results may be used to realise new generation and fast switching
energy-saving bistable devices in ultrathin surface treated BPI wafers.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev. Let
Spectral catalogue of bright gamma-ray bursts detected with the BeppoSAX/GRBM
The emission process responsible for the so-called "prompt" emission of
gamma-ray bursts is still unknown. A number of empirical models fitting the
typical spectrum still lack a satisfactory interpretation. A few GRB spectral
catalogues derived from past and present experiments are known in the
literature and allow to tackle the issue of spectral properties of gamma-ray
bursts on a statistical ground. We extracted and studied the time-integrated
photon spectra of the 200 brightest GRBs observed with the Gamma-Ray Burst
Monitor which flew aboard the BeppoSAX mission (1996-2002) to provide an
independent statistical characterisation of GRB spectra. The spectra were fit
with three models: a simple power-law, a cut-off power law or a Band function.
The typical photon spectrum of a bright GRB consists of a low-energy index
around 1.0 and a peak energy of the nuFnu spectrum E_p~240 keV in agreement
with previous results on a sample of bright CGRO/BATSE bursts. Spectra of ~35%
of GRBs can be fit with a power-law with a photon index around 2, indicative of
peak energies either close to or outside the GRBM energy boundaries. We confirm
the correlation between E_p and fluence, with a logarithmic dispersion of 0.13
around the power-law with index 0.21+-0.06. The low-energy and peak energy
distributions are not yet explained in the current literature. The capability
of measuring time-resolved spectra over a broadband energy range, ensuring
precise measurements of parameters such as E_p, will be crucial for future
experiments (abridged).Comment: 28 pages, 20 figures, 3 tables, accepted to A&
BeppoSAX observations of the X-ray binary pulsar 4U1626-67
We report on observations of the low-mass X-ray binary 4U1626-67 performed
during the BeppoSAX Science Verification Phase. We present the broad-band
0.1-100 keV pulse averaged spectrum, that is well fit by a two-component
function: a 0.27 +/- 0.02 keV blackbody and an absorbed power law with a photon
index of 0.89 +/- 0.02. A very deep and narrow absorption feature at 38 keV,
attributable to electron cyclotron resonance, is clearly visible in the
broad-band spectrum. It corresponds to a neutron star magnetic field strength
of 3.3 x 10^{12} G. The 4U1626-67 pulse profiles show a dramatic dependance on
energy: the transition between the low energy (E<10 keV) "bi-horned" shape to
the high-energy (E>10 keV) sinusoidal profile is clearly visible in our data.
The modulation index shows a monotonic increase with energy.Comment: 4 pages, 2 figures. Uses espcrc2.sty (included). To appear in
Proceedings of "The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE
The 1998 outburst of the X-ray transient XTE J2012+381 as observed with BeppoSAX
We report on the results of a series of X-ray observations of the transient
black hole candidate XTE J2012+381 during the 1998 outburst performed with the
BeppoSAX satellite. The observed broad-band energy spectrum can be described
with the superposition of an absorbed disk black body, an iron line plus a high
energy component, modelled with either a power law or a Comptonisation tail.
The source showed pronounced spectral variability between our five
observations. While the soft component in the spectrum remained almost
unchanged throughout our campaign, we detected a hard spectral tail which
extended to 200 keV in the first two observations, but became barely detectable
up to 50 keV in the following two. A further re-hardening is observed in the
final observation. The transition from a hard to a soft and then back to a hard
state occurred around an unabsorbed 0.1-200 keV luminosity of 10^38 erg/s (at
10 kpc). This indicates that state transitions in XTE 2012+281 are probably not
driven only by mass accretion rate, but additional physical parameters must
play a role in the evolution of the outburst.Comment: Paper accepted for publication on A&A (macro included, 9 pages, 5
figures
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