568 research outputs found
Synchronization of Chaotic Systems by Common Random Forcing
We show two examples of noise--induced synchronization. We study a 1-d map
and the Lorenz systems, both in the chaotic region. For each system we give
numerical evidence that the addition of a (common) random noise, of large
enough intensity, to different trajectories which start from different initial
conditions, leads eventually to the perfect synchronization of the
trajectories. The largest Lyapunov exponent becomes negative due to the
presence of the noise terms.Comment: 5 pages, uses aipproc.cls and aipproc.sty (included). Five double
figures are provided as ten separate gif files. Version with (large)
postscript figures included available from
http://www.imedea.uib.es/PhysDept/publicationsDB/date.htm
A search for x-ray counterparts of gamma-ray bursts with the ROSAT PSPC
We search for faint X-ray bursts with duration 10--300 seconds in the ROSAT
PSPC pointed observations with a total exposure of 1.6e7 seconds. We do not
detect any events shorter than ~100s, i.e. those that could be related to the
classic gamma-ray bursts. At the same time, we detect a number of long flares
with durations of several hundred seconds. Most, but not all, of the long
flares are associated with stars. If even a small number of those long flares,
that cannot identified with stars, are X-ray afterglows of GRB, the number of
X-ray afterglows greatly exceeds the number of BATSE GRB. This would imply that
the beaming factor of gamma-rays from the burst should be >100. The
non-detection of any short bursts in our data constrains the GRB counts at the
fluences 1--2.5 orders of magnitude below the BATSE limit. The constrained
burst counts are consistent with the extrapolation of the BATSE log N - log S
relation. Finally, our results do not confirm a reality of short X-ray flashes
found in the Einstein IPC data by Gotthelf, Hamilton and Helfand.Comment: Accepted to ApJ Letters. 4 pages with 3 figures, LaTeX2
Emission Spectra from Internal Shocks in Gamma-Ray-Burst Sources
Unsteady activity of gamma-ray burst sources leads to internal shocks in
their emergent relativistic wind. We study the emission spectra from such
shocks, assuming that they produce a power-law distribution of relativistic
electrons and posses strong magnetic fields. The synchrotron radiation emitted
by the accelerated electrons is Compton up-scattered multiple times by the same
electrons. A substantial component of the scattered photons acquires high
energies and produces e+e- pairs. The pairs transfer back their kinetic energy
to the radiation through Compton scattering. The generic spectral signature
from pair creation and multiple Compton scattering is highly sensitive to the
radius at which the shock dissipation takes place and to the Lorentz factor of
the wind. The entire emission spectrum extends over a wide range of photon
energies, from the optical regime up to TeV energies. For reasonable values of
the wind parameters, the calculated spectrum is found to be in good agreement
with the burst spectra observed by BATSE.Comment: 12 pages, latex, 2 figures, submitted to ApJ
BeppoSAX observations of Mrk 841 and Mrk335
We present and discuss BeppoSAX observations of Mrk841 and Mrk335, two
Seyfert 1 galaxies in which previous observations have established the presence
of soft excesses. We confirm the soft excess in both sources, even if for
Mrk~841 a warm absorber provides a fit almost as good as the one with a true
excess. As far as the hard X-ray continuum is concerned, a Comptonization model
provides a fit as good as a power law and a physically sound solution for
Mrk841. For Mrk335, the Comptonization model gives a result which is somewhat
better on statistical ground, but rather problematic on physical ground. The
most interesting results regard the reprocessing components. For Mrk841 we find
a very large reflection continuum but an almost normal iron line equivalent
width even if, within the errors, a solution in which both components are a
factor ~2 larger than expected for an accretion disc is still marginally
acceptable. If this is the case, an anisotropy of the primary emission seems
the best explanation. On the contrary, in Mrk335 we find a very large iron line
EW but a reflection component not accordingly large. In this case, the best
solution seems to be in terms of reflection from an ionized disc.Comment: Accepted for publication in A&
Unstable Nonradial Oscillations on Helium Burning Neutron Stars
Material accreted onto a neutron star can stably burn in steady state only
when the accretion rate is high (typically super-Eddington) or if a large flux
from the neutron star crust permeates the outer atmosphere. For such situations
we have analyzed the stability of nonradial oscillations, finding one unstable
mode for pure helium accretion. This is a shallow surface wave which resides in
the helium atmosphere above the heavier ashes of the ocean. It is excited by
the increase in the nuclear reaction rate during the oscillations, and it grows
on the timescale of a second. For a slowly rotating star, this mode has a
frequency of approximately 20-30 Hz (for l=1), and we calculate the full
spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The
short period X-ray binary 4U 1820--30 is accreting helium rich material and is
the system most likely to show this unstable mode,especially when it is not
exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally
stable atmosphere shows that nonradial perturbations have a different stability
criterion than the spherically symmetric thermal perturbations that generate
type I X-ray bursts.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 14
figure
A Long Observation of NGC 5548 by BeppoSAX: the High Energy Cut-off, Intrinsic Spectral Variability and a Truly Warm Absorber
NGC 5548 was observed by BeppoSAX in a single long (8 day) observation from
0.2 to 200 keV. We find (1) the spectral variation of the source is produced by
a change of the intrinsic power law slope; (2) a high energy cut-off at keV with a hint of change of with flux; (3) OVII and
OVIII absorption K edges, and a possible blended OVII-OVIII K
emission feature at keV, inconsistent with a purely
photoionized gas in equilibrium. We propose that the temperature of the
absorbing and emitting gas is K so that both collisional ionization
and photoionization contribute.Comment: 23 pages, 8 figures. Accepted for publication in The Astrophysical
Journa
Compton reflection and iron fluorescence in BeppoSAX observations of Seyfert type 1 galaxies
A sample of nine bright Seyfert 1 and NELG type galaxies, observed with
BeppoSAX, is analyzed to assess on a truly broad band basis (0.1-200 keV) the
issue of the spectral contributions of Compton reflection and iron line
fluorescence from circumnuclear gas. The empirical description adopted for the
direct continuum is the commonly used power law with an exponential cut-off.
The most direct test of the theoretical predictions, namely that the equivalent
width of the line, Walpha, and the strength R of the reflection relative to the
direct continuum are closely related to each other, gives a substantially
positive result, that is their mean ratio is very close to expectation, and
only a modest spread in the iron abundance seems implied. The existence of a
steep correlation between R and the slope Gamma of the power law is not
confirmed. A weak evidence is found that the existence of a very shallow trend
to increase on average with Gamma cannot be altogether excluded in both R and
Walpha, but needs to be tested with a larger sample. The energy Ef in the
exponential cut-off spans a range from about 80 to more than 300 keV. A
possible correlation is found, with Ef increasing on average with Gamma: if
ignored, for instance by keeping Ef at a fixed value in a sample study, it
could be cause of artificial steepening in a correlation between R and Gamma.Comment: accepted for publication in A&
COLON MICROBIAL COMPOSITION IS CORRELATED WITH THE SEVERITY OF COLITIS INDUCED BY 2,4,6-TRINITROBENZESULFONIC ACID IN MICE.
The objective of this study was to evaluate the changes in some bacterial species of colonic microbiota, the clinical signs and the intestinal changes in mice with 2,4,6-trinitrobenzenesulfonic acid (TNBS) induced colitis. CD-1 male mice were randomly divided into three groups and inoculated intrarectally with saline ethanol or TNBS solutions. ethanol and TNBS treatments induced weight loss accompained by mild and severe inflammation of the colon mucosa, respectively. however, TNBS-treated mice displayed significant differences compared to the saline group in terms of disease activity index and histological scoring. Both ethanol and TNBS groups showed an increased prevalence of escherichia coli and Clostridium supp., a decrease in lactobacilli and Bifidobacteria counts, as well as changes in the relative proportions of bacteria in the colon. The results confirm the validity of TNBS treatment to study the mechanism involved in the pathogenesis and progression of inflammatory bowel diseases (IBD) in CD-1 mice. Gut microbiota may become a diagnostic biomarker with therapeutic potential for IBD in the future
Type Ia supernovae and the ^{12}C+^{12}C reaction rate
The experimental determination of the cross-section of the ^{12}C+^{12}C
reaction has never been made at astrophysically relevant energies (E<2 MeV).
The profusion of resonances throughout the measured energy range has led to
speculation that there is an unknown resonance at E\sim1.5 MeV possibly as
strong as the one measured for the resonance at 2.14 MeV. We study the
implications that such a resonance would have for the physics of SNIa, paying
special attention to the phases that go from the crossing of the ignition curve
to the dynamical event. We use one-dimensional hydrostatic and hydrodynamic
codes to follow the evolution of accreting white dwarfs until they grow close
to the Chandrasekhar mass and explode as SNIa. In our simulations, we account
for a low-energy resonance by exploring the parameter space allowed by
experimental data. A change in the ^{12}C+^{12}C rate similar to the one
explored here would have profound consequences for the physical conditions in
the SNIa explosion, namely the central density, neutronization, thermal
profile, mass of the convective core, location of the runaway hot spot, or time
elapsed since crossing the ignition curve. For instance, with the largest
resonance strength we use, the time elapsed since crossing the ignition curve
to the supernova event is shorter by a factor ten than for models using the
standard rate of ^{12}C+^{12}C, and the runaway temperature is reduced from
\sim8.14\times10^{8} K to \sim4.26\times10^{8} K. On the other hand, a
resonance at 1.5 MeV, with a strength ten thousand times smaller than the one
measured at 2.14 MeV, but with an {\alpha}/p yield ratio substantially
different from 1 would have a sizeable impact on the degree of neutronization
of matter during carbon simmering. We conclude that a robust understanding of
the links between SNIa properties and their progenitors will not be attained
until the ^{12}C+^{12}C reaction rate is measured at energies \sim1.5 MeV.Comment: 15 pages, 6 tables, 10 figures, accepted for Astronomy and
Astrophysic
Bailout Embeddings, Targeting of KAM Orbits, and the Control of Hamiltonian Chaos
We present a novel technique, which we term bailout embedding, that can be
used to target orbits having particular properties out of all orbits in a flow
or map. We explicitly construct a bailout embedding for Hamiltonian systems so
as to target KAM orbits. We show how the bailout dynamics is able to lock onto
extremely small KAM islands in an ergodic sea.Comment: 3 figures, 9 subpanel
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