43,463 research outputs found
Pulsar scintillation patterns and strangelets
We propose that interstellar extreme scattering events, usually observed as
pulsar scintillations, may be caused by a coherent agent rather than the
usually assumed turbulence of clouds. We find that the penetration of
a flux of ionizing, positively charged strangelets or quark nuggets into a
dense interstellar hydrogen cloud may produce ionization trails. Depending on
the specific nature and energy of the incoming droplets, diffusive propagation
or even capture in the cloud are possible. As a result, enhanced electron
densities may form and constitute a lens-like scattering screen for radio
pulsars and possibly for quasars.Comment: 5 pages, 2 figure
Quantum phase transitions in a two-dimensional quantum XYX model: Ground-state fidelity and entanglement
A systematic analysis is performed for quantum phase transitions in a
two-dimensional anisotropic spin 1/2 anti-ferromagnetic XYX model in an
external magnetic field. With the help of an innovative tensor network
algorithm, we compute the fidelity per lattice site to demonstrate that the
field-induced quantum phase transition is unambiguously characterized by a
pinch point on the fidelity surface, marking a continuous phase transition. We
also compute an entanglement estimator, defined as a ratio between the
one-tangle and the sum of squared concurrences, to identify both the
factorizing field and the critical point, resulting in a quantitative agreement
with quantum Monte Carlo simulation. In addition, the local order parameter is
"derived" from the tensor network representation of the system's ground state
wave functions.Comment: 4+ pages, 3 figure
On Anomaly-Free Dark Matter Models
We investigate the predictions of anomaly-free dark matter models for direct
and indirect detection experiments. We focus on gauge theories where the
existence of a fermionic dark matter candidate is predicted by anomaly
cancellation, its mass is defined by the new symmetry breaking scale, and its
stability is guaranteed by a remnant symmetry after the breaking of the gauge
symmetry. We find an upper bound on the symmetry breaking scale by applying the
relic density and perturbative constraints. The anomaly-free property of the
theories allows us to perform a full study of the gamma lines from dark matter
annihilation. We investigate the correlation between predictions for final
radiation processes and gamma lines. Furthermore, we demonstrate that the
latter can be distinguished from the continuum gamma ray spectrum.Comment: 21 pages, 9 figures. v2: minor changes to the text, references added,
version to appear in PR
More on the circumbinary disk of SS 433
Certain lines in spectra of the Galactic microquasar SS 433, in particular
the brilliant H alpha line, have been interpreted as emission from a
circumbinary disk. In this interpretation the orbital speed of the glowing
material is in excess of 200 km/s and the mass of the binary system in excess
of 40 solar masses. A very simple model of excitation of disk material is in
remarkable agreement with the observations, yet it seems that the very
existence of a circumbinary disk is regarded as controversial.
Published spectra, taken almost nightly over two orbital periods of the
binary system, show H alpha and He I lines; these were analysed as
superpositions of Gaussian components. A model in which the excitation of any
given patch of putative circumbinary material is proportional to the inverse
square of its instantaneous distance from the compact object was constructed
and compared with observations.
The new model provides an excellent description of the observations. The
variation of the H alpha and He I spectra with orbital phase are described
quantitatively provided the radius of the emitting ring is not much greater
than the radius of the closest stable circumbinary orbit.
The new analysis has greatly strengthened the case for a circumbinary disk
orbiting the SS 433 system with a speed of over 200 km/s and presents supposed
alternative explanations with major difficulties. If the circumbinary disk
scenario is essentially correct, the mass of the binary system must exceed 40
solar masses and the compact object must be a rather massive black hole. The
case is so strong that this possibility should be taken seriously.Comment: 7 pages, 7 figures. The second version has two additional figures and
an extended discussion. To appear in A &
Some topics in the kinetics of protein aggregation
Preliminary results are presented for the kinetics of phase separation in
three distinct models of protein aggregation. The first is a model of the
formation of spherical microcrystals of insulin via an initial formation of
fractal clusters of insulin. The results of our Brownian dynamics study of this
model are in qualitative agreement with a recent experimental study (Biophys.
Jour. 89 (2005) 3424-3433) of microcrystal formation from aqueous mixtures of
insulin. A second work involves a theory for the formation of metastable
bundles of sickle hemoglobin from fibers, based on a recent generic theory of
bundle formation (Phy. Rev. Lett. 99 (2007) 098101). We also discuss a model
for the microscopic formation of these fibers. Finally, we discuss preliminary
results for the kinetics of cluster formation for a six patch model of protein
crystallization
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