3,688 research outputs found
The stellar wind velocity field of HD 77581
The early acceleration of stellar winds in massive stars is poorly
constrained. The scattering of hard X-ray photons emitted by the pulsar in the
high-mass X-ray binary Vela X-1 can be used to probe the stellar wind velocity
and density profile close to the surface of its supergiant companion HD 77581.
We built a high signal-to-noise and high resolution hard X-ray lightcurve of
Vela X-1 measured by Swift/BAT over 300 orbital periods of the system and
compared it with the predictions of a grid of hydrodynamic simulations.
We obtain a very good agreement between observations and simulations for a
narrow set of parameters, implying that the wind velocity close to the stellar
surface is twice larger than usually assumed with the standard beta law.
Locally a velocity gradient of is favoured. Even if still
incomplete, hydrodynamic simulations are successfully reproducing several
observational properties of Vela X-1.Comment: submitted to A&A, comments are welcom
Neutron star masses from hydrodynamical effects in obscured sgHMXBs
A population of obscured supergiant High Mass X-ray Binaries (sgHMXBs) has
been discovered by INTEGRAL. X-ray wind tomography of IGR J17252-3616 inferred
a slow wind velocity to account for the enhanced obscuration. The main goal of
this study is to understand under which conditions high obscuration could
occur. We have used an hydrodynamical code to simulate the flow of the stellar
wind around the neutron star. A grid of simulations was used to study the
dependency of the absorbing column density and of the X-ray light-curves on the
model parameters. A comparison between the simulation results and the
observations of IGR J17252-3616 provides an estimate on these parameters. We
have constrained the wind terminal velocity to 500-600 km/s and the neutron
star mass to 1.75-2.15 solar masses. We have confirmed that the initial
hypothesis of a slow wind velocity with a moderate mass loss rate is valid. The
mass of the neutron star can be constrained by studying its impact on the
accretion flow.Comment: A&A in pres
Quantum disorder due to singlet formation: The Plaquette lattice
I study the order/disorder transition due to singlet formation in a quantum
spin system by means of exact diagonalization. The systems is build by spin 1/2
on a two-dimensional square lattice with two different kinds of
antiferromagnetic Heisenberg interactions. The interaction J_p connects 4
nearest neighbor spins on a plaquette. The interaction J_n connects the
plaquettes with each other. If J_p=J_n the systems reduces to the simple square
lattice case. If one of the interactions becomes sufficiently larger then the
other the purely quantum effect of singlet formation drives the system into a
disordered phase with only short range correlations in the plaquettes and a
spin gap. I study the transition point by evaluating the spin gap and spin-spin
correlations. I compare the results with previously calculated data from a
non-linear sigma model approach, spin wave theory and series expansion
calculations. I confirm a critical value of J_n \approx 0.6 for the quantum
phase transition point.Comment: 5 pages (Revtex), 7 figure
Universal Magnetic Properties of at Intermediate Temperatures
We present the theory of two-dimensional, clean quantum antiferromagnets with
a small, positive, zero temperature () stiffness , but with the
ratio arbitrary. Universal scaling forms for the uniform
susceptibility (), correlation length(), and NMR relaxation rate
() are proposed and computed in a expansion and by Mont\'{e}-Carlo
simulations. For large , and asymptote
to universal values, while is nearly -independent. We find good
quantitative agreement with experiments and some numerical studies on
.Comment: 14 pages, REVTEX, 1 postscript figure appende
Origin of the X-ray off-states in Vela X-1
: Vela X-1 is the prototype of the classical sgHMXB systems. Recent
continuous and long monitoring campaigns revealed a large hard X-rays
variability amplitude with strong flares and off-states. This activity has been
interpreted invoking clumpy stellar winds and/or magnetic gating mechanisms.
: We are probing if the observed behaviour could be explained by
unstable hydrodynamic flows close to the neutron star rather than the more
exotic phenomena.
: We have used the hydrodynamic code VH-1 to simulate the flow of
the stellar wind with high temporal resolution and to compare the predicted
accretion rate with the observed light-curves.
: The simulation results are similar to the observed variability.
Off-states are predicted with a duration of 5 to 120 minutes corresponding to
transient low density bubbles forming around the neutron star. Oscillations of
the accretion rate with a typical period of 6800 sec are generated in
our simulations and observed. They correspond to the complex motion of a bow
shock, moving either towards or away from the neutron star. Flares are also
produced by the simulations up to a level of erg/s.
: We have qualitatively reproduced the hard X-ray variations
observed in Vela X-1 with hydrodynamic instabilities predicted by a simple
model. More sophisticated phenomena, such as clumpy winds or the magnetic
gating mechanism, are not excluded but not required to explain the basic
phenomenology.Comment: Accepted for publication in A&
Nearly frozen Coulomb Liquids
We show that very long range repulsive interactions of a generalized
Coulomb-like form , with (-dimensionality),
typically introduce very strong frustration, resulting in extreme fragility of
the charge-ordered state. An \textquotedbl{}almost frozen\textquotedbl{} liquid
then survives in a broad dynamical range above the (very low) melting
temperature which is proportional to . This
\textquotedbl{}pseudogap\textquotedbl{} phase is characterized by unusual
insulating-like, but very weakly temperature dependent transport, similar to
experimental findings in certain low carrier density systems.Comment: 5 pages,4 figure
Signatures of the superfluid to Mott insulator transition in equilibrium and in dynamical ramps
We investigate the equilibrium and dynamical properties of the Bose-Hubbard
model and the related particle-hole symmetric spin-1 model in the vicinity of
the superfluid to Mott insulator quantum phase transition. We employ the
following methods: exact-diagonalization, mean field (Gutzwiller), cluster
mean-field, and mean-field plus Gaussian fluctuations. In the first part of the
paper we benchmark the four methods by analyzing the equilibrium problem and
give numerical estimates for observables such as the density of double
occupancies and their correlation function. In the second part, we study
parametric ramps from the superfluid to the Mott insulator and map out the
crossover from the regime of fast ramps, which is dominated by local physics,
to the regime of slow ramps with a characteristic universal power law scaling,
which is dominated by long wavelength excitations. We calculate values of
several relevant physical observables, characteristic time scales, and an
optimal protocol needed for observing universal scaling.Comment: 23 pages, 13 figure
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