1,968 research outputs found
Oscillations of relativistic axisymmetric tori and implications for modelling kHz-QPOs in neutron-star X-ray binaries
We perform a global linear perturbative analysis, and investigate the
oscillation properties of relativistic, non-selfgravitating tori orbiting
around neutron stars in the slow rotation limit approximation. Extending the
work done in Schwarzschild and Kerr backgrounds, we consider the axisymmetric
oscillations of vertically integrated tori in the Hartle-Thorne spacetime. The
equilibrium models are constructed by selecting a number of different
non-Keplerian distributions of specific angular momentum, allowing for disc
sizes gravitational radii. Our results, obtained after
solving a global eigenvalue problem to compute the xisymmetric -modes,
indicate that such oscillation modes could account with most observed lower
() and upper () high frequency quasi-periodic oscillations for
Sco X-1, and for some Z sources and Atoll sources with Hz.
However, when Hz, -modes oscillations do not account
for the linear relation , between the upper and
the lower high frequency quasi-periodic oscillations that are observed in
neutron star low-mass X-ray binaries.Comment: 8 pages, 4 figures, matches accepted version for publication in MNRA
Molecular simulation study of the heat capacity of metastable water between 100K and 300K
Molecular simulation study of the heat capacity of metastable water between
100K and 300K Molecular simulations have been used to study the heat capacity
of metastable liquid water at low temperature adsorbed on a smooth surface.
These calculations aim at modelling water properties measured by experiments
performed on water films adsorbed on Vycor nanoporous silica at low
temperature. In particular, the study focuses on the non-monotonous variation
of the heat capacity around between 100 and 300 K
General Relativistic Magnetospheres of Slowly Rotating and Oscillating Magnetized Neutron Stars
We study the magnetosphere of a slowly rotating magnetized neutron star
subject to toroidal oscillations in the relativistic regime. Under the
assumption of a zero inclination angle between the magnetic moment and the
angular momentum of the star, we analyze the Goldreich-Julian charge density
and derive a second-order differential equation for the electrostatic
potential. The analytical solution of this equation in the polar cap region of
the magnetosphere shows the modification induced by stellar toroidal
oscillations on the accelerating electric field and on the charge density. We
also find that, after decomposing the oscillation velocity in terms of
spherical harmonics, the first few modes with are responsible for
energy losses that are almost linearly dependent on the amplitude of the
oscillation and that, for the mode , can be a factor
larger than the rotational energy losses, even for a velocity oscillation
amplitude at the star surface as small as . The results
obtained in this paper clarify the extent to which stellar oscillations are
reflected in the time variation of the physical properties at the surface of
the rotating neutron star, mainly by showing the existence of a relation
between and the oscillation amplitude. Finally, we propose a
qualitative model for the explanation of the phenomenology of intermittent
pulsars in terms of stellar oscillations that are periodically excited by star
glitches.Comment: 13 pages, 4 figures, submitted to MNRA
Dynamics of thick discs around Schwarzschild-de Sitter black holes
We consider the effects of a cosmological constant on the dynamics of
constant angular momentum discs orbiting Schwarzschild-de Sitter black holes.
The motivation behind this study is to investigate whether the presence of a
radial force contrasting the black hole's gravitational attraction can
influence the occurrence of the runaway instability, a robust feature of the
dynamics of constant angular momentum tori in Schwarzschild and Kerr
spacetimes. In addition to the inner cusp near the black hole horizon through
which matter can accrete onto the black hole, in fact, a positive cosmological
constant introduces also an outer cusp through which matter can leave the torus
without accreting onto the black hole. To assess the impact of this outflow on
the development of the instability we have performed time-dependent and
axisymmetric hydrodynamical simulations of equilibrium initial configurations
in a sequence of background spacetimes of Schwarzschild-de Sitter black holes
with increasing masses. The simulations have been performed with an unrealistic
value for the cosmological constant which, however, yields sufficiently small
discs to be resolved accurately on numerical grids and thus provides a first
qualitative picture of the dynamics. The calculations, carried out for a wide
range of initial conditions, show that the mass-loss from the outer cusp can
have a considerable impact on the instability, with the latter being rapidly
suppressed if the outflow is large enough.Comment: 12 pages; A&A, in pres
Explaining the subpulse drift velocity of pulsar magnetosphere within the space-charge limited flow model
We try to explain the subpulse drift phenomena adopting the space-charge
limited flow (SCLF) model and comparing the plasma drift velocity in the inner
region of pulsar magnetospheres with the observed velocity of drifting
subpulses. We apply the approach described in a recent paper of van Leeuwen &
Timokhin (2012), where it was shown that the standard estimation of the
subpulse drift velocity through the total value of the scalar potential drop in
the inner gap gives inaccurate results, while the exact expression relating the
drift velocity to the gradient of the scalar potential should be used instead.
After considering a selected sample of sources taken from the catalog of
Weltevrede, Edwards & Stappers (2006) with coherently drifting subpulses and
reasonably known observing geometry, we show that their subpulse drift
velocities would correspond to the drift of the plasma located very close or
above the pair formation front. Moreover, a detailed analysis of PSR B0826-34
and PSR B0818-41 reveals that the variation of the subpulse separation with the
pulse longitude can be successfully explained by the dependence of the plasma
drift velocity on the angular coordinates.Comment: 14 pages, 6 figures, 2 table
A New Simple Model for High Frequency Quasi Periodic Oscillations in Black Hole Candidates
Observations of X-ray emissions from binary systems have long since been
considered important tools to test General Relativity in strong-field regimes.
The high frequency quasi-periodic oscillations (HFQPOs) observed in binaries
containing a black hole candidate, in particular, have been proposed as a means
to measure more directly the black hole properties such as its mass and spin.
Numerous models have been suggested to explain the HFQPOs and the rich
phenomenology accompanying them. Many of these models rest on a number of
assumptions and are at times in conflict with the most recent observations. We
here propose a new, simple model in which the HFQPOs result from basic -mode
oscillations of a small accretion torus orbiting close to the black hole. We
show that within this model the key properties of the HFQPOs can be explained
simply, given a single reasonable assumption. We also discuss observational
tests that can falsify the model.Comment: 5 pages; MNRAS Letters, in pres
Hacia una fenomenología de las ciencias sociales
Este ensayo fue escrito en enero de 1997. El autor quiere agradecer las sugerencias, enseñanzas y consejos -anteriores a la redacción- de Francisco Leocata, si bien, obviamente, este agradecimiento lo exime de toda responsabilidad en los posibles errores cometidos. También agradecemos los comentarios de Luciano Elizalde, Carlos Álvarez y Ricardo Crespo
Dynamics of magnetized relativistic tori oscillating around black holes
We present a numerical study of the dynamics of magnetized, relativistic,
non-self-gravitating, axisymmetric tori orbiting in the background spacetimes
of Schwarzschild and Kerr black holes. The initial models have a constant
specific angular momentum and are built with a non-zero toroidal magnetic field
component, for which equilibrium configurations have recently been obtained. In
this work we extend our previous investigations which dealt with purely
hydrodynamical thick discs, and study the dynamics of magnetized tori subject
to perturbations which, for the values of the magnetic field strength
considered here, trigger quasi-periodic oscillations lasting for tens of
orbital periods. Overall, we have found that the dynamics of the magnetized
tori analyzed is very similar to that found in the corresponding unmagnetized
models. The spectral distribution of the eigenfrequencies of oscillation shows
the presence of a fundamental p mode and of a series of overtones in a harmonic
ratio 2:3:.... These simulations, therefore, extend the validity of the model
of Rezzolla et al.(2003a) for explaining the high-frequency QPOs observed in
the spectra of LMXBs containing a black-hole candidate also to the case of
magnetized discs with purely toroidal magnetic field distribution. If
sufficiently compact and massive, these oscillations can also lead to the
emission of intense gravitational radiation which is potentially detectable for
sources within the Galaxy.Comment: 10 pages,7 figures, submitted to MNRA
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