410,970 research outputs found
Turning Models Inside Out
We present an approach for change-based (as opposed to state-based) model persistence that can facilitate highperformance incremental model processing (e.g. validation, transformation) by minimising the cost of change identification when models evolve. We illustrate a prototype that implements the proposed approach on top of the Eclipse Modelling Framework and we present a roadmap for further work in this direction
Synthetic NLTE accretion disc spectra for the dwarf nova SS Cyg during an outburst cycle
Dwarf nova outbursts result from enhanced mass transport through the
accretion disc of a cataclysmic variable system.
We assess the question of whether these outbursts are caused by an enhanced
mass transfer from the late-type main sequence star onto the white dwarf
(so-called mass transfer instability model, MTI) or by a thermal instability in
the accretion disc (disc instability model, DIM).
We compute non-LTE models and spectra of accretion discs in quiescence and
outburst and construct spectral time sequences for discs over a complete
outburst cycle. We then compare our spectra to published optical spectroscopy
of the dwarf nova SS Cygni. In particular, we investigate the hydrogen and
helium line profiles that are turning from emission into absorption during the
rise to outburst.
The evolution of the hydrogen and helium line profiles during the rise to
outburst and decline clearly favour the disc-instability model. Our spectral
model sequences allow us to distinguish inside-out and outside-in moving
heating waves in the disc of SS Cygni, which can be related to symmetric and
asymmetric outburst light curves, respectively.Comment: 8 pages, 8 figures; accepted to A&
Forecasting Capacity of ARIMA Models; A Study on Croatian Industrial Production and its Sub-sectors
As one of the most important indicator for monitoring the production in industry as well as for directing investment decisions, industrial production plays important role within growth perspectives. Not only does the composition and/or fluctuation of the goods produced indicate the course of economic activity but it also reflects the changes in cyclical development of the economy thereby providing opportunity to macro-manage with early signs of (short-term) turning-points and (long-term) trend variations. In this paper, we compare univariate autoregressive integrated moving average (ARIMA) models of the Croatian industrial production and its subsectors in order to evaluate their forecasting features within short and long-term data evolution. The aim of this study is not to forecast industrial production but to analyze the out-of-sample predictive performance of ARIMA models on aggregated and disaggregated level inside different forecasting horizons. Our results suggest that ARIMA models do perform very well over the whole rage of the prediction horizons. It is mainly because univariate models often improve the predictive ability of their single component over the short horizons. In that manner ARIMA modelling could be used at least as a benchmark for more complex forecasting methods in predicting the movements of industrial production in Croatia
Cosmology vs. Holography
The most radical version of the holographic principle asserts that all
information about physical processes in the world can be stored on its surface.
This formulation is at odds with inflationary cosmology, which implies that
physical processes in our part of the universe do not depend on the boundary
conditions. Also, there are some indications that the radical version of the
holographic theory in the context of cosmology may have problems with unitarity
and causality. Another formulation of the holographic principle, due to
Fischler and Susskind, implies that the entropy of matter inside the
post-inflationary particle horizon must be smaller than the area of the
horizon. Their conjecture was very successful for a wide class of open and flat
universes, but it did not apply to closed universes. Bak and Rey proposed a
different holographic bound on entropy which was valid for closed universes of
a certain type. However, as we will show, neither proposal applies to open,
flat and closed universes with matter and a small negative cosmological
constant. We will argue, in agreement with Easther, Lowe, and Veneziano, that
whenever the holographic constraint on the entropy inside the horizon is valid,
it follows from the Bekenstein-Hawking bound on the black hole entropy. These
constraints do not allow one to rule out closed universes and other universes
which may experience gravitational collapse, and do not impose any constraints
on inflationary cosmology.Comment: 8 pages, we added one reference and comments about possible problems
with unitarity and causality of the holographic theory in cosmolog
Gravitational waves from a test particle scattered by a neutron star: Axial mode case
Using a metric perturbation method, we study gravitational waves from a test
particle scattered by a spherically symmetric relativistic star. We calculate
the energy spectrum and the waveform of gravitational waves for axial modes.
Since metric perturbations in axial modes do not couple to the matter fluid of
the star, emitted waves for a normal neutron star show only one peak in the
spectrum, which corresponds to the orbital frequency at the turning point,
where the gravitational field is strongest. However, for an ultracompact star
(the radius ), another type of resonant periodic peak appears in
the spectrum. This is just because of an excitation by a scattered particle of
axial quasinormal modes, which were found by Chandrasekhar and Ferrari. This
excitation comes from the existence of the potential minimum inside of a star.
We also find for an ultracompact star many small periodic peaks at the
frequency region beyond the maximum of the potential, which would be due to a
resonance of two waves reflected by two potential barriers (Regge-Wheeler type
and one at the center of the star). Such resonant peaks appear neither for a
normal neutron star nor for a Schwarzschild black hole. Consequently, even if
we analyze the energy spectrum of gravitational waves only for axial modes, it
would be possible to distinguish between an ultracompact star and a normal
neutron star (or a Schwarzschild black hole).Comment: 21 pages, revtex, 11 figures are attached with eps files Accepted to
Phys. Rev.
Magneto-elastic oscillations of neutron stars: exploring different magnetic field configurations
We study magneto-elastic oscillations of highly magnetized neutron stars
(magnetars) which have been proposed as an explanation for the quasi-periodic
oscillations (QPOs) appearing in the decaying tail of the giant flares of soft
gamma-ray repeaters (SGRs). We extend previous studies by investigating various
magnetic field configurations, computing the Alfv\'en spectrum in each case and
performing magneto-elastic simulations for a selected number of models. By
identifying the observed frequencies of 28 Hz (SGR 1900+14) and 30 Hz (SGR
1806-20) with the fundamental Alfv\'en QPOs, we estimate the required surface
magnetic field strength. For the magnetic field configurations investigated
(dipole-like poloidal, mixed toroidal-poloidal with a dipole-like poloidal
component and a toroidal field confined to the region of field lines closing
inside the star, and for poloidal fields with an additional quadrupole-like
component) the estimated dipole spin-down magnetic fields are between 8x10^14 G
and 4x10^15 G, in broad agreement with spin-down estimates for the SGR sources
producing giant flares. A number of these models exhibit a rich Alfv\'en
continuum revealing new turning points which can produce QPOs. This allows one
to explain most of the observed QPO frequencies as associated with
magneto-elastic QPOs. In particular, we construct a possible configuration with
two turning points in the spectrum which can explain all observed QPOs of SGR
1900+14. Finally, we find that magnetic field configurations which are entirely
confined in the crust (if the core is assumed to be a type I superconductor)
are not favoured, due to difficulties in explaining the lowest observed QPO
frequencies (f<30 Hz).Comment: 21 pages, 16 figures, 6 tables, matched to version accepted by MNRAS
with extended comparison/discussion to previous wor
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