580 research outputs found
The randomly driven Ising ferromagnet, Part II: One and two dimensions
We consider the behavior of an Ising ferromagnet obeying the Glauber dynamics
under the influence of a fast switching, random external field. In Part I, we
introduced a general formalism for describing such systems and presented the
mean field theory. In this article we derive results for the one dimensional
case, which can be only partially solved. Monte Carlo simulations performed on
a square lattice indicate that the main features of the mean field theory
survive the presence of strong fluctuations.Comment: 10 pages in REVTeX/LaTeX format, 17 eps/ps figures. Submitted to
Journal of Physics
Pushing the limits, episode 2: K2 observations of extragalactic RR Lyrae stars in the dwarf galaxy Leo IV
We present the first observations of extragalactic pulsating stars in the K2
ecliptic survey of the Kepler space telescope. Variability of all three RR
Lyrae stars in the dwarf spheroidal galaxy Leo IV were successfully detected,
at a brightness of Kp~21.5 mag, from data collected during Campaign 1. We
identified one modulated star and another likely Blazhko candidate with periods
of 29.8+-0.9 d and more than 80 d, respectively. EPIC 210282473 represents the
first star beyond the Magellanic Clouds for which the Blazhko period and
cycle-to-cycle variations in the modulation were unambiguously measured.The
photometric [Fe/H] indices of the stars agree with earlier results that Leo IV
is a very metal-poor galaxy. Two out of three stars blend with brighter
background galaxies in the K2 frames. We demonstrate that image subtraction can
be reliably used to extract photometry from faint confused sources that will be
crucial not only for the K2 mission but for future space photometric missions
as well.Comment: 8 pages, 6 figures, accepted for publication in the Astrophysical
Journal. Light curves can be downloaded from http://konkoly.hu/KIK/data.htm
First observations of W Virginis stars with K2: detection of period doubling
We present the first analysis of W Vir stars observed by the Kepler space
telescope in the K2 mission. Clear cycle-to-cycle variation were detected in
the light curves of KT Sco and the globular cluster member M80-V1. While the
variations in the former star seems to be irregular on the short time scale of
the K2 data, the latter appears to experience period doubling in its pulsation.
Ground-based colour data confirmed that both stars are W Vir-type pulsators,
while a comparison with historical photometric time-series data revealed
drastic period changes in both stars. For comparison we reexamine ground-based
observations of W Vir, the prototype of the class, and conclude that it shows
period doubling instead of mode beating. These results support the notion that
nonlinear dynamics plays an important role in the pulsation of W Virginis-type
stars.Comment: 8 pages, 7 figures, accepted for publication in MNRA
The contact binary VW Cephei revisited: surface activity and period variation
Context. Despite the fact that VW Cephei is one of the well-studied contact
binaries in the literature, there is no fully consistent model available that
can explain every observed property of this system.
Aims. Our motivation is to obtain new spectra along with photometric
measurements, to analyze what kind of changes may have happened in the system
in the past two decades, and to propose new ideas for explaining them.
Methods. For the period analysis we determined 10 new times of minima from
our light curves, and constructed a new OC diagram of the system. Radial
velocities of the components were determined using the cross-correlation
technique. The light curves and radial velocities were modelled simultaneously
with the PHOEBE code. All observed spectra were compared to synthetic spectra
and equivalent widths of the H line were measured on their differences.
Results. We have re-determined the physical parameters of the system
according to our new light curve and spectral models. We confirm that the
primary component is more active than the secondary, and there is a correlation
between spottedness and the chromospheric activity. We propose that flip-flop
phenomenon occurring on the primary component could be a possible explanation
of the observed nature of the activity. To explain the period variation of VW
Cep, we test two previously suggested scenarios: presence of a fourth body in
the system, and the Applegate-mechanism caused by periodic magnetic activity.
We conclude that although none of these mechanisms can be ruled out entirely,
the available data suggest that mass transfer with a slowly decreasing rate
gives the most likely explanation for the period variation of VW Cep.Comment: 13 pages, 18 figures, 9 tables, accepted for publication in Astronomy
and Astrophysic
Necessary and sufficient condition on global optimality without convexity and second order differentiability
The main goal of this paper is to give a necessary and sufficient condition
of global optimality for unconstrained optimization problems, when the objective
function is not necessarily convex. We use Gâteaux differentiability of the objective
function and its bidual (the latter is known from convex analysis)
Two-point theory for the differential self-interrogation Feynman-alpha method
A Feynman-alpha formula has been derived in a two region domain pertaining
the stochastic differential self-interrogation (DDSI) method and the
differential die-away method (DDAA). Monte Carlo simulations have been used to
assess the applicability of the variance to mean through determination of the
physical reaction intensities of the physical processes in the two domains.
More specifically, the branching processes of the neutrons in the two regions
are described by the Chapman - Kolmogorov equation, including all reaction
intensities for the various processes, that is used to derive a variance to
mean relation for the process. The applicability of the Feynman-alpha or
variance to mean formulae are assessed in DDSI and DDAA of spent fuel
configurations.Comment: 15 pages, 5 figures. Submitted to EPJ Plu
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