1,838 research outputs found
Dwarf Novae in the Shortest Orbital Period Regime: II. WZ Sge Stars as the Missing Population near the Period Minimum
WZ Sge-type dwarf novae are characterized by long recurrence times of
outbursts (~10 yr) and short orbital periods (<~ 85 min). A significant part of
WZ Sge stars may remain undiscovered because of low outburst activity.
Recently, the observed orbital period distribution of cataclysmic variables
(CVs) has changed partly because outbursts of new WZ Sge stars have been
discovered routinely. Hence, the estimation of the intrinsic population of WZ
Sge stars is important for the study of the population and evolution of CVs. In
this paper, we present a Bayesian approach to estimate the intrinsic period
distribution of dwarf novae from observed samples. In this Bayesian model, we
assumed a simple relationship between the recurrence time and the orbital
period which is consistent with observations of WZ Sge stars and other dwarf
novae. As a result, the minimum orbital period was estimated to be ~70 min. The
population of WZ Sge stars exhibited a spike-like feature at the shortest
period regime in the orbital period distribution. These features are consistent
with the orbital period distribution previously predicted by population
synthesis studies. We propose that WZ Sge stars and CVs with a low
mass-transfer rate are excellent candidates for the missing population
predicted by the evolution theory of CVs.Comment: 9 pages, 5 figures, accepted for publication in PAS
Koszul binomial edge ideals
It is shown that if the binomial edge ideal of a graph defines a Koszul
algebra, then must be chordal and claw free. A converse of this statement
is proved for a class of chordal and claw free graphs
Markov basis and Groebner basis of Segre-Veronese configuration for testing independence in group-wise selections
We consider testing independence in group-wise selections with some
restrictions on combinations of choices. We present models for frequency data
of selections for which it is easy to perform conditional tests by Markov chain
Monte Carlo (MCMC) methods. When the restrictions on the combinations can be
described in terms of a Segre-Veronese configuration, an explicit form of a
Gr\"obner basis consisting of moves of degree two is readily available for
performing a Markov chain. We illustrate our setting with the National Center
Test for university entrance examinations in Japan. We also apply our method to
testing independence hypotheses involving genotypes at more than one locus or
haplotypes of alleles on the same chromosome.Comment: 25 pages, 5 figure
Infrared/optical - X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506
We investigate the origin of short X-ray flares which are occasionally
observed in early stages of afterglows of gamma-ray bursts (GRBs). We observed
two events, GRB 071112C and GRB 080506, before the start of X-ray flares in the
optical and near-infrared (NIR) bands with the 1.5-m Kanata telescope. In
conjunction with published X-ray and optical data, we analyzed densely sampled
light curves of the early afterglows and spectral energy distributions (SEDs)
in the NIR-X-ray ranges. We found that the SEDs had a break between the optical
and X-ray bands in the normal decay phases of both GRBs regardless of the model
for the correction of the interstellar extinction in host galaxies of GRBs. In
the X-ray flares, X-ray flux increased by 3 and 15 times in the case of GRB
071112C and 080506, respectively, and the X-ray spectra became harder than
those in the normal decay phases. No significant variation in the optical-NIR
range was detected together with the X-ray flares. These results suggest that
the X-ray flares were associated with either late internal shocks or external
shocks from two-component jets.Comment: 10 pages, 5 figures, accepted to Astronomy and Astrophysic
Multigap Superconductivity in YC: A C-NMR Study
We report on the superconducting (SC) properties of YC with a
relatively high transition temperature K investigated by
C nuclear-magnetic-resonance (NMR) measurements under a magnetic field.
The C Knight shift has revealed a significant decrease below , suggesting a spin-singlet superconductivity. From an analysis of the
temperature dependence of the nuclear spin-lattice relaxation rate in
the SC state, YC is demonstrated to be a multigap superconductor that
exhibits a large gap at the main band and a
small gap at other bands. These results have
revealed that YC is a unique multigap s-wave superconductor similar to
MgB.Comment: 4 pages, 5 figure
Electronic structure of the incommensurate compound
We extracted, from strongly-correlated ab-initio calculations, a complete
model for the chain subsystem of the
incommensurate compound. A second neighbor model has been determined as
a function of the fourth crystallographic parameter , for both low and
room temperature crystallographic structures. The analysis of the obtained
model shows the crucial importance of the structural modulations on the
electronic structure through the on-site energies and the magnetic
interactions. The structural distortions are characterized by their long range
effect on the cited parameters that hinder the reliability of analyses such as
BVS. One of the most striking results is the existence of antiferromagnetic
nearest-neighbor interactions for metal-ligand-metal angles of . A
detailed analysis of the electron localization and spin arrangement is
presented as a function of the chain to ladder hole transfer and of the
temperature. The obtained spin arrangement is in agreement with
antiferromagnetic correlations in the chain direction at low temperature
A simultaneous learning method for both activation functions and connection weights of multilayer nueral networks
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