1,668 research outputs found
The Applegate mechanism in Post-Common-Envelope Binaries: Investigating the role of rotation
Eclipsing time variations (ETVs) are observed in many close binary systems.
In particular, for several post-common-envelope binaries (PCEBs) that consist
of a white dwarf and a main sequence star, the O-C diagram suggests that real
or apparent orbital period variations are driven by Jupiter-mass planets or as
a result of magnetic activity, the so-called Applegate mechanism. The latter
explains orbital period variations as a result of changes in the stellar
quadrupole moment due to magnetic activity. We explore the feasibility of
driving ETVs via the Applegate mechanism for a sample of PCEB systems,
including a range of different rotations. Using the MESA code we evolve 12
stars with different masses and rotation rates. We apply a simple dynamo model
to their radial profiles to investigate on which scale the predicted activity
cycle matches the observed modulation period, and quantify the uncertainty, and
further calculate the required energies to drive que Applegate mechanism. We
show that the Applegate mechanism is energetically feasible in 5 PCEB systems,
and note that these are the systems with the highest rotation rate compared to
the critical rotation rate of the main-sequence star. The results suggest that
the ratio of physical to critical rotation in the main sequence star is an
important indicator for the feasibility of Applegate's mechanism, but exploring
larger samples will be necessary to probe this hypothesis.Comment: 9 pages, 5 figures. Accepted for publication in A&
Inducing nonclassical lasing via periodic drivings in circuit quantum electrodynamics
We show how a pair of superconducting qubits coupled to a microwave cavity mode can be used to engineer a single-atom laser that emits light into a nonclassical state. Our scheme relies on the dressing of the qubit-field coupling by periodic modulations of the qubit energy. In the dressed basis, the radiative decay of the first qubit becomes an effective incoherent pumping mechanism that injects energy into the system, hence turning dissipation to our advantage. A second, auxiliary qubit is used to shape the decay within the cavity, in such a way that lasing occurs in a squeezed basis of the cavity mode. We characterize the system both by mean-field theory and exact calculations. Our work may find applications in the generation of squeezing and entanglement in circuit QED, as well as in the study of dissipative few- and many-body phase transitions
Simulating quantum-optical phenomena with cold atoms in optical lattices
We propose a scheme involving cold atoms trapped in optical lattices to
observe different phenomena traditionally linked to quantum-optical systems.
The basic idea consists of connecting the trapped atomic state to a non-trapped
state through a Raman scheme. The coupling between these two types of atoms
(trapped and free) turns out to be similar to that describing light-matter
interaction within the rotating-wave approximation, the role of matter and
photons being played by the trapped and free atoms, respectively. We explain in
particular how to observe phenomena arising from the collective spontaneous
emission of atomic and harmonic oscillator samples such as superradiance and
directional emission. We also show how the same setup can simulate Bose-Hubbard
Hamiltonians with extended hopping as well as Ising models with long-range
interactions. We believe that this system can be realized with state of the art
technology
Symbiotic Stars in OGLE Data I. Large Magellanic Cloud Systems
Symbiotic stars are long-orbital-period interacting-binaries characterized by
extended emission over the whole electromagnetic range and by complex
photometric and spectroscopic variability. In this paper, the first of a
series, we present OGLE light curves of all the confirmed symbiotic stars in
the Large Magellanic Cloud, with one exception. By careful visual inspection
and combined time-series analysis techniques, we investigate for the first time
in a systematic way the photometric properties of these astrophysical objects,
trying in particular to distinguish the nature of the cool component (e.g.,
Semi-Regular Variable vs. OGLE Small-Amplitude Red Giant), to provide its
first-order pulsational ephemerides, and to link all this information with the
physical parameters of the binary system as a whole. Among the most interesting
results, there is the discovery of a 20-year-long steady fading of Sanduleak's
star, a peculiar symbiotic star known to produce the largest stellar jet ever
discovered. We discuss by means of direct examples the crucial need for
long-term multi-band observations to get a real understanding of symbiotic and
other interacting binary stars. We eventually introduce BOMBOLO, a multi-band
simultaneous imager for the SOAR 4m Telescope, whose design and construction we
are currently leading.Comment: 16 pages, 4 Tables, 12 Figures. Accepted for publication in MNRA
Quantum coherent control of highly multipartite continuous-variable entangled states by tailoring parametric interactions
The generation of continuous-variable multipartite entangled states is
important for several protocols of quantum information processing and
communication, such as one-way quantum computation or controlled dense coding.
In this article we theoretically show that multimode optical parametric
oscillators can produce a great variety of such states by an appropriate
control of the parametric interaction, what we accomplish by tailoring either
the spatio-temporal shape of the pump, or the geometry of the nonlinear medium.
Specific examples involving currently available optical parametric oscillators
are given, hence showing that our ideas are within reach of present technology.Comment: 14 pages, 5 figure
Long-term variations in the X-ray activity of HR 1099
Although timing variations in close binary systems have been studied for a
long time, their underlying causes are still unclear. A possible explanation is
the so-called Applegate mechanism, where a strong, variable magnetic field can
periodically change the gravitational quadrupole moment of a stellar component,
thus causing observable period changes. One of the systems exhibiting such
strong orbital variations is the RS CVn binary HR 1099, whose activity cycle
has been studied by various authors via photospheric and chromospheric activity
indicators, resulting in contradicting periods. We aim at independently
determining the magnetic activity cycle of HR 1099 using archival X-ray data to
allow for a comparison to orbital period variations. Archival X-ray data from
80 different observations of HR 1099 acquired with 12 different X-ray
facilities and covering almost four decades were used to determine X-ray fluxes
in the energy range of 2-10 keV via spectral fitting and flux conversion. Via
the Lomb-Scargle periodogram we analyze the resulting long-term X-ray light
curve to search for periodicities. We do not detect any statistically
significant periodicities within the X-ray data. An analysis of optical data of
HR 1099 shows that the derivation of such periods is strongly dependent on the
time coverage of available data, since the observed optical variations strongly
deviate from a pure sine wave. We argue that this offers an explanation as to
why other authors derive such a wide range of activity cycle periods based on
optical data. We conclude that our analysis constitutes the longest stellar
X-ray activity light curve acquired to date, yet the still rather sparse
sampling of the X-ray data, along with stochastic flaring activity, does not
allow for the independent determination of an X-ray activity cycle.Comment: 8 pages, 6 figures, 2 tables accepted for publication in A&
Generating highly squeezed Hybrid Laguerre-Gauss modes in large-Fresnel-number Degenerate Optical Parametric Oscillators
We theoretically describe the quantum properties of a large Fresnel number
degenerate optical parametric oscillator with spherical mirrors that is pumped
by a Gaussian beam. The resonator is tuned so that the resonance frequency of a
given transverse mode family coincides with the down-converted frequency. After
demonstrating that only the lower orbital angular momentum (OAM) Laguerre-Gauss
modes are amplified above threshold, we focus on the quantum properties of the
rest of (classically empty) modes. We find that combinations of opposite OAM
(Hybrid Laguerre-Gauss modes) can exhibit arbitrary large quadrature squeezing
for the lower OAM non amplified modes.Comment: 10 pages, 3 figures and 2 table
An efficient semiparametric maxima estimator of the extremal index
The extremal index , a measure of the degree of local dependence in
the extremes of a stationary process, plays an important role in extreme value
analyses. We estimate semiparametrically, using the relationship
between the distribution of block maxima and the marginal distribution of a
process to define a semiparametric model. We show that these semiparametric
estimators are simpler and substantially more efficient than their parametric
counterparts. We seek to improve efficiency further using maxima over sliding
blocks. A simulation study shows that the semiparametric estimators are
competitive with the leading estimators. An application to sea-surge heights
combines inferences about with a standard extreme value analysis of
block maxima to estimate marginal quantiles.Comment: 17 pages, 7 figures. Minor edits made to version 1 prior to journal
publication. The final publication is available at Springer via
http://dx.doi.org/10.1007/s10687-015-0221-
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