39,629 research outputs found

### Magnetism in the Square Kilometre Array Era

The unprecedented sensitivity, angular resolution and broad bandwidth
coverage of Square Kilometre Array (SKA) radio polarimetric observations will
allow us to address many long-standing mysteries in cosmic magnetism science. I
will highlight the unique capabilities of the SKA to map the warm hot
intergalactic medium, reveal detailed 3-dimensional structures of magnetic
fields in local galaxies and trace the redshift evolution of galactic magnetic
fields.Comment: 4 pages, Invited paper, Proceedings of FM8 "New Insights in
Extragalactic Magnetic Fields", XXXth General Assembly of the IAU, Vienna,
August 20-31, 201

### Delay-dependent robust stability of stochastic delay systems with Markovian switching

In recent years, stability of hybrid stochastic delay systems, one of the important issues in the study of stochastic systems, has received considerable attention. However, the existing results do not deal with the structure of the diffusion but estimate its upper bound, which induces conservatism. This paper studies delay-dependent robust stability of hybrid stochastic delay systems. A delay-dependent criterion for robust exponential stability of hybrid stochastic delay systems is presented in terms of linear matrix inequalities (LMIs), which exploits the structure of the diffusion. Numerical examples are given to verify the effectiveness and less conservativeness of the proposed method

### Microlensing of Sub-parsec Massive Binary Black Holes in Lensed QSOs: Light Curves and Size-Wavelength Relation

Sub-parsec binary massive black holes (BBHs) are long anticipated to exist in
many QSOs but remain observationally elusive. In this paper, we propose a novel
method to probe sub-parsec BBHs through microlensing of lensed QSOs. If a QSO
hosts a sub-parsec BBH in its center, it is expected that the BBH is surrounded
by a circum-binary disk, each component of the BBH is surrounded by a small
accretion disk, and a gap is opened by the secondary component in between the
circum-binary disk and the two small disks. Assuming such a BBH structure, we
generate mock microlensing light curves for some QSO systems that host BBHs
with typical physical parameters. We show that microlensing light curves of a
BBH QSO system at the infrared-optical-UV bands can be significantly different
from those of corresponding QSO system with a single massive black hole (MBH),
mainly because of the existence of the gap and the rotation of the BBH (and its
associated small disks) around the center of mass. We estimate the half-light
radii of the emission region at different wavelengths from mock light curves
and find that the obtained half-light radius vs. wavelength relations of BBH
QSO systems can be much flatter than those of single MBH QSO systems at a
wavelength range determined by the BBH parameters, such as the total mass, mass
ratio, separation, accretion rates, etc. The difference is primarily due to the
existence of the gap. Such unique features on the light curves and half-light
radius-wavelength relations of BBH QSO systems can be used to select and probe
sub-parsec BBHs in a large number of lensed QSOs to be discovered by current
and future surveys, including the Panoramic Survey Telescope and Rapid Response
System (Pan-STARRS), the Large Synoptic Survey telescope (LSST) and Euclid.Comment: 18 pages, 17 figures, accepted for publication in the Astrophysical
Journa

### Gravitational lensing effects on sub-millimetre galaxy counts

We study the effects on the number counts of sub-millimetre galaxies due to
gravitational lensing. We explore the effects on the magnification cross
section due to halo density profiles, ellipticity and cosmological parameter
(the power-spectrum normalisation $\sigma_8$). We show that the ellipticity
does not strongly affect the magnification cross section in gravitational
lensing while the halo radial profiles do. Since the baryonic cooling effect is
stronger in galaxies than clusters, galactic haloes are more concentrated. In
light of this, a new scenario of two halo population model is explored where
galaxies are modeled as a singular isothermal sphere profile and clusters as a
Navarro, Frenk and White (NFW) profile. We find the transition mass between the
two has modest effects on the lensing probability. The cosmological parameter
$\sigma_8$ alters the abundance of haloes and therefore affects our results.
Compared with other methods, our model is simpler and more realistic. The
conclusions of previous works is confirm that gravitational lensing is a
natural explanation for the number count excess at the bright end.Comment: 10 pages, 10 figures, accepted by MNRA

### Fractal space frames and metamaterials for high mechanical efficiency

A solid slender beam of length $L$, made from a material of Young's modulus
$Y$ and subject to a gentle compressive force $F$, requires a volume of
material proportional to $L^{3}f^{1/2}$ [where $f\equiv F/(YL^{2})\ll 1$] in
order to be stable against Euler buckling. By constructing a hierarchical space
frame, we are able to systematically change the scaling of required material
with $f$ so that it is proportional to $L^{3}f^{(G+1)/(G+2)}$, through changing
the number of hierarchical levels $G$ present in the structure. Based on simple
choices for the geometry of the space frames, we provide expressions specifying
in detail the optimal structures (in this class) for different values of the
loading parameter $f$. These structures may then be used to create effective
materials which are elastically isotropic and have the combination of low
density and high crush strength. Such a material could be used to make
light-weight components of arbitrary shape.Comment: 6 pages, 4 figure

### Empirical Study of Simulated Two-planet Microlensing Event

We undertake the first study of two-planet microlensing models recovered from
simulations of microlensing events generated by realistic multi-planet systems
in which 292 planetary events including 16 two-planet events were detected from
6690 simulated light curves. We find that when two planets are recovered, their
parameters are usually close to those of the two planets in the system most
responsible for the perturbations. However, in one of the 16 examples, the
apparent mass of both detected planets was more than doubled by the unmodeled
influence of a third, massive planet. This fraction is larger than, but
statistically consistent with, the roughly 1.5% rate of serious mass errors due
to unmodeled planetary companions for the 274 cases from the same simulation in
which a single planet is recovered. We conjecture that an analogous effect due
to unmodeled stellar companions may occur more frequently. For seven out of 23
cases in which two planets in the system would have been detected separately,
only one planet was recovered because the perturbations due to the two planets
had similar forms. This is a small fraction (7/274) of all recovered
single-planet models, but almost a third of all events that might plausibly
have led to two-planet models. Still, in these cases, the recovered planet
tends to have parameters similar to one of the two real planets most
responsible for the anomaly.Comment: 21 pages, 9 figures, 2 tables; submitted to ApJ; for a short video
introducing the key results, see https://www.youtube.com/watch?v=qhK4a6sbfO

### Almost sure exponential stability of numerical solutions for stochastic delay differential equations

Using techniques based on the continuous and discrete semimartingale convergence theorems, this paper investigates if numerical methods may reproduce the almost sure exponential stability of the exact solutions to stochastic delay differential equations (SDDEs). The important feature of this technique is that it enables us to study the almost sure exponential stability of numerical solutions of SDDEs directly. This is significantly different from most traditional methods by which the almost sure exponential stability is derived from the moment stability by the Chebyshev inequality and the BorelâCantelli lemma

### Object DUO 2: A New Binary Lens Candidate

We present the light curve of an unusual variable object, DUO 2, detected
during the search for microlensing events by the DUO project. The star remained
stable for more than 150 days before it brightened by more than two magnitudes
in 6 days in the B and R bands. The light curves are achromatic during the
variability. We consider possible explanations of the photometric behavior,
with particular emphasis on the binary lens interpretation of the event. The
masses of the lenses are quite small, with the companion possibly in the range
of a brown dwarf or even a few times of Jupiter. We report evidence of blending
of the source by a companion through the first detection of shift in the light
centroid among all the microlensing experiments. This shift sets a lower limit
of $0.3^{\prime\prime}$ on the separation between the stars. The best lens
model obtained requires moderate blending, which was what motivated us to check
the centroid shift that was subsequently found. The best lens model predicts a
separation of $1^{\prime\prime}$ between the two blended stars. This prediction
was recently tested using two CCD images taken under good seeing conditions.
Both images show two components. Their separation and position angle are in
good agreement with our model.Comment: uuencoded, compressed PostScript, 4 pages, 4 figures (in text).
Accepted for publication in Astronomy and Astrophysics Letter

### A New Photometric Model of the Galactic Bar using Red Clump Giants

We present a study of the luminosity density distribution of the Galactic bar
using number counts of red clump giants (RCGs) from the OGLE-III survey. The
data were recently published by Nataf et al. (2013) for 9019 fields towards the
bulge and have $2.94\times 10^6$ RC stars over a viewing area of $90.25
\,\textrm{deg}^2$. The data include the number counts, mean distance modulus
($\mu$), dispersion in $\mu$ and full error matrix, from which we fit the data
with several tri-axial parametric models. We use the Markov Chain Monte Carlo
(MCMC) method to explore the parameter space and find that the best-fit model
is the $E_3$ model, with the distance to the GC is 8.13 kpc, the ratio of
semi-major and semi-minor bar axis scale lengths in the Galactic plane
$x_{0},y_{0}$, and vertical bar scale length $z_0$, is $x_0:y_0:z_0 \approx
1.00:0.43:0.40$ (close to being prolate). The scale length of the stellar
density profile along the bar's major axis is $\sim$ 0.67 kpc and has an angle
of $29.4^\circ$, slightly larger than the value obtained from a similar study
based on OGLE-II data. The number of estimated RC stars within the field of
view is $2.78 \times 10^6$, which is systematically lower than the observed
value. We subtract the smooth parametric model from the observed counts and
find that the residuals are consistent with the presence of an X-shaped
structure in the Galactic centre, the excess to the estimated mass content is
$\sim 5.8%$. We estimate the total mass of the bar is $\sim 1.8 \times 10^{10}
M_\odot$. Our results can be used as a key ingredient to construct new density
models of the Milky Way and will have implications on the predictions of the
optical depth to gravitational microlensing and the patterns of hydrodynamical
gas flow in the Milky Way.Comment: 15 pages, 6 figures, 4 tables. MNRAS accepte

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