1,492 research outputs found
Strong Lensing Reconstruction
We present a general linear algorithm for measuring the surface mass density
1-\kappa from the observable reduced shear g=\gamma/(1-\kappa) in the strong
lensing regime. We show that in general, the observed polarization field can be
decomposed into ``electric'' and ``magnetic'' components, which have
independent and redundant solutions, but perfectly orthogonal noise properties.
By combining these solutions, one can increase the signal-to-noise ratio by
\sqrt{2}. The solutions allow dynamic optimization of signal and noise, both in
real and Fourier space (using arbitrary smoothing windows). Boundary conditions
have no effect on the reconstructions, apart from its effect on the
signal-to-noise. Many existing reconstruction techniques are recovered as
special cases of this framework. The magnetic solution has the added benefit of
yielding the global and local parity of the reconstruction in a single step.Comment: final accepted version for ApJ
Efficient approximations of neutrino physics for three-dimensional simulations of stellar core collapse
Neutrino transport in spherically symmetric models of stellar core collapse
and bounce has achieved a technically complete level, rewarded by the agreement
among independent groups that a multi-dimensional treatment of the
fluid-instabilities in the post-bounce phase is indispensable to model
supernova explosions. While much effort is required to develop a reliable
neutrino transport technique in axisymmetry, we explore neutrino physics
approximations and parameterizations for an efficient three-dimensional
simulation of the fluid-instabilities in the shock-heated matter that
accumulates between the accretion shock and the protoneutron star. We
demonstrate the reliability of a simple parameterization scheme in the collapse
phase and extend our 3D magneto-hydrodynamical collapse simulations to a
preliminary postbounce evolution. The growth of magnetic fields is
investigated.Comment: 5 pages, 4 figures, in Proceedings of "Nuclei in the Cosmos IX,
Geneva, Jun 25-30", associated movies are displayed at
http://www.physik.unibas.ch/~liebend/displa
Comparison between the Blue and the Red Galaxy Alignments Detected in the Sloan Digital Sky Survey
We measure the intrinsic alignments of the blue and the red galaxies
separately by analyzing the spectroscopic data of the Sloan Digital Sky Survey
Data Release 6 (SDSS DR6). For both samples of the red and the blue galaxies
with axial ratios of b/a <= 0.8, we detect a 3 sigma signal of the ellipticity
correlation in the redshift range of 0 <= z <= 0.4 for r-band absolute (model)
magnitude cut of M_r <= -19.2 (no K correction). We note a difference in the
strength and the distance scale for the red and the blue galaxy correlation
eta_{2D}(r): For the bright blue galaxies, it behaves as a quadratic scaling of
the linear density correlation of xi(r) as eta_{2D}(r) proportional to
xi^{2}(r) with strong signal detected only at small distance bin of r <= 3
Mpc/h. While for the bright red galaxies it follows a linear scaling as
eta_{2D}(r) proportional to xi(r) with signals detected at larger distance out
to r~6 Mpc/h. We also test whether the detected correlation signal is intrinsic
or spurious by quantifying the systematic error and find that the effect of the
systematic error on the ellipticity correlation is negligible. It is finally
concluded that our results will be useful for the weak lensing measurements as
well as the understanding of the large scale structure formation.Comment: accepted by ApJL, revised version, 12 pages, 2 figures, 1 table,
systematic error analyzed and beaten down, cross-correlations between the
blue and red galaxies shown, clearer discussion on the different generation
mechanism for the blue and red galaxy alignments adde
Pulsar scintillation patterns and strangelets
We propose that interstellar extreme scattering events, usually observed as
pulsar scintillations, may be caused by a coherent agent rather than the
usually assumed turbulence of clouds. We find that the penetration of
a flux of ionizing, positively charged strangelets or quark nuggets into a
dense interstellar hydrogen cloud may produce ionization trails. Depending on
the specific nature and energy of the incoming droplets, diffusive propagation
or even capture in the cloud are possible. As a result, enhanced electron
densities may form and constitute a lens-like scattering screen for radio
pulsars and possibly for quasars.Comment: 5 pages, 2 figure
Variation in habitat choice and delayed reproduction: Adaptive queuing strategies or individual quality differences?
In most species, some individuals delay reproduction or occupy inferior breeding positions. The queue hypothesis tries to explain both patterns by proposing that individuals strategically delay breeding (queue) to acquire better breeding or social positions. In 1995, Ens, Weissing, and Drent addressed evolutionarily stable queuing strategies in situations with habitat heterogeneity. However, their model did not consider the non - mutually exclusive individual quality hypothesis, which suggests that some individuals delay breeding or occupy inferior breeding positions because they are poor competitors. Here we extend their model with individual differences in competitive abilities, which are probably plentiful in nature. We show that including even the smallest competitive asymmetries will result in individuals using queuing strategies completely different from those in models that assume equal competitors. Subsequently, we investigate how well our models can explain settleme! nt patterns in the wild, using a long-term study on oystercatchers. This long-lived shorebird exhibits strong variation in age of first reproduction and territory quality. We show that only models that include competitive asymmetries can explain why oystercatchers' settlement patterns depend on natal origin. We conclude that predictions from queuing models are very sensitive to assumptions about competitive asymmetries, while detecting such differences in the wild is often problematic.
Approaching the dynamics of hot nucleons in supernovae
All recent numerical simulations agree that stars in the main sequence mass
range of 9-40 solar masses do not produce a prompt hydrodynamic ejection of the
outer layers after core collapse and bounce. Rather they suggest that stellar
core collapse and supernova explosion are dynamically distinct astrophysical
events, separated by an unspectacular accretion phase of at least ~40 ms
duration. As long as the neutrinospheres remain convectively stable, the
explosion dynamics is determined by the neutrons, protons, electrons and
neutrinos in the layer of impact-heated matter piling up on the protoneutron
star. The crucial role of neutrino transport in this regime has been emphasized
in many previous investigations. Here, we search for efficient means to address
the role of magnetic fields and fluid instabilities in stellar core collapse
and the postbounce phase.Comment: 4 pages, contribution to Nuclei in the Cosmos VIII, Jul. 19-23,
submitted to Nucl. Phys.
FISH: A 3D parallel MHD code for astrophysical applications
FISH is a fast and simple ideal magneto-hydrodynamics code that scales to ~10
000 processes for a Cartesian computational domain of ~1000^3 cells. The
simplicity of FISH has been achieved by the rigorous application of the
operator splitting technique, while second order accuracy is maintained by the
symmetric ordering of the operators. Between directional sweeps, the
three-dimensional data is rotated in memory so that the sweep is always
performed in a cache-efficient way along the direction of contiguous memory.
Hence, the code only requires a one-dimensional description of the conservation
equations to be solved. This approach also enable an elegant novel
parallelisation of the code that is based on persistent communications with MPI
for cubic domain decomposition on machines with distributed memory. This scheme
is then combined with an additional OpenMP parallelisation of different sweeps
that can take advantage of clusters of shared memory. We document the detailed
implementation of a second order TVD advection scheme based on flux
reconstruction. The magnetic fields are evolved by a constrained transport
scheme. We show that the subtraction of a simple estimate of the hydrostatic
gradient from the total gradients can significantly reduce the dissipation of
the advection scheme in simulations of gravitationally bound hydrostatic
objects. Through its simplicity and efficiency, FISH is as well-suited for
hydrodynamics classes as for large-scale astrophysical simulations on
high-performance computer clusters. In preparation for the release of a public
version, we demonstrate the performance of FISH in a suite of astrophysically
orientated test cases.Comment: 27 pages, 11 figure
Pulsar timing arrays as imaging gravitational wave telescopes: angular resolution and source (de)confusion
Pulsar timing arrays (PTAs) will be sensitive to a finite number of
gravitational wave (GW) "point" sources (e.g. supermassive black hole
binaries). N quiet pulsars with accurately known distances d_{pulsar} can
characterize up to 2N/7 distant chirping sources per frequency bin \Delta
f_{gw}=1/T, and localize them with "diffraction limited" precision \delta\theta
\gtrsim (1/SNR)(\lambda_{gw}/d_{pulsar}). Even if the pulsar distances are
poorly known, a PTA with F frequency bins can still characterize up to
(2N/7)[1-(1/2F)] sources per bin, and the quasi-singular pattern of timing
residuals in the vicinity of a GW source still allows the source to be
localized quasi-topologically within roughly the smallest quadrilateral of
quiet pulsars that encircles it on the sky, down to a limiting resolution
\delta\theta \gtrsim (1/SNR) \sqrt{\lambda_{gw}/d_{pulsar}}. PTAs may be
unconfused, even at the lowest frequencies, with matched filtering always
appropriate.Comment: 7 pages, 1 figure, matches Phys.Rev.D versio
Generating Cosmological Gaussian Random Fields
We present a generic algorithm for generating Gaussian random initial
conditions for cosmological simulations on periodic rectangular lattices. We
show that imposing periodic boundary conditions on the real-space correlator
and choosing initial conditions by convolving a white noise random field
results in a significantly smaller error than the traditional procedure of
using the power spectrum. This convolution picture produces exact correlation
functions out to separations of L/2, where L is the box size, which is the
maximum theoretically allowed. This method also produces tophat sphere
fluctuations which are exact at radii . It is equivalent to
windowing the power spectrum with the simulation volume before discretizing,
thus bypassing sparse sampling problems. The mean density perturbation in the
volume is no longer constrained to be zero, allowing one to assemble a large
simulation using a series of smaller ones. This is especially important for
simulations of Lyman- systems where small boxes with steep power
spectra are routinely used.
We also present an extension of this procedure which generates exact initial
conditions for hierarchical grids at negligible cost.Comment: 12 pages incl 3 figures, accepted in ApJ Letter
The Sunyaev Zel'dovich effect: simulation and observation
The Sunyaev Zel'dovich effect (SZ effect) is a complete probe of ionized
baryons, the majority of which are likely hiding in the intergalactic medium.
We ran a CDM simulation using a moving mesh hydro code to
compute the statistics of the thermal and kinetic SZ effect such as the power
spectra and measures of non-Gaussianity. The thermal SZ power spectrum has a
very broad peak at multipole with temperature fluctuations
K. The power spectrum is consistent with available
observations and suggests a high and a possible role of
non-gravitational heating. The non-Gaussianity is significant and increases the
cosmic variance of the power spectrum by a factor of for .
We explore optimal driftscan survey strategies for the AMIBA CMB
interferometer and their dependence on cosmology. For SZ power spectrum
estimation, we find that the optimal sky coverage for a 1000 hours of
integration time is several hundred square degrees. One achieves an accuracy
better than 40% in the SZ measurement of power spectrum and an accuracy better
than 20% in the cross correlation with Sloan galaxies for . For
cluster searches, the optimal scan rate is around 280 hours per square degree
with a cluster detection rate 1 every 7 hours, allowing for a false positive
rate of 20% and better than 30% accuracy in the cluster SZ distribution
function measurement.Comment: 34 pages, 20 figures. Submitted to ApJ. Simulation maps have been
replaced by high resolution images. For higher resolution color images,
please download from http://www.cita.utoronto.ca/~zhangpj/research/SZ/ We
corrected a bug in our analysis. the SZ power spectrum decreases 50% and y
parameter decrease 25
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