1,565 research outputs found
Shifted Laplacian multigrid for the elastic Helmholtz equation
The shifted Laplacian multigrid method is a well known approach for
preconditioning the indefinite linear system arising from the discretization of
the acoustic Helmholtz equation. This equation is used to model wave
propagation in the frequency domain. However, in some cases the acoustic
equation is not sufficient for modeling the physics of the wave propagation,
and one has to consider the elastic Helmholtz equation. Such a case arises in
geophysical seismic imaging applications, where the earth's subsurface is the
elastic medium. The elastic Helmholtz equation is much harder to solve than its
acoustic counterpart, partially because it is three times larger, and partially
because it models more complicated physics. Despite this, there are very few
solvers available for the elastic equation compared to the array of solvers
that are available for the acoustic one. In this work we extend the shifted
Laplacian approach to the elastic Helmholtz equation, by combining the complex
shift idea with approaches for linear elasticity. We demonstrate the efficiency
and properties of our solver using numerical experiments for problems with
heterogeneous media in two and three dimensions
The Cosmic History of Black Hole Growth from Deep Multiwavelength Surveys
Significant progress has been made in the last few years on understanding how
supermassive black holes form and grow. In this paper, we begin by reviewing
the spectral signatures of Active Galactic Nuclei (AGN) ranging from radio to
hard X-ray wavelengths. We then describe the most commonly used methods to find
these sources, including optical/UV, radio, infrared and X-ray emission and
optical emission lines. We then describe the main observational properties of
the obscured and unobscured AGN population. Finally, we summarize the cosmic
history of black hole accretion, i.e., when in the history of the Universe
supermassive black holes were getting most of their mass. We finish with a
summary of open questions and a description of planned and future observatories
that are going to help answer them.Comment: 33 pages, 20 figures, review paper accepted for the Advances in
Astronomy Special Issue "Seeking for the Leading Actor on the Cosmic Stage:
Galaxies versus Supermassive Black Holes
The Evolution and Eddington Ratio Distribution of Compton Thick Active Galactic Nuclei
Previous studies of the active galactic nuclei (AGN) contribution to the
cosmic X-ray background (CXB) consider only observable parameters such as
luminosity and absorbing column. Here, for the first time, we extend the study
of the CXB to physical parameters including the Eddington ratio of the sources
and the black hole mass. In order to calculate the contribution to the CXB of
AGN accreting at various Eddington ratios, an evolving Eddington ratio space
density model is calculated. In particular, Compton thick (CT) AGN are modeled
as accreting at specific, physically motivated Eddington ratios instead of as a
simple extension of the Compton thin type 2 AGN population. Comparing against
the observed CT AGN space densities and log N-log S relation indicates that CT
AGN are likely a composite population of AGN made up of sources accreting
either at >90% or <1% of their Eddington rate.Comment: accepted ApJ
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