1,783,477 research outputs found
Past and future gauge in numerical relativity
Numerical relativity describes a discrete initial value problem for general
relativity. A choice of gauge involves slicing space-time into space-like
hypersurfaces. This introduces past and future gauge relative to the
hypersurface of present time. Here, we propose solving the discretized Einstein
equations with a choice of gauge in the future and a dynamical gauge in the
past. The method is illustrated on a polarized Gowdy wave.Comment: To appear in Class Quantum Grav, Let
Numerical Integration of Nonlinear Wave Equations for General Relativity
A second-order numerical implementation is given for recently derived
nonlinear wave equations for general relativity. The Gowdy T cosmology is
used as a test bed for studying the accuracy and convergence of simulations of
one-dimensional nonlinear waves. The complete freedom in space-time slicing in
the present formulation is exploited to compute in the Gowdy line-element.
Second-order convergence is found by direct comparison of the results with
either analytical solutions for polarized waves, or solutions obtained from
Gowdy's reduced wave equations for the more general unpolarized waves. Some
directions for extensions are discussed.Comment: 19 pages (LaTex), 3 figures (ps
The Chandra X-ray view of the power sources in Cepheus A
The central part of the massive star-forming region Cepheus A contains
several radio sources which indicate multiple outflow phenomena, yet the
driving sources of the individual outflows have not been identified. We present
a high-resolution Chandra observation of this region that shows the presence of
bright X-ray sources, consistent with active pre-main sequence stars, while the
strong absorption hampers the detection of less luminous objects. A new source
has been discovered located on the line connecting H_2 emission regions at the
eastern and western parts of Cepheus A. This source could be the driving source
of HH 168. We present a scenario relating the observed X-ray and radio
emission.Comment: 7 pages, 6 figures, accepted for publication in A&
Topological Black Holes of Einstein-Yang-Mills dilaton Gravity
We present the topological solutions of Einstein-dilaton gravity in the
presence of a non-Abelian Yang-Mills field. In 4 dimensions, we consider the
and semisimple group as the Yang-Mills gauge group, and
introduce the black hole solutions with spherical and hyperbolic horizons,
respectively. The solution in the absence of dilaton potential is
asymptotically flat and exists only with spherical horizon. Contrary to the
non-extreme Reissner-Nordstrom black hole, which has two horizons with a
timelike and avoidable singularity, here the solution may present a black hole
with a null and unavoidable singularity with only one horizon. In the presence
of dilaton potential, the asymptotic behavior of the solutions is neither flat
nor anti-de Sitter. These solutions contain a null and avoidable singularity,
and may present a black hole with two horizons, an extreme black hole or a
naked singularity. We also calculate the mass of the solutions through the use
of a modified version of Brown and York formalism, and consider the first law
of thermodynamics.Comment: 13 pages, 3 figure
The evolution of the X-ray emission of HH 2 - Investigating heating and cooling processes
Young stellar objects often drive powerful bipolar outflows which evolve on
time scales of a few years. An increasing number of these outflows has been
detected in X-rays implying the existence of million degree plasma almost
co-spatial with the lower temperature gas observed in the optical and near-IR.
The details of the heating and cooling processes of the X-ray emitting part of
these so-called Herbig-Haro objects are still ambiguous, e.g., whether the
cooling is dominated by expansion, radiation or thermal conduction.
We present a second epoch Chandra observation of the first X-ray detected
Herbig-Haro object (HH 2) and derive the proper-motion of the X-ray emitting
plasma and its cooling history. We argue that the most likely explanation for
the constancy of the X-ray luminosity, the alignment with the optical emission
and the proper-motion is that the cooling is dominated by radiative losses
leading to cooling times exceeding a decade. We explain that a strong shock
caused by fast material ramming into slower gas in front of it about ten years
ago can explain the X-ray emission while being compatible with the available
multi-wavelength data of HH 2.Comment: 5 pages with 4 figures; accepted for publication by Astronomy and
Astrophysic
Cluster magnetic fields from active galactic nuclei
Active galactic nuclei (AGN) found at the centers of clusters of galaxies are
a possible source for weak cluster-wide magnetic fields. To evaluate this
scenario, we present 3D adaptive mesh refinement MHD simulations of a cool-core
cluster that include injection of kinetic, thermal, and magnetic energy via an
AGN-powered jet. Using the MHD solver in FLASH 2, we compare several
sub-resolution approaches that link the estimated accretion rate as measured on
the simulation mesh to the accretion rate onto the central black hole and the
resulting feedback. We examine the effects of magnetized outflows on the
accretion history of the black hole and discuss the ability of these models to
magnetize the cluster medium.Comment: 4 pages, 2 figures, submitted to conference proceedings "The
Monster's Fiery Breath: Feedback in Groups, Galaxies, and Clusters
Some geometry and combinatorics for the S-invariant of ternary cubics
Given a real cubic form f(x,y,z), there is a pseudo-Riemannian metric given
by its Hessian matrix, defined on the open subset of R^3 where the Hessian
determinant h is non-zero. We determine the full curvature tensor of this
metric in terms of h and the S-invariant of f, obtaining in the process various
different characterizations of S. Motivated by the case of intersection forms
associated with complete intersection threefolds in the product of three
projective spaces, we then study ternary cubic forms which arise as follows: we
choose positive integers d1, d2, d3, set r = d1 + d2 + d3 - 3, and consider the
coefficient F(x,y,z) of H1^d1 H2^d2 H3^d3 in the product (x H1 + y H2 + z H3)^3
(a_1 H1 + b_1 H2 + c_1 H3) ... (a_r H1 + b_r H2 + c_r H3), the a_j, b_j and c_j
denoting non-negative real numbers; we assume also that F is non-degenerate.
Previous work of the author on sectional curvatures of Kahler moduli suggests a
number of combinatorial conjectures concerning the invariants of F. It is
proved here for instance that the Hessian determinant, considered as a
polynomial in x,y,z and the a_j, b_j, c_j, has only positive coefficients. The
same property is also conjectured to hold for the S-invariant; the evidence and
background to this conjecture is explained in detail in the paper.Comment: 23 pages, plain Tex, updated and shortened, final versio
Finite size corrections to the blackbody radiation laws
We investigate the radiation of a blackbody in a cavity of finite size. For a
given geometry, we use semiclassical techniques to obtain explicit expressions
of the modified Planck's and Stefan-Boltzmann's blackbody radiation laws as a
function of the size and shape of the cavity. We determine the range of
parameters (temperature, size and shape of the cavity) for which these effects
are accessible to experimental verification. Finally we discuss potential
applications of our findings in the physics of the cosmic microwave background
and sonoluminescence.Comment: 5 pages, 1 figure, journal versio
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