846 research outputs found
The Surface Brightness Fluctuations and Globular Cluster Populations of M87 and its Companions
Using the surface brightness fluctuations in HST WFPC-2 images, we determine
that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC
4476 lies in the background at ~21 Mpc. We also examine the globular clusters
of M87 using archived HST fields. We detect the bimodal color distribution, and
find that the amplitude of the red peak relative to the blue peak is greatest
near the center. This feature is in good agreement with the merger model of
elliptical galaxy formation, where some of the clusters originated in
progenitor galaxies while other formed during mergers.Comment: 5 pages, 2 figure
Recursive Encoding and Decoding of Noiseless Subsystem and Decoherence Free Subspace
When the environmental disturbace to a quantum system has a wavelength much
larger than the system size, all qubits localized within a small area are under
action of the same error operators. Noiseless subsystem and decoherence free
subspace are known to correct such collective errors. We construct simple
quantum circuits, which implement these collective error correction codes, for
a small number of physical qubits. A single logical qubit is encoded with
and , while two logical qubits are encoded with . The recursive
relations among the subspaces employed in noiseless subsystem and decoherence
free subspace play essential r\^oles in our implementation. The recursive
relations also show that the number of gates required to encode logical
qubits increases linearly in .Comment: 9 pages, 3 figure
Critical Collapse of an Ultrarelativistic Fluid in the Limit
In this paper we investigate the critical collapse of an ultrarelativistic
perfect fluid with the equation of state in the limit of
. We calculate the limiting continuously self similar (CSS)
solution and the limiting scaling exponent by exploiting self-similarity of the
solution. We also solve the complete set of equations governing the
gravitational collapse numerically for and
compare them with the CSS solutions. We also investigate the supercritical
regime and discuss the hypothesis of naked singularity formation in a generic
gravitational collapse. The numerical calculations make use of advanced methods
such as high resolution shock capturing evolution scheme for the matter
evolution, adaptive mesh refinement, and quadruple precision arithmetic. The
treatment of vacuum is also non standard. We were able to tune the critical
parameter up to 30 significant digits and to calculate the scaling exponents
accurately. The numerical results agree very well with those calculated using
the CSS ansatz. The analysis of the collapse in the supercritical regime
supports the hypothesis of the existence of naked singularities formed during a
generic gravitational collapse.Comment: 23 pages, 16 figures, revised version, added new results of
investigation of a supercritical collapse and the existence of naked
singularities in generic gravitational collaps
Coherent Ro-vibrational Revivals in a Thermal Molecular Ensemble
We report an experimental and theoretical study of the evolution of
vibrational coherence in a thermal ensemble of nitrogen molecules. Rotational
dephasing and rephasing of the vibrational coherence is detected by coherent
anti-Stokes Raman scattering. The existence of ro-vibrational coupling and the
discrete energy spectrum of the rotational bath lead to a whole new class of
full and fractional ro-vibrational revivals. Following the rich ro-vibrational
dynamics on a nanosecond time scale with sub-picosecond time resolution enables
us to determine the second-order ro-vibrational constant and assess
new possibilities of controlling decoherence.Comment: submitted at Physical Review
Criticality and convergence in Newtonian collapse
We study through numerical simulation the spherical collapse of isothermal
gas in Newtonian gravity. We observe a critical behavior which occurs at the
threshold of gravitational instability leading to core formation. For a given
initial density profile, we find a critical temperature, which is of the same
order as the virial temperature of the initial configuration. For the exact
critical temperature, the collapse converges to a self-similar form, the first
member in Hunter's family of self-similar solutions. For a temperature close to
the critical value, the collapse first approaches this critical solution. Later
on, in the supercritical case, the collapse converges to another self-similar
solution, which is called the Larson-Penston solution. In the subcritical case,
the gas bounces and disperses to infinity. We find two scaling laws: one for
the collapsed mass in the supercritical case and the other for the maximum
density reached before dispersal in the subcritical case. The value of the
critical exponent is measured to be in the supercritical case,
which agrees well with the predicted value . These critical
properties are quite similar to those observed in the collapse of a radiation
fluid in general relativity. We study the response of the system to temperature
fluctuation and discuss astrophysical implications for the insterstellar medium
structure and for the star formation process. Newtonian critical behavior is
important not only because it provides a simple model for general relativity
but also because it is relevant for astrophysical systems such as molecular
clouds.Comment: 15 pages, 8 figures, accepted for publication in PRD, figures 1 and 3
at lower resolution than in journal version, typos correcte
Relativistic MHD with Adaptive Mesh Refinement
This paper presents a new computer code to solve the general relativistic
magnetohydrodynamics (GRMHD) equations using distributed parallel adaptive mesh
refinement (AMR). The fluid equations are solved using a finite difference
Convex ENO method (CENO) in 3+1 dimensions, and the AMR is Berger-Oliger.
Hyperbolic divergence cleaning is used to control the
constraint. We present results from three flat space tests, and examine the
accretion of a fluid onto a Schwarzschild black hole, reproducing the Michel
solution. The AMR simulations substantially improve performance while
reproducing the resolution equivalent unigrid simulation results. Finally, we
discuss strong scaling results for parallel unigrid and AMR runs.Comment: 24 pages, 14 figures, 3 table
Analysis of ``Gauge Modes'' in Linearized Relativity
By writing the complete set of (ADM) equations for linearized waves,
we are able to demonstrate the properties of the initial data and of the
evolution of a wave problem set by Alcubierre and Schutz. We show that the
gauge modes and constraint error modes arise in a straightforward way in the
analysis, and are of a form which will be controlled in any well specified
convergent computational discretization of the differential equations.Comment: 11pages LaTe
Operationally Invariant Measure of the Distance between Quantum States by Complementary Measurements
We propose an operational measure of distance of two quantum states, which
conversely tells us their closeness. This is defined as a sum of differences in
partial knowledge over a complete set of mutually complementary measurements
for the two states. It is shown that the measure is operationally invariant and
it is equivalent to the Hilbert-Schmidt distance. The operational measure of
distance provides a remarkable interpretation of the information distance
between quantum states.Comment: 4 page
Simulating binary neutron stars: dynamics and gravitational waves
We model two mergers of orbiting binary neutron stars, the first forming a
black hole and the second a differentially rotating neutron star. We extract
gravitational waveforms in the wave zone. Comparisons to a post-Newtonian
analysis allow us to compute the orbital kinematics, including trajectories and
orbital eccentricities. We verify our code by evolving single stars and
extracting radial perturbative modes, which compare very well to results from
perturbation theory. The Einstein equations are solved in a first order
reduction of the generalized harmonic formulation, and the fluid equations are
solved using a modified convex essentially non-oscillatory method. All
calculations are done in three spatial dimensions without symmetry assumptions.
We use the \had computational infrastructure for distributed adaptive mesh
refinement.Comment: 14 pages, 16 figures. Added one figure from previous version;
corrected typo
WhiskyMHD: a new numerical code for general relativistic magnetohydrodynamics
The accurate modelling of astrophysical scenarios involving compact objects
and magnetic fields, such as the collapse of rotating magnetized stars to black
holes or the phenomenology of gamma-ray bursts, requires the solution of the
Einstein equations together with those of general-relativistic
magnetohydrodynamics. We present a new numerical code developed to solve the
full set of general-relativistic magnetohydrodynamics equations in a dynamical
and arbitrary spacetime with high-resolution shock-capturing techniques on
domains with adaptive mesh refinements. After a discussion of the equations
solved and of the techniques employed, we present a series of testbeds carried
out to validate the code and assess its accuracy. Such tests range from the
solution of relativistic Riemann problems in flat spacetime, over to the
stationary accretion onto a Schwarzschild black hole and up to the evolution of
oscillating magnetized stars in equilibrium and constructed as consistent
solutions of the coupled Einstein-Maxwell equations.Comment: minor changes to match the published versio
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