3,339 research outputs found
The Complexity of Simultaneous Geometric Graph Embedding
Given a collection of planar graphs on the same set of
vertices, the simultaneous geometric embedding (with mapping) problem, or
simply -SGE, is to find a set of points in the plane and a bijection
such that the induced straight-line drawings of
under are all plane.
This problem is polynomial-time equivalent to weak rectilinear realizability
of abstract topological graphs, which Kyn\v{c}l (doi:10.1007/s00454-010-9320-x)
proved to be complete for , the existential theory of the
reals. Hence the problem -SGE is polynomial-time equivalent to several other
problems in computational geometry, such as recognizing intersection graphs of
line segments or finding the rectilinear crossing number of a graph.
We give an elementary reduction from the pseudoline stretchability problem to
-SGE, with the property that both numbers and are linear in the
number of pseudolines. This implies not only the -hardness
result, but also a lower bound on the minimum size of a
grid on which any such simultaneous embedding can be drawn. This bound is
tight. Hence there exists such collections of graphs that can be simultaneously
embedded, but every simultaneous drawing requires an exponential number of bits
per coordinates. The best value that can be extracted from Kyn\v{c}l's proof is
only
Inhomogeneous extragalactic magnetic fields and the second knee in the cosmic ray spectrum
Various experiments indicate the existence of a second knee around energy
E=3.10^{17} eV in the cosmic ray spectrum. This feature could be the signature
of the end of the galactic component and of the emergence of the extragalactic
one, provided that the latter cuts off at low energies. Recent analytical
calculations have shown that this cut-off could be a consequence of the
existence of extragalactic magnetic fields: low energy protons diffuse on
extragalactic magnetic fields and cannot reach the observer within a given
time. We study the influence of inhomogeneous magnetic fields on the magnetic
horizon, using a new semi-analytical propagation code. Our results indicate
that, at a fixed value of the volume averaged magnetic field , the amplitude
of the low energy cut-off is mainly controled by the strength of magnetic
fields in the voids of the large scale structure distribution.Comment: 15 pages, 10 figures. Version to appear in PRD (minor changes
Erratic Jet Wobbling in the BL Lacertae Object OJ287 Revealed by Sixteen Years of 7mm VLBA Observations
We present the results from an ultra-high-resolution 7mm Very Long Baseline
Array (VLBA) study of the relativistic jet in the BL Lacertae object OJ287 from
1995 to 2011 containing 136 total intensity images. Analysis of the image
sequence reveals a sharp jet-position-angle swing by >100 deg. during
[2004,2006], as viewed in the plane of the sky, that we interpret as the
crossing of the jet from one side of the line of sight to the other during a
softer and longer term swing of the inner jet. Modulating such long term swing,
our images also show for the first time a prominent erratic wobbling behavior
of the innermost ~0.4mas of the jet with fluctuations in position angle of up
to ~40 deg. over time scales ~2yr. This is accompanied by highly superluminal
motions along non-radial trajectories, which reflect the remarkable
non-ballistic nature of the jet plasma on these scales. The erratic nature and
short time scales of the observed behavior rules out scenarios such as binary
black hole systems, accretion disk precession, and interaction with the ambient
medium as possible origins of the phenomenon on the scales probed by our
observations, although such processes may cause longer-term modulation of the
jet direction. We propose that variable asymmetric injection of the jet flow;
perhaps related to turbulence in the accretion disk; coupled with hydrodynamic
instabilities, leads to the non-ballistic dynamics that cause the observed
non-periodic changes in the direction of the inner jet.Comment: Accepted for Publication in The Astrophysical Journal. 11 pages, 6
figures, 4 tables. High resolution images on figure 1 and complete tables 1
and 2 may be provided on reques
Locations of Auroral Kilometric Radiation Bursts Inferred From Multi-Spacecraft Wideband Cluster VLBI Observations i: Description of Technique and Initial Results
The Cluster Wideband Data instrument has been used to determine the locations
of auroral kilometric radiation (AKR) using very long baseline interferometry.
The technique involves cross-correlating individual AKR bursts from all six
Cluster baselines using time and frequency filtered waveforms. We report the
locations of over 1,700 individual AKR bursts during six observing epochs
between 10 July 2002 and 22 January 2003 when the Cluster constellation was
high above the southern or northern hemisphere. In general we find that the AKR
burst locations lie along magnetic field lines which map onto the nighttime
auroral zone as expected from previous AKR studies. The distribution of AKR
auroral footprint locations at each epoch had a overall spatial scale between
1000 - 2000 km, much larger than the positional uncertainty of an individual
AKR burst location magnetic footprint, but a small fraction of the auroral
oval. For two of the six epochs, there was a significant drift in the mean
location of AKR activity over a period of 1-2 hours. The drift was
predominantly in latitude at one epoch and in longitude at the other, with
average drift speed V ~ 80-90 m s-1 at the AKR emission location.Comment: 31 pages, 9 figures, accepted for publication 19 June 2003 in JGR
Space Physics. accepted for publicatio
Splitting neutrino masses and showering into Sky
Neutrino masses might be as light as a few time the atmospheric neutrino mass
splitting. High Energy ZeV cosmic neutrinos (in Z-Showering model) might hit
relic ones at each mass in different resonance energies in our nearby Universe.
This non-degenerated density and energy must split UHE Z-boson secondaries (in
Z-Burst model) leading to multi injection of UHECR nucleons within future
extreme AUGER energy. Secondaries of Z-Burst as neutral gamma, below a few tens
EeV are better surviving local GZK cut-off and they might explain recent Hires
BL-Lac UHECR correlations at small angles. A different high energy resonance
must lead to Glashow's anti-neutrino showers while hitting electrons in matter.
In air, Glashow's anti-neutrino showers lead to collimated and directional
air-showers offering a new Neutrino Astronomy. At greater energy around PeV,
Tau escaping mountains and Earth and decaying in flight are effectively
showering in air sky. These Horizontal showering is splitting by geomagnetic
field in forked shapes. Such air-showers secondaries release amplified and
beamed gamma bursts (like observed TGF), made also by muon and electron pair
bundles, with their accompanying rich Cherenkov flashes. Also planet' s largest
(Saturn, Jupiter) atmosphere limbs offer an ideal screen for UHE GZK and
Z-burst tau neutrino, because their largest sizes. Titan thick atmosphere and
small radius are optimal for discovering up-going resonant Glashow resonant
showers. Earth detection of Neutrino showering by twin Magic Telescopes on top
mountains, or by balloons and satellites arrays facing the limbs are the
simplest and cheapest way toward UHE Neutrino Astronomy .Comment: 4 pages, 7 figures; an author's name correction and Journal Referenc
Ultra-High Energy Cosmic Ray Propagation in the Local Supercluster
We present detailed numerical simulations and analytical approximations of
the propagation of nucleons above 10**(19) eV in the Local Supercluster,
assuming that the ambient magnetic field is turbulent, and its strength 0.01 <
B_rms < 1 micro-Gauss. In such strong magnetic fields, protons in the low
energy part of the spectrum, 10**(19) eV < E < E_C diffuse, while the higher
energy particles, with E > E_C propagate along nearly straight lines. The
magnitude of the transition energy E_C depends mainly on the strength of the
magnetic field, the coherence length, and the distance to the source; for
B_rms=0.1 micro-Gauss, a largest eddy of length 10 Mpc, and a distance to the
source of 10 Mpc, E_C=100 EeV. Our numerical treatment substantially improves
on previous analytical approximations, as it allows to treat carefully the
transition between the two propagation regimes, as well as the effects due to
inhomogeneities expected on scales of a few Mpc. We show that a turbulent
magnetic field B_rms=0.1 micro-Gauss, close to equipartition, would allow to
reproduce exactly the observed spectrum of ultra high energy cosmic rays, up to
the highest energy observed, for a distance to the source below 10 Mpc, for the
geometry of the Local Supercluster, i.e. a sheet of thickness 10 Mpc.
Diffusion, in this case, allows to reproduce the high flux beyond the Greisen
Zatsepin Kuzmin cut-off, with a soft injection spectrum proportional to
E**(-2.4). Moreover, the large deflection angles at the highest energies
observed, typically 10 degrees for the above values, would explain why no
close-by astrophysical counterpart could be associated with these events.Comment: 17 latex pages (tightened format), 9 updated postscript figures, uses
revtex.sty and epsf.sty, extended discussion of numerical results, to appear
in Astroparticle Physic
Maximizing the Total Resolution of Graphs
A major factor affecting the readability of a graph drawing is its
resolution. In the graph drawing literature, the resolution of a drawing is
either measured based on the angles formed by consecutive edges incident to a
common node (angular resolution) or by the angles formed at edge crossings
(crossing resolution). In this paper, we evaluate both by introducing the
notion of "total resolution", that is, the minimum of the angular and crossing
resolution. To the best of our knowledge, this is the first time where the
problem of maximizing the total resolution of a drawing is studied.
The main contribution of the paper consists of drawings of asymptotically
optimal total resolution for complete graphs (circular drawings) and for
complete bipartite graphs (2-layered drawings). In addition, we present and
experimentally evaluate a force-directed based algorithm that constructs
drawings of large total resolution
Reconstruction of cosmological density and velocity fields in the Lagrangian Zel'dovich Approximation
We present a method for reconstructing cosmological densityn for and velocity
fields using the Lagrangian Zel'dovich formalism. . The method involves finding
the least action solution for straight line particle paths in an evolving
density field. Our starting point is the final, evolved density , so that we
are in effect carrying out the standard Zel'dovich Approximation based process
in reverse. Using a simple numerical algorithm we are able to minimise the
action for the trajectories of several million particles. We apply our method
to the evolved density taken from N-body simulations of different cold dark
matter dominated universes, testing both the prediction for the present day
velocity field and for the initial density field. The method is easy to apply,
reproduces the accuracy of the forward Zel'dovich Approximation, and also works
directly in redshift space with minimal modification.Comment: 13 pages with only 2 (out 9) figures. MNRAS in press. New Appendix
shows the relation between shell crossing and PIZA. A completed version with
all 9 figures available by anonymous ftp at
ftp://bessel.mps.ohio-state.edu/pub/racc/piza.ps.gz (USA) or
ftp://ftp-astro.physics.ox.ac.uk/pub/eg/piza3.ps.gz (UK
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