7,408 research outputs found
Torus-based compression by factor 4 and 6
We extend the torus-based compression technique for cyclotomic subgroups and show how the elements of certain subgroups in characteristic two and three fields can be compressed by a factor of 4 and 6, respectively. Our compression and decompression functions can be computed at a negligible cost. In particular, our techniques lead to very efficient exponentiation algorithms that work with the compressed representations of elements and can be easily incorporated into pairing-based protocols that require exponentiations or products of pairings
The quenching of star formation in accretion-driven clumpy turbulent tori of active galactic nuclei
Galactic gas-gas collisions involving a turbulent multiphase ISM share common
ISM properties: dense extraplanar gas visible in CO, large linewidths (>= 50
km/s), strong mid-infrared H_2 line emission, low star formation activity, and
strong radio continuum emission. Gas-gas collisions can occur in the form of
ICM ram pressure stripping, galaxy head-on collisions, compression of the
intragroup gas and/or galaxy ISM by an intruder galaxy which flies through the
galaxy group at a high velocity, or external gas accretion on an existing gas
torus in a galactic center. We suggest that the common theme of all these
gas-gas interactions is adiabatic compression of the ISM leading to an increase
of the turbulent velocity dispersion of the gas. The turbulent gas clouds are
then overpressured and star formation is quenched. Within this scenario we
developed a model for turbulent clumpy gas disks where the energy to drive
turbulence is supplied by external infall or the gain of potential energy by
radial gas accretion within the disk. The cloud size is determined by the size
of a C-type shock propagating in dense molecular clouds with a low ionization
fraction at a given velocity dispersion. We give expressions for the expected
volume and area filling factors, mass, density, column density, and velocity
dispersion of the clouds. The latter is based on scaling relations of
intermittent turbulence whose open parameters are estimated for the CND in the
Galactic Center. The properties of the model gas clouds and the external mass
accretion rate necessary for the quenching of the star formation rate due to
adiabatic compression are consistent with those derived from high-resolution
H_2 line observations. Based on these findings, a scenario for the evolution of
gas tori in galactic centers is proposed and the implications for star
formation in the Galactic Center are discussed.Comment: 13 pages, 1 figure, accepted for publication by A&
Radio Galaxy NGC 1265 unveils the Accretion Shock onto the Perseus Galaxy Cluster
We present a consistent 3D model for the head-tail radio galaxy NGC 1265 that
explains the complex radio morphology and spectrum by a past passage of the
galaxy and radio bubble through a shock wave. Using analytical solutions to the
full Riemann problem and hydrodynamical simulations, we study how this passage
transformed the plasma bubble into a toroidal vortex ring. Adiabatic
compression of the aged electron population causes it to be energized and to
emit low-surface brightness and steep-spectrum radio emission. The large infall
velocity of NGC 1265 and the low Faraday rotation measure values and variance
of the jet strongly argue that this transformation was due to the accretion
shock onto Perseus situated roughly at R_200. Estimating the volume change of
the radio bubble enables inferring a shock Mach number of M =
4.2_{-1.2}^{+0.8}, a density jump of 3.4_{-0.4}^{+0.2}, a temperature jump of
6.3_{-2.7}^{+2.5}, and a pressure jump of 21.5 +/- 10.5 while allowing for
uncertainties in the equation of state of the radio plasma and volume of the
torus. Extrapolating X-ray profiles, we obtain upper limits on the gas
temperature and density in the infalling warm-hot intergalactic medium of kT <
0.4 keV and n < 5e-5 / cm^3. The orientation of the ellipsoidally shaped radio
torus in combination with the direction of the galaxy's head and tail in the
plane of the sky is impossible to reconcile with projection effects. Instead,
this argues for post-shock shear flows that have been caused by curvature in
the shock surface with a characteristic radius of 850 kpc. The energy density
of the shear flow corresponds to a turbulent-to-thermal energy density of 14%.
The shock-injected vorticity might be important in generating and amplifying
magnetic fields in galaxy clusters. Future LOFAR observations of head-tail
galaxies can be complementary probes of accretion shocks onto galaxy clusters.Comment: 14 pages, 4 figures, ApJ, in print; v3: typos corrected to match the
published version; v2: improved presentation, added 2D numerical simulations
and exact solution to the 1D Riemann problem of a shock overrunning a
spherical bubble that gets transformed into a vortex rin
Shock waves from non-spherical cavitation bubbles
We present detailed observations of the shock waves emitted at the collapse
of single cavitation bubbles using simultaneous time-resolved shadowgraphy and
hydrophone pressure measurements. The geometry of the bubbles is systematically
varied from spherical to very non-spherical by decreasing their distance to a
free or rigid surface or by modulating the gravity-induced pressure gradient
aboard parabolic flights. The non-spherical collapse produces multiple shocks
that are clearly associated with different processes, such as the jet impact
and the individual collapses of the distinct bubble segments. For bubbles
collapsing near a free surface, the energy and timing of each shock are
measured separately as a function of the anisotropy parameter , which
represents the dimensionless equivalent of the Kelvin impulse. For a given
source of bubble deformation (free surface, rigid surface or gravity), the
normalized shock energy depends only on , irrespective of the bubble
radius and driving pressure . Based on this finding, we
develop a predictive framework for the peak pressure and energy of shock waves
from non-spherical bubble collapses. Combining statistical analysis of the
experimental data with theoretical derivations, we find that the shock peak
pressures can be estimated as jet impact-induced hammer pressures, expressed as
at . The same approach is found to explain the shock energy quenching as a
function of .Comment: Accepted for publication in Physical Review Fluid
On the Formation of Cluster Radio Relics
(abridged) We present detailed 3-dimensional magneto-hydrodynamical
simulations of the passage of a radio plasma cocoon filled with turbulent
magnetic fields through a shock wave. Taking into account synchrotron, inverse
Compton and adiabatic energy losses and gains we evolved the relativistic
electron population to produce synthetic polarisation radio maps. On contact
with the shock wave the radio cocoons are first compressed and finally torn
into filamentary structures, as is observed in several cluster radio relics. In
the synthetic radio maps the electric polarisation vectors are mostly
perpendicular to the filamentary radio structures. If the magnetic field inside
the cocoon is not too strong, the initially spherical radio cocoon is
transformed into a torus after the passage of the shock wave. Very recent,
high-resolution radio maps of cluster radio relics seem to exhibit such
toroidal geometries in some cases. This supports the hypothesis that cluster
radio relics are fossil radio cocoons that have been revived by a shock wave.
For a late-stage relic the ratio of its global diameter to the filament
diameter should correlate with the shock strength. Finally, we argue that the
total radio polarisation of radio relic should be well correlated with the
3-dimensional orientation of the shock wave that produced the relic.Comment: accepted by MNRAS, 10 pages, 13 figures, some modifications due to
comments of a refere
The length of unknotting tunnels
We show there exist tunnel number one hyperbolic 3-manifolds with arbitrarily
long unknotting tunnel. This provides a negative answer to an old question of
Colin Adams.Comment: 20 pages, 6 figures. 20 pages, 6 figures. Daryl Cooper added as
author. V2 contains two new sections, including a second proof of the main
result, and a proof that the result holds for knots in homology sphere
MHD models of Pulsar Wind Nebulae
Pulsar Wind Nebulae (PWNe) are bubbles or relativistic plasma that form when
the pulsar wind is confined by the SNR or the ISM. Recent observations have
shown a richness of emission features that has driven a renewed interest in the
theoretical modeling of these objects. In recent years a MHD paradigm has been
developed, capable of reproducing almost all of the observed properties of
PWNe, shedding new light on many old issues. Given that PWNe are perhaps the
nearest systems where processes related to relativistic dynamics can be
investigated with high accuracy, a reliable model of their behavior is
paramount for a correct understanding of high energy astrophysics in general. I
will review the present status of MHD models: what are the key ingredients,
their successes, and open questions that still need further investigation.Comment: 18 pages, 5 figures, Invited Review, Proceedings of the "ICREA
Workshop on The High-Energy Emission from Pulsars and their Systems", Sant
Cugat, Spain, April 12-16, 201
Theory of Pulsar Wind Nebulae
Our understanding of Pulsar Wind Nebulae (PWNe), has greatly improved in the
last years thanks to unprecedented high resolution images taken from the
HUBBLE, CHANDRA and XMM satellites. The discovery of complex but similar inner
features, with the presence of unexpected axisymmetric rings and jets, has
prompted a new investigation into the dynamics of the interaction of the pulsar
winds with the surrounding SNR, which, thanks to the improvement in the
computational resources, has let to a better understanding of the properties of
these objects. On the other hand the discovery of non-thermal emission from bow
shock PWNe, and of systems with a complex interaction between pulsar and SNR,
has led to the development of more reliable evolutionary models. I will review
the standard theory of PWNe, their evolution, and the current status in the
modeling of their emission properties, in particular I will show that our
evolutionary models are able to describe the observations, and that the X-ray
emission can now be reproduced with sufficient accuracy, to the point that we
can use these nebulae to investigate fundamental issues as the properties of
relativistic outflows and particle acceleration.Comment: 9 page, 5 figures, Proceeding of the conference "40 Years of
Pulsars", 12-17 August 2007, Montreal, Canada. (figures are not properly
displayed in .ps or .pdf version please download archive for them
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