13,281 research outputs found
Infinite primitive and distance transitive directed graphs of finite out-valency
We give certain properties which are satisfied by the descendant set of a vertex in an infinite, primitive, distance transitive digraph of finite out-valency and provide a strong structure theory for digraphs satisfying these properties. In particular, we show that there are only countably many possibilities for the isomorphism type of such a descendant set, thereby confirming a conjecture of the first Author. As a partial converse, we show that certain related conditions on a countable digraph are sufficient for it to occur as the descendant set of a primitive, distance transitive digraph
Antiquities Theft: The Role of the Museum in Modern Symbolic Violence
Humans have been collecting artifacts for centuries, whether it is for their aesthetic value or for the acquisition of knowledge. However, these artifacts have, in most cases, been taken without permission from the countries of origin. Today, museums are struggling with the issue of repatriation and many refuse to return their priceless possessions. Western museums and their supporters are arguing that repatriation will put the artifacts in danger and hurt the chances for humanity to learn from them. The arguments of these museums are an attempt of symbolic violence on non-Western nations, who are seen as unfit or unable to care for their own history
Classification of some countable descendant-homogeneous digraphs
For finite q, we classify the countable, descendant-homogeneous digraphs in
which the descendant set of any vertex is a q-valent tree. We also give
conditions on a rooted digraph G which allow us to construct a countable
descendant-homogeneous digraph in which the descendant set of any vertex is
isomorphic to G.Comment: 16 page
High energy cosmic ray self-confinement close to extragalactic sources
The ultra-high energy cosmic rays observed at the Earth are most likely
accelerated in extra-galactic sources. For the typical luminosities invoked for
such sources, the electric current associated to the flux of cosmic rays that
leave them is large. The associated plasma instabilities create magnetic
fluctuations that can efficiently scatter particles. We argue that this
phenomenon forces cosmic rays to be self-confined in the source proximity for
energies , where GeV
for low background magnetic fields (). For larger values of
, cosmic rays are confined close to their sources for energies GeV, where
is the field in units of nG, is its coherence
lengths in units of 10 Mpc and is the source luminosity in units of
erg/s.Comment: To Appear in Physical Review Letter
Dynamical effects of self-generated magnetic fields in cosmic ray modified shocks
Recent observations of greatly amplified magnetic fields () around supernova shocks are consistent with the predictions of the
non-linear theory of particle acceleration (NLT), if the field is generated
upstream of the shock by cosmic ray induced streaming instability. The high
acceleration efficiencies and large shock modifications predicted by NLT need
however to be mitigated to confront observations, and this is usually assumed
to be accomplished by some form of turbulent heating. We show here that
magnetic fields with the strength inferred from observations have an important
dynamical role on the shock, and imply a shock modification substantially
reduced with respect to the naive unmagnetized case. The effect appears as soon
as the pressure in the turbulent magnetic field becomes comparable with the
pressure of the thermal gas. The relative importance of this unavoidable effect
and of the poorly known turbulent heating is assessed. More specifically we
conclude that even in the cases in which turbulent heating may be of some
importance, the dynamical reaction of the field cannot be neglected, as instead
is usually done in most current calculations.Comment: 4 pages, 1 figure, accepted for publication in ApJ Letter
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