15,768 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
Contribution to diffuse gamma-ray emission coming from self-confined CRs around their Galactic sources
Recent observations of the diffuse Galactic gamma-ray emission by the
Fermi-LAT satellite have shown significant deviations from models which assume
the same diffusion properties for cosmic rays (CR) throughout the Galaxy. We
explore the possibility that a fraction of this diffuse Galactic emission could
be due to hadronic interactions of CRs self-confined in the region around their
sources. In fact, freshly accelerated CRs that diffuse away from the
acceleration region can trigger the streaming instability able to amplify
magnetic disturbance and to reduce the particle diffusion. When this happen,
CRs are trapped in the near source region for a time longer than expected and
an extended gamma-ray halo is produces around each source. Here we calculate
the contribution to the diffuse gamma-ray background due to the overlap along
lines of sight of several of these extended halos. We find that if the density
of neutrals is low, the halos can account for a substantial fraction of the
diffuse emission observed by Fermi-LAT, depending on the orientation of the
line of sight with respect to the direction of the galactic center.Comment: 8 pages, 2 figs. Proceeding the 35th International Cosmic Ray
Conference (ICRC2017), Bexco, Busan, Kore
- …