38,754 research outputs found
Cluster Accretion Shocks as Possible Acceleration Sites for Ultra High Energy Protons below the Greisen Cutoff
Three-dimensional hydrodynamic simulations of large scale structure in the
Universe have shown that accretion shocks form during the gravitational
collapse of one-dimensional caustics, and that clusters of galaxies formed at
intersections of the caustics are surrounded by these accretion shocks.
Estimated speed and curvature radius of the shocks are 1000-3000 \kms and about
5 Mpc, respectively, in the CDM universe. Assuming that energetic
protons are accelerated by these accretion shocks via the first-order Fermi
process and modeling particle transport around the shocks through Bohm
diffusion, we suggest that protons can be accelerated up to the {\it Greisen
cutoff energy} near eV, provided the mean magnetic field
strength in the region around the shocks is at least of order a microgauss. We
have also estimated the proton flux at earth from the Virgo cluster. Assuming a
few (1-10) \% of the ram pressure of the infalling matter would be transferred
to the cosmic-rays, the estimated flux for eV is consistent
with observations, so that such clusters could be plausible sources of the UHE
CRs.Comment: 14 pages, uuencoded compressed postscript file. Accepted for Jan. 1,
1996 issue of Ap
Winds and Shocks in Galaxy Clusters: Shock Acceleration on an Intergalactic Scale
We review the possible roles of large scale shocks as particle accelerators
in clusters of galaxies. Recent observational and theoretical work has
suggested that high energy charged particles may constitute a substantial
pressure component in clusters. If true that would alter the expected dynamical
evolution of clusters and increase the dynamical masses consistent with
hydrostatic equilibrium. Moderately strong shocks are probably common in
clusters, through the actions of several agents. The most obvious of these
agents include winds from galaxies undergoing intense episodes of starbursts,
active galaxies and cosmic inflows, such as accretion and cluster mergers. We
describe our own work derived from simulations of large scale structure
formation, in which we have, for the first time, explicitly included passive
components of high energy particles. We find, indeed that shocks associated
with these large scale flows can lead to nonthermal particle pressures big
enough to influence cluster dynamics. These same simulations allow us also to
compute nonthermal emissions from the clusters. Here we present resulting
predictions of gamma-ray fluxes.Comment: 12 pages, uses aipproc.cls and aipproc.sty, to appear in Proc. of the
International Symposium on "High Energy Gamma-Ray Astrophysics" (published as
a volume of AIP Conference Series) eds. F. Aharonian and H. Voel
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