Kavli Affiliate: Roger Blandford
| First 5 Authors: Paul Simeon, Noémie Globus, Kirk S. S. Barrow, Roger Blandford,
| Summary:
We propose that a hierarchical shock model$unicode{x2014}$including
supernova remnant shocks, galactic wind termination shocks, and accretion
shocks around cosmic filaments and galaxy clusters$unicode{x2014}$can
naturally explain the cosmic ray spectrum from ~1 GeV up to ~200 EeV. While
this framework applies to the entire cosmic ray spectrum, in this work, we
focus on its implications for ultra-high-energy cosmic rays (UHECRs). We
perform a hydrodynamic cosmological simulation to investigate the power
processed at shocks around clusters and filaments. The downstream flux from
nearby shocks around the local filament accounts for the softer, lower-energy
extragalactic component around the ankle, and the upstream escaping flux from
nearby clusters accounts for the transition to a hard spectral component at the
highest energies. This interpretation is in agreement with UHECR observations.
We suggest that a combination of early-Universe galactic outflows, cosmic ray
streaming instabilities, and a small-scale turbulent dynamo can increase
magnetic fields enough to attain the required rigidities. Our simulation
suggests that the available volume-averaged power density of accretion shocks
exceeds the required UHECR luminosity density by three orders of magnitude. We
show that microgauss magnetic fields at these shocks could explain both the
origin of UHECRs and the as-yet unidentified source of the diffuse radio
synchrotron background below 10 GHz. The shock-accelerated electrons produce a
hard radio background without overproducing diffuse inverse Compton emission.
These results motivate further observational tests with upcoming facilities to
help distinguish accretion shocks from other UHECR sources.
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