UHECR Acceleration around Filaments of Cosmological Structure Formation

摘要

A mechanism for proton acceleration to 10 21 eV is suggested. It may operate in accretion flows onto thin darkmatter filaments of cosmic structure formation. The flow compresses the ambient magnetic field to strongly increase andalign it with the filament. Particles begin the acceleration by the E × B drift with the accretion flow. The energy gain in thedrift regime is limited by the conservation of the adiabatic invariant p 2/B(r). Upon approaching the filament, the drift turnsinto the gyro-motion around the filament so that the particle moves parallel to the azimuthal electric field. In this 'betatron'regime the acceleration speeds up to rapidly reach the electrodynamic limit cpmax = eBR for an accelerator with magneticfield B and the orbit radius R (Larmor radius). The periodic orbit becomes unstable and the particle slings out of the filamentto the region of a weak (uncompressed) magnetic field, which terminates the acceleration.To escape the filament, accelerated particles must have gyro-radii comparable with the filament radius. Therefore, themechanism requires pre-acceleration that is likely to occur in structure formation shocks upstream or nearby the filamentaccretion flow. Previous studies identify such shocks as efficient proton accelerators to a firm upper limit 10 19.5 eV placedby the catastrophic photo-pion losses. The present mechanism combines explosive energy gain in its final (betatron) phasewith prompt particle release from the region of strong magnetic field. It is this combination that allows protons to overcomeboth the photo-pion and the synchrotron-Compton losses and therefore attain energy 10 21 eV.