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J. A. Eilek, J. C. Weatherall (New Mexico Tech)
Radio haloes in clusters present a severe test of current particle acceleration models. The relativistic electrons suffer radiative losses on the microwave background, and thus must be locally reaccelerated. Diffusion from an active galaxy is too slow to fill the extended haloes, and most halo clusters show little sign of large-scale shocks. One is left with MHD turbulence as the only source of acceleration. But this model also appears to have problems: acceleration by turbulent Alfven waves is a slow process. We find that the rate of acceleration by resonant Alfven waves, in the turbulent cluster environment, is much too slow to account for these haloes.
We propose an alternative model. We suspect the turbulence contains strong lower hybrid (LH) waves, which have been driven by cascade from longer-wavelength MHD turbulence. Transit time damping of strong LH turbulence is known to be effective in accelerating particles in the terrestrial magnetosphere; we show that it is also capable of acceleration of the relativistic electrons which produce the radio haloes. We are undertaking numerical simulations to follow the detailed evolution of the cascade, and of the LH wave packet collapse and its interaction with relativistic plasma particles. We estimate the LH acceleration rate in the cluster environment, given observational constraints on the level of turbulence, and derive conditions under which strong LH turbulent acceleration can maintain the relativistic electron supply in the haloes.
We speculate that the occurence of strong cluster-scale haloes may be a consequence of favorable conditions which allow the MHD cascade effectively to drive strong LH turbulence.