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Astronomy Colloquium
Cosmic ray electron re-acceleration in the intracluster medium via magnetic pumping
Date: Thursday, October 26th
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Aaron Tran, UW-Madison
Abstract: Clusters of galaxies are embedded in a hot (T~10^7 K), low-density halo of gas called the intracluster medium (ICM).  In some clusters, the ICM hosts low-frequency (MHz–GHz) radio synchrotron emission associated with radio galaxies, jets, and cluster merger-induced shocks and turbulence.  The radio emission comes from GeV cosmic ray electrons (CRe).  Some CRe may be "fossils", i.e., previously-accelerated CRe that then radiated and cooled to long-lived MeV energies.  Fossil CRe are invisible until some perturbation, like adiabatic compression or turbulence, re-accelerates the CRe back to GeV energies.  I will present a recent study of CRe re-acceleration via magnetic pumping, a mechanism in which small-scale (nanoparsec) plasma waves coupled to large-scale (kiloparsec to Megaparsec) motions can efficiently energize CRe.  In 1D kinetic plasma simulations, we show that ion cyclotron wave scattering with background compression leads to resonant CRe gaining ~10-30% of their initial energy in one compress/expand cycle, assuming adiabatic expansion without further scattering.  I will comment briefly on the applicability and limitations of this re-acceleration mechanism, and on prospects for future work.
Host: Ke Zhang
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