Suprathermal electrons in hot interstellar/intracluster media

We discuss stochastic acceleration of electrons in the Galactic ridge and clusters of galaxies. The first-order acceleration is expected to work at shocks in supernova remnants or in the envelope of merging clusters, whereas the second-order acceleration by irregular fields left behind shocks expected in the hot interstellar medium or in the cluster core. The spectrum of accelerated electrons is composed of bulk thermal (Maxwellian), nonthermal (power-law) and intermediate quasi-thermal components. Unlike nonthermal electrons, quasi-thermal electrons interact with background thermal ions and produce nonequilibrium 'thermal' emission, which looks like multi-temperature emission but at a much lower cooling rate. (author)