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thermal bremsstrahlung

(type of EMR produced by thin, hot plasma)

Thermal bremsstrahlung is a type of thermal emission that hot, thin plasma can emit, that has a spectrum differing from a black-body spectrum. Specifically it occurs at higher temperatures if the plasma particles are in Maxwell-Boltzmann distribution, but the plasma is optically thin, with the photons not coupled to the particles those formed by bremsstrahlung escape the gas without having undergone sufficient processing (absorption, re-emission, and scattering by the particles) to transform the electromagnetic radiation (EMR) into a black body spectrum, and contribute to the shape of the spectral energy distribution (SED). The SED is "flattened", not reaching as high a specific intensity as a black body spectrum for the same the temperature, but generally black-body-like at lower frequencies, adhering to the Rayleigh-Jeans law (due to the plasma being more opaque to these frequencies, contributed to by bremsstrahlung self-absorption) and with an exponential drop at a higher frequency, i.e., its cut-off frequency, a good sign of the plasma temperature (a black body spectrum has an analogous cut-off). The thermal bremsstrahlung characteristics become more evident the hotter and thinner the plasma and generally dominates in plasma clouds above 7,000 K. The two common astronomy cases where the thermal bremsstrahlung is evident are HII regions, from which the thermal bremsstrahlung EMR is basically radio/microwave and intracluster medium (ICM) plasma, which can be millions of K, emitting thermal bremsstrahlung into the X-ray spectrum, which are one means of identifying distant galaxy clusters, and is a dominant cooling mechanism for the ICM (bremsstrahlung cooling).

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thermal emission