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Transrelativistic Synchrotron Emissivity, Cross-Section, and Polarization

The spectrum and polarization produced by particles spiraling in a magnetic field undergo dramatic changes as the emitters transition from nonrelativistic to relativistic energies. However, none of the currently available methods for calculating the characteristics of this radiation field are adequate for the purpose of sustaining accuracy and speed of computation in the intensity, and none even attempt to provide a means of determining the polarization fraction other than in the cyclotron or synchrotron limits. But the transrelativistic regime, which we here find to lie between $5\times 10^7$ K and $5\times 10^9$ K for a thermal plasma, is becoming increasingly important in high-energy astrophysical environments, such as in the intra-cluster medium, and in the accretion flows of supermassive black holes. In this paper, we present simple, yet highly accurate, fitting formulae for the magnetobremsstrahlung (also known as cyclo-synchrotron) emissivity, its polarization fraction (and content), and the absorption cross-section. We demonstrate that both the harmonic and high-energy limiting behavior are well represented, incurring at most an error of $\sim 5%$ throughout the transition region.