Bremsstrahlung, German for 'braking radiation', is the continuum emission created by the interaction of two "free" charged particles. Since unlike radiative or dielectronic recombination the interacting particle begins and ends the process without being 'bound', bremsstrahlung is often called 'free-free' radiation. In a typical hot astrophysical plasma, the thermal bremsstrahlung continuum created by electrons with a Maxwell-Boltzmann distribution interacting with hydrogen and helium nuclei is at least significant and is often the dominant source of energy loss (see Radiated Power).
A vast literature exists for calculations of bremsstrahlung radiation in unusual circumstances. However, APEC includes only a few different methods of calculating the bremsstrahlung continuum, as the majority of astrophysical situations require little else. These include:
"A fast and accurate method for evaluating the nonrelativistic free-free Gaunt factor for hydrogenic ions" by D. G. Hummer (1988, ApJ, 327, 477)
"Relativistic Thermal Bremsstrahlung Gaunt Factor for the Intracluster Plasma" by Itoh, N. et al.(1998, ApJ, 507, 530)
for purely non-relativistic and relativistic (electron-proton) collisions. There is a significant (>10%) difference between these two calculations, due to relativistic effects. However, APEC primarily uses a calculation by Kellogg et al. (1975), itself a fit to data from Karzas & Latter (1961). This is enhanced with a unpublished correction for low temperatures from Robert Kurucz. The choice of this method was driven by a desire to match the 'brems' model of XSPEC, since this is by far the most common method. A check was made between the three different calculations, which shows that for a 9 keV plasma, the Kellogg result agrees with the relativistic calculation to within ~1%. At T~25 keV, the relativistic correction is about 5%, and up at 87 keV, it's 12%. Since APEC is meant to be useful with line-emitting plasmas, some disagreement at 87 keV was considered acceptable. However, in the future we may switch to using the full relativisitic correction.
Although thermal bremsstrahlung is the most important emission mechanism for optically-thin thermal plasmas, there are other types of bremsstrahlung that can affect astrophysical plasmas. At sufficiently high energies (> 50 keV), electron-electron bremsstrahlung may be significant (Stepney & Guilbert, 1983, MNRAS, 204, 1269; Haug, 1989, A&A, 218, 330), in shocks non-thermal bremsstrahlung can be important (Vink, J. 2008, A&A, 486, 837), and stellar flares can show emission due to thick-target bremsstrahlung (Osten et al. 2007, ApJ, 654, 1052).