Accurate determination of the free-free Gaunt factor - I. Non-relativistic Gaunt factors

Modern spectral synthesis codes need the thermally averaged free-free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum emitted by an ionized plasma. Until now no set of data exists that would meet this need in a fully satisfactory way. We have therefore undertaken to produce a table of very accurate non-relativistic Gaunt factors over a much wider range of parameters than has ever been produced before. We first produced a table of non-averaged Gaunt factors, covering the parameter space (10)log is an element of(i) = -20 to +10 and (10)log w = -30 to +25. We then continued to produce a table of thermally averaged Gaunt factors covering the parameter space (10)log gamma(2) = -6 to +10 and (10)log u = -16 to +13. Finally, we produced a table of the frequency integrated Gaunt factor covering the parameter space (10)log gamma(2) = -6 to +10. All the data presented in this paper are available online.