Context: Continuum emission is a fundamental component of the solar and stellar spectra between 1 Å and 2000 Å and has important applications to plasma diagnostics. However, the importance of X-ray free-bound continuum radiation has been overlooked in recent years, and no assessment of the accuracy of different data and approximations underlying the calculation of continuum emissivity has been carried out to understand the reliability of diagnostic results.
Aims: The importance of free-bound radiation in the X-rays will be demonstrated and its effects on plasma diagnostics will be discussed. The importance of user-chosen parameters such as ion and element abundances, necessary to the calculation of continuum emissivity, will be assessed. The uncertainties in the atomic data underlying continuum calculations will be investigated.
Methods: We will use the CHIANTI spectral code to investigate the relative importance of the free-free, free-bound and two-photon radiation as a function of wavelength in the 1-2000 Å spectral range, and to assess the effects of user-chosen parameters on the calculation of the continuum emission. A comparison between continuum emissivity of two of the most widely used spectral codes, developed using very different atomic data and approximations, will give us an indication of their reliability.
Results: The effects of element abundances and of the neglect of free-bound radiation in the X-rays are shown to be significant, with important consequences for plasma diagnostics results. The total continuum emissivities of the two spectral codes we compared are found to be in agreement to better than 40% at all wavelengths and temperatures of interest, with only a few exceptions.