5 Discussion and conclusions

Our primary goal in this study was to benchmark the MEKAL spectral code using high-resolution solar flare X-ray spectra. The two sets of solar spectra selected from the SMM data are representative of a flare in a late stage of development, thus showing relatively low-excitation lines (e.g. those due to n=3-2 transitions in Fe ions up to Fe XXI) and an energetic flare at the peak of its development, thus showing high-excitation lines (e.g. n=4-2 and n=5-2 lines of Fe ions up to Fe XXIV). The comparison was made by splitting the observed spectral scans into several small scans, so that the large instrumental background varies in a simple way with wavelength.

As the wavelength precision from the SMM spectra is estimated to be very high (2-3 mÅ for line wavelengths less than about 13 Å), the comparison with the MEKAL synthetic spectra not surprisingly showed some need for refinements in the wavelengths used in the code. Wavelength adjustments of up to 15 mÅ were applied to the MEKAL data for lines near 15 Å, though most adjustments were much less than this. The statistical fits of the MEKAL spectra to the observed were consequently greatly improved (see Table 2). The value of reduced $\chi^2$ in fact is acceptably low for short-wavelength ranges though for longer-wavelength ranges the reduced $\chi^2$ remained higher than 1. Further improvement was made by the addition of up to thirteen lines either not included or too weak in ("missing" from) the MEKAL code in each spectral range. The fact that the reduced $\chi^2$ is still larger than unity is probably mostly due to the poor fits to the background emission. An additional source of error is the fact that atomic data other than line wavelengths are most likely in need of revision. Future work on this is proceeding.

Figures 3 to 6 are a graphical summary of the comparison of the solar spectra and MEKAL fits, while Appendix I gives full details of the lines in the flare spectra, the MEKAL wavelengths as previously adopted (see [24, Mewe et al. 1995)], and the line identifications. Many of the lines belong to the iron L-shell complex between 7 and 17 Å, wavelengths of which are available from measurements of high-precision laboratory plasma spectra. The present study complements these laboratory data, since there is generally good agreement of wavelengths between the solar and laboratory data. Also, the HULLAC identifications generally used for the laboratory data are practically the same as those given by [26, Phillips et al. (1982)] and [12, Fawcett et al. (1987)] for the solar data. The only exceptions are cases of line blends in the SMM data. Appendix II gives the energy level notation used in this work.

Analysis of EUVE and ASCA spectra has shown that current spectral synthesis codes suffer from a number of inadequacies, but owing to the poor spectral resolution and sometimes also poor statistical quality of the observed data, it is difficult to trace the exact nature of the problems in the codes. It is likely that with the advent of the next generation of spectrometers onboard Chandra, XMM, and ASTRO-E, there will be a need for more accurate spectral codes to make better determinations of physical parameters, in particular temperature and emission measure. Our benchmark study in which the widely used MEKAL spectral code has been compared with solar flare spectra from the SMM spacecraft having a generally high statistical quality and a spectral resolution higher than non-solar spectrometers previously flown (or even those on the future missions mentioned for wavelengths shorter than 13 Å) has provided a greatly improved spectral code. This improved code should help users to make better comparisons with future X-ray spectral data and hence obtain more reliable values of temperature, emission measure, and possibly other information for the emitting sources.

We note that a test of the improved code will be the first analysis of Chandra data for three selected stars (Procyon, Capella, and HR 1099) as part of the so-called X-ray Emission Line Project (XELP), initiated by Nancy Brickhouse and Jeremy Drake, and to be conducted by the AXAF Science Center (ASC).