ON CORONAL ABUNDANCES DERIVED WITH THE SAX LECS AND ASCA/SIS DETECTORS/

We have studied the performance of global chi(2) fitting of low-resolu tion X-ray spectra in retrieving intrinsic source parameters, with emp hasis on the coronal metallicity. The study has been conducted by fitt ing large numbers of simulated spectra with known characteristics, and studying the distribution of best-fit parameters. We have studied the behavior of the LECS detector on board the SAX satellite and the SIS detector on board the ASCA satellite. The fitted source spectra have e ither two discrete temperature components or a power-law temperature d istribution, with metallicity variations modeled by a single global ab undance parameter. The model used for the fitting has always been a tw o-temperature one, with global varying abundance, to explore the influ ence of the a priori ignorance of the actual temperature stratificatio n in the source being observed. The simulations performed explore the influence of varying statistics in the observed spectrum (spanning a r ealistic range of values) as well as the effect of varying the intrins ic source metallicity, with values in the range 0.15-1.0 times the sol ar value. We find that the source metallicity can be retrieved to with in a few tens of percent from ASCA/SIS spectra of typical signal to no ise ratio, and within few percent from SAX/LECS spectra at the same si gnal to noise ratio. However, relatively small uncertainties in the de tector calibrations and in the plasma emission codes are likely to pot entially cause large systematic off-sets in the value of the best-fit parameters. Similar systematic off-sets may derive from assuming too s implistic a temperature distribution for the source plasma. In additio n we have re-analyzed the ASCA/SIS spectra of the active giants beta C et and Capella with the same set of assumptions used in the simulation s, showing how the best-fit metallicity in these two real cases depend s on the details of the fitting process, and in particular on the chos en energy range.