SIMBAD references

We have studied the performance of global χ² fitting of low-resolution X-ray spectra in retrieving intrinsic source parameters, with emphasis on the coronal metallicity. The study has been conducted by fitting 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 either two discrete temperature components or a power-law temperature distribution, with metallicity variations modeled by a single global abundance parameter. The model used for the fitting has always been a two-temperature one, with global varying abundance, to explore the influence of the a priori ignorance of the actual temperature stratification in the source being observed. The simulations performed explore the influence of varying statistics in the observed spectrum (spanning a realistic range of values) as well as the effect of varying the intrinsic source metallicity, with values in the range 0.15-1.0 times the solar value. We find that the source metallicity can be retrieved to within a few tens of percent from ASCA/SIS spectra of typical signal to noise ratio, and within few percent from SAX/LECS spectra at the same signal to noise ratio. However, relatively small uncertainties in the detector calibrations and in the plasma emission codes are likely to potentially cause large systematic off-sets in the value of the best-fit parameters. Similar systematic off-sets may derive from assuming too simplistic a temperature distribution for the source plasma. In addition we have re-analyzed the ASCA/SIS spectra of the active giants β Cet and Capella with the same set of assumptions used in the simulations, showing how the best-fit metallicity in these two real cases depends on the details of the fitting process, and in particular on the chosen energy range.