Continuous vs. Discrete Emission Components in X-ray Spectrum of AR Lac

We present analysis of X-ray spectra of an RS CVn binary, AR Lac, based on simultaneous observations with the ROSAT PSPC and the ASCA SIS and GIS detectors. The X-ray spectra from all the detectors were fitted jointly with the plasma emission models assuming (a) discrete emission components with different temperatures, and (b) a continuous emission measure of the power-law type ( EM(T) ~ (T/T_max)(alpha ) ). We demonstrate that (i) both ASCA and ROSAT data can be best-fitted with the same plasma emission model viz., two-temperature discrete emission components with an under-abundance of the O, Mg, Si, S, Ar, Ca and Fe relative to the solar photospheric values, as was used in the preliminary analysis of data from a single detector (SIS0) by White et al. (1994), and (ii) the plasma models with continuous emission measure do not provide acceptable fits. We also present an analysis of the spectral data during the primary and secondary eclipses and show that the the emission measures of both the components in the two-temperature models vary by about the same amount during the eclipses, and the temperatures remain unchanged. N. E. White et al. 1994, PASJ, 46, L97.