Emission measure analysis methods: the corona of AR Lacertae revisited.

A simultaneous ROSAT/ASCA observation of the RS CVn binary AR Lac has been re-analysed using updated calculations for the plasma emission. Several analysis methods are applied that serve to reconstruct the emission measure distribution of AR Lac. In particular we describe the regularisation method, a Chebyshev polynomial method, a clean algorithm, a genetic algorithm and a method based upon broadened discrete temperature components. We confirm earlier results that the abundances are non-solar; for most elements (O, Mg, Si, S, Ar, Ca and Fe), we find abundances that are consistent with 1/3 of the solar photospheric abundances. The abundances of Ne (0.7) and Ni (1.1) are somewhat larger. The emission measure analysis shows that there are at least two and probably three temperature components: a cool, intermediate and hot component at temperatures of 0.6, 1 and 2.4keV, respectively. The cool component is rather narrow (less than 50% relative width) and there is no significant emission below 0.3keV down to our detection limit at about 0.03keV. The intermediate and hot component may be separate structures, but could also be the dominant features of a more continuous emission measure distribution between 1-4keV. High-temperature emission above 5keV is limited to at most 5% of the total