The X-ray coronal emission of lambda Andromedae observed with ASCA and ROSAT

We present an observation of the long-period RS CVn binary lambda And obtained with ASCA. We also discuss a previous observation by ROSAT of the same star, that we have retrieved from the public archive. There was little variation in the quiescent X-ray flux of lambda And between these two observations almost 5 years apart, but they offer different, and in part complementary, views of its corona. The ASCA spectra of lambda And are well described by a 2-T model with a main component at T-1 similar or equal to 0.9 keV and a smaller component at T-2 similar or equal to 3 keV, with EM2/EM1 similar or equal to 0.15. The fitted coronal metallicity is Z similar or equal to 0.1, much lower than solar but consistent within a factor of 2 with the reported photospheric metallicity Z(phot) similar or equal to 0.2 On the contrary, a successful 2-T fit of the ROSAT spectrum yields a cool component at similar or equal to 0.5 keV and a hotter component at similar or equal to 1.4 keV, with EM2/EM1 similar or equal to 2 and Z similar or equal to 0.25. Because of these differences, we were unable to find an acceptable fit to the joint ASCA and ROSAT data. We discuss possible causes for the discrepancy, including spectral time variability, uncertainties in the plasma emission codes and in the instrumental calibration, and limitations in the adopted fitting model. The hydrogen column density toward lambda And, well constrained by ROSAT, is N-H similar or equal to 1 x 10(19) cm(-2), a factor similar to 2 larger than estimated from UV observations from Copernicus and the Hubble Space Telescope. Albeit small, this difference may indicate the presence of substantial ionization of the interstellar medium.