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

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Title:		The X-ray coronal emission of λ Andromedae observed with ASCA and ROSAT.
Authors:		Ortolani, A.; Maggio, A.; Pallavicini, R.; Sciortino, S.; Drake, J. J.; Drake, S. A.
Publication:		Astronomy and Astrophysics, v.325, p.664-676 (A&A Homepage)
Publication Date:		09/1997
Origin:		A&A via CDS
A&A Keywords:		STARS: CORONAE, STARS: LATE-TYPE, STARS: ABUNDANCES, STARS: INDIVIDUAL: {LAMBDA} AND, X-RAYS: STARS
Bibliographic Code:		1997A&A...325..664O

Abstract

We present an observation of the long-period RS CVn binary λ 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 λ 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 λ And are well described by a 2-T model with a main component at T_1_=~0.9keV and a smaller component at T_2_=~3keV, with EM_2_/EM_1_=~0.15. The fitted coronal metallicity is Z=~0.1, much lower than solar but consistent within a factor of 2 with the reported photospheric metallicity Z_phot_=~0.2. On the contrary, a successful 2-T fit of the ROSAT spectrum yields a cool component at =~0.5keV and a hotter component at =~1.4keV, with EM_2_/EM_1_=~2 and Z=~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 λ And, well constrained by ROSAT, is N_H_=~1x10^19^cm^-2^, a factor ~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.

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