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Astron. Astrophys. 325, 664-676


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

A. Ortolani1, A. Maggio2, R. Pallavicini2, S. Sciortino2, J.J. Drake3, and S.A. Drake4

1Osservatorio Astrofisico di Arcetri, Largo Fermi 5, I-50125 Firenze, Italy
2Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy
3Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
4NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received 7 January 1997 / Accepted 24 March 1997

Abstract

We present an observation of the long-period RS CVn binary FORMULA 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 FORMULA 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 FORMULA And are well described by a 2-T model with a main component at FORMULA keV and a smaller component at FORMULA keV, with FORMULA . The fitted coronal metallicity is FORMULA , much lower than solar but consistent within a factor of 2 with the reported photospheric metallicity FORMULA . On the contrary, a successful 2-T fit of the ROSAT spectrum yields a cool component at FORMULA keV and a hotter component at FORMULA keV, with FORMULA and FORMULA . 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 FORMULA And, well constrained by ROSAT, is FORMULA cm-2, a factor FORMULA 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.

Key words: stars: coronae - stars: late-type - stars: abundances - stars: individual: FORMULA And - X-rays: stars

Send offprint requests to: A. Maggio


© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 28, 1998
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