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Astron. Astrophys. 324, L37-L40


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Letter to the Editor

A SAX/LECS X-ray observation of the active binary Capella

F. Favata1, R. Mewe2, N.S. Brickhouse3, R. Pallavicini4, and G. Micela4, and A.K. Dupree3

1Astrophysics Division - Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
3Harvard-Smithsonian Center for Astrophysics, 60 Garden St., 02138 Cambridge, Mass., USA
4Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy

Received 21 April 1997 / Accepted 27 May 1997

Abstract

We present a SAX/LECS X-ray observation of the active binary Capella, the first coronal source observed in the SAX Guest Investigator program. The analysis of this observation, performed using the MEKAL plasma emission code, shows that the LECS spectrum is well fit by a two-component optically-thin plasma model. A differential emission measure (DEM) obtained by direct inversion of the X-ray spectrum shows no additional features in addition to the double-peaked structure implied by the direct two-temperature analysis. Such a simple temperature stratification is however not compatible with the EUVE emission from the same object, which is well represented by a more complex DEM, with a power-law-like tail toward the low temperatures. At the same time, the EUVE-derived DEM predicts well the softer part of the Capella LECS spectrum, but it fails to correctly reproduce the higher energy part of the Capella LECS spectrum. Possible causes for this discrepancy are discussed. The coronal metallicity derived from the SAX observation is compatible both with the EUVE-derived metallicity as well as with the photospheric metallicity of Capella, thus showing no evidence for coronal under-abundances.

Key words: Stars: individual: Capella; stars: late-type; stars: activity; X-rays: stars

Send offprint requests to: F. Favata (fabio.favata@astro.estec.esa.nl)


© European Southern Observatory (ESO) 1997

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