Astronomy & Astrophysics

Free access article

A&A 442, 315-321 (2005)
DOI: 10.1051/0004-6361:20053314

X-rays from $\alpha$ Centauri - The darkening of the solar twin

J. Robrade¹, J. H. M. M. Schmitt¹ and F. Favata²

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: jrobrade@hs.uni-hamburg.de
² Astrophysics Division - Research and Science Support Department of ESA, ESTEC, Postbus 299, 2200 AG, Noordwijk, The Netherlands

(Received 26 April 2005 / Accepted 11 July 2005 )

Abstract
We present first results from five XMM-Newton observations of the binary system $\alpha$ Centauri, which has been observed in snapshot like exposures of roughly two hours each during the last two years. In all our observations the X-ray emission of the system is dominated by $\alpha$ Cen B, a K1 star. The derived light curves of the individual components reveal variability on short timescales and a flare was discovered on $\alpha$ Cen B during one observation. A PSF fitting algorithm is applied to the event distribution to determine the brightness of each component during the observations. We perform a spectral analysis with multi-temperature models to calculate the X-ray luminosities. We investigate long term variability and possible activity cycles of both stars and find the optically brighter component $\alpha$ Cen A, a G2 star very similar to our Sun, to have fainted in X-rays by at least an order of magnitude during the observation program, a behaviour never observed before on $\alpha$ Cen A, but rather similar to the X-ray behaviour observed with XMM-Newton on HD 81809. We also compare our data with earlier spatially resolved observations performed over the last 25 years.

Key words: stars: activity -- stars: coronae -- stars: flare -- stars: late-type -- X-rays: stars

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X-rays from Centauri - The darkening of the solar twin

X-rays from $\alpha$ Centauri - The darkening of the solar twin