The Cycles of Alpha Centauri

Computer Science

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Stars And Star Formation

Scientific paper

The main AB pair of the nearby Alpha Centauri triple system has an extensive X-ray history, covering three decades. Alpha Cen A (G2V) is a near twin of the Sun, with a similarly soft (1-2 MK) corona. Alpha Cen B (K1V) is more active than the Sun, with a generally harder X-ray spectrum. Here, spatially resolved measurements from ROSAT, XMM-Newton, and Chandra are compared on a common basis. In the combined time series, Alpha Cen B shows a distinct X-ray modulation with a period of about eight years and a factor of 5 cycle depth (the latter is similar to the Sun's). Alpha Cen A showed minimal variability 1995-2000, a decrease in the initial XMM epoch 2003-2005, and nearly constant behavior in the subsequent Chandra segment (late-2005 to present), although down a factor of 2 from the mid-1990's level. A remarkable ``smoking gun'' LETGS spectrum in mid-2007 emphasized that much of the coronal luminosity of solar-activity objects falls at longer wavelengths than recorded efficiently by contemporary instruments. This makes cycle depth strongly dependent on the energy bandpass of the measurement (here 0.2-2 keV), and complicates assessments of coronal heating requirements.

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