Abstract
Beginning in 2003, XMM-Newton snapshot monitoring of α Centauri (HD 128620, 128621: G2 V, K1 V) documented a steady fading of the primary's X-ray corona, which had all but disappeared by early 2005. The steep decline in LX was at odds with the previous two decades of high-energy measurements, which showed only modest variability of the Sun-like star. A Chandra LETGS spectrum in 2007 June, however, fully resolved the source of the curious X-ray darkening: a depletion of plasma above ~2 MK had substantially depressed the line spectrum where the XMM-Newton response peaks (λ 30 A), even though the overall coronal luminosity, dominated by longer wavelength emissions, had declined only slightly. This is reminiscent of the Sun's magnetic activity cycle, where the 2-3 MK active regions of sunspot maximum give way to the spatially pervasive, but cycle-independent, 1 MK quiet corona at minimum. This emphasizes that any discussion of cyclic coronal variability in low-activity stars will depend crucially on the energy coverage of the measurements.
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