XMM-Newton's X-ray images of Alpha Centauri

Two XMM EPIC MOS images of Alpha Centauri A+B, taken in March 2003 (left) and Feb. 2005 (right). Alpha Centauri is our nearest stellar system consisting of a G2V (A) and a K1V (B) star at a distance of about four light-years, the M dwarf star Proxima Centauri is not in the field of view. Alpha Cen B is the X-ray brighter object at lower right and exhibits a comparable X-ray luminosity in both exposures. In contrast Alpha Cen A, a star very similar to our Sun, is only visible in the left image. It has faded in X-rays by at least an order of magnitude, a behaviour never observed before despite several observations of the Alpha Centauri system over the last 25 years. Is this an irregular event or do we see a coronal activity cycle?

Our Sun, a relatively inactive star, exhibits a well-known activity cycle with a period of 11 years. While chromospheric activity cycles on low-activity stars are established from optical measurements of Ca II emission lines, coronal X-ray activity cycles are known for very few objects. A long-term XMM-Newton monitoring programme of solar-like stars, including Alpha Centauri A+B, was initiated to put some more light on this topic. No activity cycle was ever detected on a component of Alpha Centauri. Since also no chromospheric data of Alpha Centauri is available and all previous resolved X-ray observations (Einstein, ROSAT, Chandra) revealed a similar situation as this March 2003 XMM-Newton observation, a definite explanation of this astonishing finding can only be given by future observations.