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| | | | | | | ESA’s Integral and XMM-Newton missions extended
5 December 2005
| The SPI instrument on board ESA's Integral has performed a search for 511 keV emission (resulting from positron-electron annihilation) all over the sky. The figure represents the results of this search: the all-sky map in galactic co-ordinates shows that 511 keV emission is - so far - only seen towards the center of our Galaxy. The SPI data are equally compatible with galactic bulge or halo distributions, the combination of a bulge and a disk component, or a combination of a number of point sources. Such distributions are expected if positrons originate either from low-mass X-ray binaries, novae, Type Ia supernovae, or possibly light 'dark matter'.
Credits: ESA/J. Knödlseder (CESR) and SPI team | |  |  | |
| | | | | | 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.
Credits: ESA/Jan Robrade | | | | |
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