Benvenuti in queste pagine dedicate a scienza ed arte. Amelia Carolina Sparavigna

Friday, January 14, 2011

The 'millicrab'

"One of the most studied objects in the sky, the Crab Nebula is the remnant of an exploded star 6500 light-years away from Earth. At its core is a neutron star that spins 30 times per second, driving processes that are responsible for it X-ray and gamma-ray emissions. Until recently the X-ray intensity of the Crab was considered to be so stable that it is used as a "standard candle" to judge the relative brightness of other objects in the sky. Indeed, X-ray brightness is often expressed in units of "millicrab"." X-ray astronomers have for decades calibrated their detectors using the Crab Nebula, but now an international team of astronomers has discovered that the X-ray output of the Crab has dropped by 7% in the last two years.
Astronomers say goodbye to the 'millicrab' - physicsworld.com