"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