When a Standard Candle Flickers

The Crab is the only bright steady source in the X-ray sky. The Crab consists of a pulsar wind nebula, a synchrotron nebula, and a cloud of expanding ejecta. On small scales, the Crab is extremely complex and turbulent. X-ray astronomers have often used the Crab as a standard candle to calibrate instruments, assuming its spectrum and overall flux remains constant over time. Four instruments (Fermi/GBM, RXTE/PCA, Swift/BAT, INTEGRAL/ISGRI) show a approx.5% (50 m Crab) decline in the Crab from 2008-2010. This decline appears to be larger with increasing energy and is not present in the pulsed flux, implying changes in the shock acceleration, electron population or magnetic field in the nebula. The Crab is known to be dynamic on small scales, so it is not too surprising that its total flux varies as well. Caution should be taken when using the Crab for in-orbit calibrations.