  The Crab Nebula is the only hard X-ray source in the sky that is both bright
enough and steady enough to be easily used as a standard candle. As a result,
it has been used as a normalization standard by most X-ray/gamma ray
telescopes. Although small-scale variations in the nebula are well-known, since
the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in
August 2008, a ~ 7% (70 mcrab) decline has been observed in the overall Crab
Nebula flux in the 15 - 50 keV band, measured with the Earth occultation
technique. This decline is independently confirmed with three other
instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray
Timing Explorer Proportional Counter Array (RXTE/PCA), and the INTErnational
Gamma-Ray Astrophysics Laboratory Imager on Board INTEGRAL (IBIS). A similar
decline is also observed in the ~3 - 15 keV data from the RXTE/PCA and INTEGRAL
Joint European Monitor (JEM-X) and in the 50 - 100 keV band with GBM and
INTEGRAL/IBIS. Observations from 100 to 500 keV with GBM suggest that the
decline may be larger at higher energies. The pulsed flux measured with
RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that
the observed changes are nebular. Correlated variations in the Crab Nebula flux
on a ~3 year timescale are also seen independently with the PCA, BAT, and IBIS
from 2005 to 2008, with a flux minimum in April 2007. As of August 2010, the
current flux has declined below the 2007 minimum.
