title: Crab (unit)
text: # Crab (unit)
  
  A Crab is a standard astrophotometrical unit for measurement of the intensity of Astrophysical X-ray sources. One Crab is defined as the intensity of the Crab Nebula at the corresponding X-ray photon energy.
  
  The Crab Nebula, and the Crab Pulsar within it, is an intense space X-ray source. It is used as a standard candle in the calibration procedure of X-ray instruments in space. However, because of the Crab Nebula's variable intensity at different X-ray energies, conversion of the Crab to another units depends on the X-ray energy range of interest.
  
  In the photon energy range from 2 to 10 keV, 1 Crab equals 2.4 · $10^{−8}$ erg $cm^{−2}$ $s^{−1}$ = 15 keV $cm^{−2}$ $s^{−1}$ = 2.4 · $10^{−11}$ W $m^{−2}.$ For energies greater than ~30 keV, the Crab Nebula becomes unsuitable for calibration purposes, as its flux can no longer be characterized by a single coherent model.
  
  The unit mCrab, or milliCrab, is sometimes used instead of the Crab.
  
  