Crab (unit)

A Crab is a standard astrophotometrical unit for measurement of the intensity of Astrophysical X-ray sources.

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.

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.

[1] The unit mCrab, or milliCrab, is sometimes used instead of the Crab.