The Crab Nebula as a standard candle

Because of its stable and intense emission and its well-known spectrum, the Crab Nebula is commonly used in X-ray astronomy as a standard candle for in-flight calibration of instruments response. Several measurements of the total (nebula + pulsar) emission have been performed in X and gamma rays. In the X-rays, the canonical spectrum is taken to be the one measured by Toor & Seward (1974) in the 2-60 keV band, i.e. an absorbed single power-law with photon index 2.1$\pm$0.03, normalization at 1 keV 9.7$\pm$1.0 photons cm^-2 s^-1 keV^-1 and column density N_H=3x10²¹ cm^-2. Other measurements in similar energy ranges give photon index values close to this. Extending the measurements to soft gamma-rays and still adopting the single power-law model, softer values of the photon index are obtained. In the 40-700 keV energy band typical photon index and 100 keV flux values are around 2.15-2.2 and (55-65)x10^-5 photons cm^-2 s^-1 keV^-1 respectively (e.g. [Bartlett 1994]). Moreover, the variable pulsar emission becomes a more significant percentage of the total emission as the energy increases. Thus, the hard X-rays to gamma-rays spectrum of the Crab is more likely to be a broken power-law with variations of the high energy component due to the variability of the pulsar emission.
The GRIS experiment ([Bartlett 1994]), which makes use of high energy resolution Germanium detectors, found that the Crab Nebula total spectrum between 20 keV and 8 MeV is best fitted by a broken power-law with the following average values: break energy at 61$\pm$10 keV, low energy photon index 2.00 $\pm$0.03 and high energy photon index 2.22$\pm$0.03.