A multi-mission study of the Crab nebula: spectral response of hard X-ray detectors

We present a multi-mission study of the 0.5 - 180 keV spectrum of the Crab nebula and pulsar. We obtain a consistent hard X-ray spectral index for the X-ray data obtained from the various missions like RXTE, ASCA and BeppoSAX. We find that all hard X-ray data are consistent with a simple power-law. The deviations from the power-law are mainly due to the systematic errors involved in each of the detector systems used for the observations. Implications of these results on the use of Crab as a standard candle for calibration in future missions like INTEGRAL are briefly discussed.