The interstellar oxygen edge in the X-ray spectrum of the Crab Nebula

Measurement of the X-ray spectrum of the Crab Nebula by the HEAO 1 A-2 low-energy (0.15-3 keV) detectors is reported. The spectral restoration techniques described by Blissett and Cruise (1979) and Kahn and Blissett (1979) are used to demonstrate explicitly the existence of a 'negative' feature near 0.5 keV that is consistent with the edge expected due to absorption by interstellar oxygen. For a simple power-law model with photon index -2.08, the data require a column density of 3.0 + or - 0.1 x 10 to the 21st per sq cm and an oxygen abundance of 1.0 + or - 0.1 times the Brown and Gould (1970) value. Satisfactory fits can also be achieved with the more sophisticated Fireman (1974) models, but no model can be adjusted to fit the data without the inclusion of interstellar oxygen.