Interstellar oxygen edge in the x-ray spectrum of the Crab Nebula

We report the measurement of the X-ray spectrum of the Crab Nebula by the HEAO 1 A-2 low-energy (0.15--3 keV) detectors. Using the spectral restoration techniques described by Blissett and Cruise and Kahn and Blissett, we explicitly demonstrate the existence of a ''negative'' feature near 0.5 keV that is consistent with the edge expected due to absorption by interstellar oxygen. Assuming a simple power-law model with photon index -2.08, the data require a column density of 3.0 +- 0.1 x 1021 cm-2 and an oxygen abundance of 1.0 +- 0.1 times the Brown and Gould value. Satisfactory fits can also be achieved with the more sophisticated Fireman models, but no model can be adjusted to fit the data without the inclusion of interstellar oxygen