High-Resolution X-Ray Spectroscopy of the Crab Nebula and the Oxygen Abundance of the Interstellar Medium

The authors have measured a high-resolution spectrum of the Crab Nebula in the 0.5 - 3 keV band using the Einstein Observatory. The oxygen K-edge is prominent in the measured spectrum. The detailed spectrum in the vicinity of this edge gives marginal evidence for a resonant 1s - 2p absorption line and hints of a K-edge from singly ionized oxygen. Model fitting to the overall spectrum yields a hydrogen column density of 3.45±0.42×10²¹cm^-2 and an oxygen column density of 2.78±0.55×10¹⁸cm^-2, corresponding to an oxygen abundance of 1.1±0.3 times solar. The absence of any evident oxygen depletion effects suggests that most interstellar grains are reasonably transparent to soft X-rays, implying sizes <0.4 μm.