High-Resolution X-Ray Spectroscopy of the Interstellar Medium: Structure at the Oxygen Absorption Edge

We present high-resolution spectroscopy of the oxygen K-shell interstellar absorption edge in seven X-ray binaries using the High Energy Transmission Grating Spectrometer (HETGS) onboard the Chandra X-Ray Observatory. Using the brightest sources as templates, we found a best-fit model of two absorption edges and five Gaussian absorption lines. All of these features can be explained by the recent predictions of K-shell absorption from neutral and ionized atomic oxygen. We identify the Kα and Kβ absorption lines from neutral oxygen, as well as the S=3/2 absorption edge. The expected S=1/2 edge is not detected in these data because of overlap with instrumental features. We also identify the Kα absorption lines from singly and doubly ionized oxygen. The O I Kα absorption line is used as a benchmark with which to adjust the absolute wavelength scale for theoretical predictions of the absorption cross sections. We find that shifts of 30-50 mÅ are required, consistent with differences previously noticed from comparisons of the theory with laboratory measurements. Significant oxygen features from dust or molecular components, as suggested in previous studies, are not required by our HETGS spectra. With these spectra, we can begin to measure the large-scale properties of the interstellar medium (ISM). We place a limit on the velocity dispersion of the neutral lines of <~200 km s^-1, consistent with measurements at other wavelengths. We also make the first measurement of the oxygen ionization fractions in the ISM. We constrain the interstellar ratio of O II/O I to ~0.1 and the ratio of O III/O I to <~0.1. This work demonstrates the utility of X-ray spectroscopy for studies of the ISM. Future work will provide measurements of the relative abundances and ionization fractions for elements from carbon to iron.