2013ApJ...771L...8M

Photoabsorption of atomic oxygen in the energy region below the 1/s threshold in X-ray spectroscopy from Chandra and XMM-Newton is observed in a variety of X-ray binary spectra. Photoabsorption cross sections determined from an R-matrix method with pseudo-states and new, high precision measurements from the Advanced Light Source (ALS) are presented. High-resolution spectroscopy with E/ΔE ~ 4250±400 was obtained for photon energies from 520 eV to 555 eV at an energy resolution of 124±12 meV FWHM. K-shell photoabsorption cross section measurements were made with a re-analysis of previous experimental data on atomic oxygen at the ALS. Natural line widths Γ are extracted for the 1/s 2s ²2p ⁴(⁴ P)np ³ P° and 1s.^–12s ²2p ⁴(² P)np ³ P° Rydberg resonances series and compared with theoretical predictions. Accurate cross sections and line widths are obtained for applications in X-ray astronomy. Excellent agreement between theory and the ALS measurements is shown which will have profound implications for the modeling of X-ray spectra and spectral diagnostics.