A Comprehensive X-Ray Absorption Model for Atomic Oxygen

Abstract

7An analytical formula is developed to represent accurately the photoabsorption cross section of O I for all energies of interest in X-ray spectral modeling.In the vicinity of the K edge, a Rydberg series expression is used to fit R-matrix results, including important orbital relaxation effects, that accurately predict the absorption oscillator strengths below threshold and merge consistently and continuously to the above-threshold cross section. Further minor adjustments are made to the threshold energies in order to reliably align the atomic Rydberg resonances after consideration of both experimental and observed line positions. At energies far below or above the K-edge region, the formulation is based on both outer-and inner-shell direct photoionization, including significant shakeup and shake-off processes that result in photoionization-excitation and double photoionization contributions to the total cross section. The ultimate purpose for developing a definitive model for oxygen absorption is to resolve standing discrepancies between the astronomically observed and laboratory measured line positions, and between the inferred atomic and molecular oxygen abundances in the interstellar medium from xstar and spex spectral models.Subject headings: X-rays: ISM -ISM: atoms -atomic processes -line: formation -8 line: profiles 9 Atomic photoionization, an important astrophysical process, has been studied for 11 more than a century since the seminal understanding of its energetics by Einstein (1905) 12 and the first calculations of quantum mechanical cross sections (Bates 1939). Over the 13 years, a plethora of experimental and theoretical investigations have managed an excellent 14 grasp of its physics (Fano & Cooper 1968; Starace 1982), together with a remarkable 15 quantitative description of the valence-shell photoionization of atoms and atomic ions 16 (Opacity Project Team 1995, 1997). However, the quantitative model of inner-shell 17 photoabsorption is less sound due to a variety of relaxation processes, namely Auger and 18 X-ray emission, that must be taken into account in order to achieve acceptable accuracy, 19 especially in the near-threshold region. 20 Inner-shell photoabsorption of metals with nuclear charge 7 ≤ Z ≤ 28 is directly 21 accessible to modern X-ray observatories such as Chandra and XMM-Newton, and, hence, 22 is of much interest in astronomy. Particularly prominent in the photoabsorption of the 23 interstellar medium (ISM) are the K-shell features (lines and edges) of atomic oxygen, 24 which is the most abundant metal and is critically important in the energetic and chemical 25 evolution of the Universe (Stasińska et al. 2012). At present, though, the unsatisfactory 26 quantitative understanding of oxygen inner-shell photoabsorption is such that there exists 27 various sets of cross sections, each one leading to different conclusions regarding the 28 ionization and atomic-to-molecular fractions in the ISM along various Galactic lines of 29 sight. 30 The first inner-shell photoabsorption cross se...

A Deep X-Ray View of the Bare Agn Ark 120. I. Revealing the Soft X-Ray Line Emission

“…The expected restframe positions of severals p 1 2 resonance lines (e.g., from the He-and H-like ions of N, O, Ne, and Mg) are marked in the figure and there appear to be no significant absorption lines at any of these positions. However, in the local (z = 0) observed frame, there appear to be several absorption features; in particular, around the position of the neutral O I edge at ∼23 Å there is a strong resonance absorption line (at 23.5 Å or 527 eV) which may be identified with the Kα absorption line due to O I in our galaxy (Gorczyca et al 2013). In addition another strong absorption line is observed near 31.2 Å, which likewise may be attributed to the neutral Kα resonance line of N. Thus it appears clear we need to account for the absorption in the ISM due to our own Galaxy in the soft X-ray spectrum of Ark 120, which we investigate below.…”

X-Ray Absorption in Young Core-collapse Supernova Remnants

“…The photoabsorption cross-sections below 10 keV are taken from the tables of Gatuzz et al (2015) 1 . They compile cross-sections obtained using different analytic methods (Bethe & Salpeter 1957), the R-matrix method (Gorczyca 2000;García et al 2005García et al , 2009Juett et al 2006;Witthoeft et al 2009Witthoeft et al , 2011Gorczyca et al 2013;Hasoglu et al 2014), semi-empirical fits (Verner & Yakovlev 1995), and experimental values (Kortright & Kim 2000). The tables cover energies from 0.01 keV to 10 keV with 650 000 logarithmically spaced steps, which results in a relative energy resolution of 10 −5 .…”

Oxygen, neon, and iron X-ray absorption in the local interstellar medium

“…Gatuzz et al (2014) fitted the O K edge in the Chandra spectra of eight LMXB with the warmabs photoionization model to determine the ISM ionization degree. In this same work, the O photoabsorption cross-section computed by García et al (2005) was compared with the most recent calculation by Gorczyca et al (2013), finding no substantial differences in the modeling of the K-shell region; it is worth emphasising the remarkable accuracy of the oxygen atomic data that was thereby established. It was therefore concluded in this work the dominance of a neutral gas in all the observed lines of sight, with ionic fractions O /O  and O /O  lower than 0.1.…”