High-Precision Determination of Oxygen Kα Transition Energy Excludes Incongruent Motion of Interstellar Oxygen

Abstract

We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, allowing us to measure the K-shell Rydberg spectrum of molecular O2 with 8 meV-uncertainty. We reveal a systematic ∼450 meV shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned O2 literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.

The XSTAR Atomic Database

“…A further consideration in assessing the reliability of the ALS measurements was that the absolute energy calibration relied on earlier molecular measurements with uncertainty estimates that seemed questionable. Since the O I study by [23], new laboratory measurements have been performed using an electron beam ion trap (EBIT) to determine the Kα resonance position at 527.26 ± 0.04 eV [52], in good agreement with the observations and the converged MCHF result. This new measurement suggests a recalibration of the ALS reference spectrum.…”

“…We have briefly recounted two projects that have emerged from the XSTAR database, namely the ISMabs absorption model and the uaDB database. The former has become an important initiative as it has led to benchmarks of the atomic data with observations and laboratory measurements; however, it has not been a smooth process as unexpected inaccuracies in the calibration of the laboratory wavelength scale were detected and finally resolved [23,52]. This outcome has given us some confidence in the identification of the spectral features and the accuracy of our atomic data.…”

“…Finally, the development of the XSTAR database has given rise to two spin-offs: the ISMabs (https://heasarc.gsfc.nasa.gov/xanadu/xspec/models/ismabs.html) absorption model [35] and the uaDB (https://heasarc.gsfc.nasa.gov/uadb/index.php) online database. The former is the result of benchmarks of the theoretical photoionization curves with observed K absorption in the interstellar medium (ISM), which has led to the implementation of a reliable model of ISM K absorption and to the matching of the theoretical K-line wavelengths with the astronomically observed and laboratory measured values, the latter two not always agreeing (e.g., O I [23,52]). The uaDB database allows the downloading of the XSTAR and other atomic rates without having to interfere with the code.…”

A new benchmark of soft X-ray transition energies of $$\mathrm {Ne}$$, $$\mathrm {CO}_2$$, and $$\mathrm {SF}_6$$: paving a pathway towards ppm accuracy

“…These experiments, however, rely on existing soft X-ray calibration standards, which have limitations to their accuracy. We recently found a discrepancy in the extensively used standard of the Rydberg transitions of molecular oxygen of almost 0.5 eV [28], thus resolving a tension between astrophysical and laboratory measurements of transitions of atomic oxygen [29], which had been calibrated against this molecular standard [30]. Such discrepancies raise the question of whether other commonly-used soft Xray standards may have errors of comparable magnitude, given that many such standards are based on similar experimental techniques using electron energy loss spectroscopy (EELS).…”

“…The anticipated high precision of line energy measurements enabled by high spectral resolution coupled with large photon fluxes in future space-based observatories, as well as in high-performance synchrotron beamlines, implies a need to reevaluate soft X-ray transition energies of common elements and materials that have been used for energy calibration using the same accurate standards used by [28]: highly charged ions (HCI) with one or two electrons. To further illustrate the capabilities of these methods, in this paper we present measurements of photoion yield spectra for CO 2 around the oxygen K-edge, SF 6 around the F K-edge, and Ne around its K-edge.…”