Astrophysical plasma study benefits from new soft X-ray transition energies

The new standard for X-ray transition energies set for neon, carbon dioxide, and sulphur hexafluoride paves a way to achieving a high accuracy analysis of astrophysical plasmas.

The analysis of astrophysical plasmas is vital in the pursuit to learning more about some of the Universe’s most powerful and mysterious objects and events, such as stellar coronae and winds, cataclysmic variables, X-ray binaries containing neutron stars and black holes, supernova remnants, or outflows in active galactic nuclei.

The success of such research could potentially lead to future astrophysical X-ray observatories, enabling scientists to access techniques that are currently not available to X-ray astronomy. A key requirement for the accurate interpretation of high-resolution X-ray spectra is the accurate knowledge of transition energies.

Astrophysical plasma study

A new paper published in EPJ D authored by J Stierhof, of the Dr Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics of Friedrich-Alexander-Universt Erlangen-Nürnberg, Bamberg, and co-authors, utilises a newly introduced experimental setup at the BESSY II synchrotron facility to provide precise calibration references in the soft X-ray regime of neon, carbon dioxide, and sulphur hexafluoride gases.

“In many research fields involving X-rays or any wavelength of light, insights are obtained by comparing measurements of emission or absorption line wavelengths with known values of transitions in various elements. A shift of the observed wavelength with respect to the known one can occur because of the velocity of the emitter or absorber,” explained Stierhof. “Our work demonstrates a setup to measure transition energies of gases simultaneous with known transitions in highly charged ions having only two remaining electrons that are precisely known from theoretical calculations.”

astrophysical plasmas
An artist’s impression of an X-ray binary, the kind of cosmic event that will benefit from a new benchmark in the study of astrophysical plasmas.

Monochromatic X-rays

Monochromatic X-rays from a synchrotron beamline can pass through an electron beam ion trap (EBIT), where they interact with the low-density plasma produced and trapped inside the EBIT. They then enter a gas photoionization cell containing the atoms or molecules under investigation. Fluorescence emission from the ions in the EBIT provides the basis for the precise calibration of the monochromator energy scale in the experiment.

In the paper, the authors discovered results for the energy transition in the k-shell of carbon dioxide that agree and compliment previous findings. The results in the transitions demonstrated by sulphur hexafluoride indicated that previous experiments have a shift of around 0.5 eV, more than twice their claimed uncertainty.

The team added that the statistical uncertainty in principle allows calibrations in the desired range of 1 to 10 meV, with systematic contributions currently limiting the uncertainty to around 40 to 100 meV.

“Our proposed setup provides an absolute calibration for the X-ray beam, but we found that the total uncertainty is dominated by relative changes of the beam,” Stierhof concluded. “Providing an additional setup to measure these relative changes will bring us closer to the resolution limit of 10 meV.”

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