An international team led by Heidelberg MPl for Nuclear Physics has solved a decade-old problem in astrophysics with a high precision experiment. The intensity ratios of key iron emission lines previously measured in the laboratory deviated from those calculated, and therefore there was uncertainty about the states of very hot gases from X-ray spectra, such as in the solar corona or near a black hole. .

With the new experimental data, agreement with the theory has now been reached. This means that X-ray data from space telescopes can be analyzed in the future with a high degree of confidence in the atomic models behind them.

Very hot gas, such as those found in the Sun’s corona or in the immediate vicinity of black holes, emit intense X-ray radiation. It indicates what physical conditions exist there, such as temperature and density. But for decades, researchers have struggled with the problem of inconsistency between measured and calculated ratios, and hence gas parameters derived from X-ray spectra. An international team led by the Max Planck Institute for Nuclear Physics in Heidelberg has now solved the problem with a highly sensitive experiment.

Almost everything we know about distant stars, gas nebulae, and galaxies is based on the analysis of the light we receive from them. More precisely, electromagnetic waves, because astronomers now have their entire spectrum at their fingertips. The spectral range in which a solid or gas emits brightest depends mainly on its temperature: the hotter it is, the more energetic the radiation.

In space, more than 99% of all visible matter is in the plasma state; it is so hot that atoms lose one or more electrons and emerge as positively charged ions. For example, in the solar corona visible during a total solar eclipse, there is extremely hot plasma with a temperature of more than one million degrees. Alternatively, they exist as intergalactic gas near black holes or between galaxies.

The X-rays emitted by this plasma show traces of the chemical elements present in it. The spectral lines (emission lines) from a large number of ionized irons, especially Fe XVII, which have lost 16 of its original 26 electrons, are very prominent. The reason: among the heavy elements, iron is common and Fe XVII exists over a wide temperature range.

When analyzing the X-ray spectrum, not only the energies of the radiation lines are compared, but also the ratio of the intensities of the characteristic lines. In order to make inferences about the properties of cosmic plasma, these density ratios should be well known. This can be done by calculating theoretically and checking experimentally in the laboratory.

And this has been a problem so far: Quantum mechanics calculations and lab results of the density ratios of the two strong lines, called 3C and 3D, have diverged by about 20%, casting doubt on our understanding of atomic structure. as well as confidence in the models used.

This was a headache not only for astronomers but also for physicists, because where was the mistake in theory or experiment? Two years ago, a team led by Steffen Kühn, postdoctoral researcher at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg conducted the most definitive experiment to date, and even then the inconsistency remained confusing.

The MPIK theory team led by Natalia Oreshkina and Zoltan Harman, as well as Marianna Safronova and Charles Cheung in the US and Julian Behrengut in Australia, ran supercomputers at full power to count the highest 3C and 3D emission lines of Fe-XVII. precision: inconsistency and also question: who was right?

“We were convinced at the time that we had controlled for all known systemic effects,” Kuhn recalls. But in the last attempt, he and a research group led by José Crespo tried to get to the root of the matter: instead of measuring the intensity ratio of the two lines, they tried to measure the absolute strength of individual transitions. called oscillator power. But in order to measure the strength of these individual lines and identify the bad guy of the two lines in a theoretical observation, the quality of the measurement data had to be significantly improved.

For this challenging measurement, as part of his doctoral thesis, Kuhn used an Electron Beam Ion Trap (PolarX-EBIT) apparatus created as part of a project by his postdoctoral fellow at MPIK, Sonia Burnitt. In it, iron ions are created by an electron beam and captured in a magnetic field. Thus, the electron beam removes the outer electrons of the iron ions until the desired Fe XVII is available. The captured iron ions are then irradiated with X-ray light of suitable energy for them to fluoresce. For this, it is necessary to vary the energy of the incoming X-ray photons until the desired lines are exactly reached.

PolarX-EBIT had to be moved to DESY in Hamburg as commercially available sources were unable to produce the required X-rays. Here, the PETRA III synchrotron produces an X-ray beam whose energy can be adjusted within a certain energy range. Thus, iron ions are encouraged to emit X-rays, which are analyzed spectrally depending on the energy of the incident photon.

Thanks to clever improvements to the apparatus and measurement scheme, Kuhn and his colleagues Moto Togawa, René Steinbrugge and Chintan Shah were able to once again double the spectrum resolution compared to their previous measurements during long days and short nights on the PETRAIII beamline. and to suppress the interfering background that occurs during each measurement a thousand times.

The significantly improved quality of the data provided a breakthrough: for the first time, the studied emission lines could be completely separated from neighboring lines. In addition, 3C and 3D lines can now be measured to the extreme.

“In previous measurements, the wings of these lines were hidden in the background, leading to a misinterpretation of density,” Kühn explains. Maurice Leitenegger of NASA’s Goddard Space Flight Center, who participated in the experiment as an X-ray astrophysicist, is also very pleased with the result: “The final result is now in perfect agreement with the theoretical predictions. This makes theorists happy.

“This strengthens the reliance on quantum mechanical calculations used to analyze astrophysical spectra. This is especially true for lines without experimental reference values,” said Kuhn, emphasizing the importance of the new result. In addition, spectra from space telescopes can now be predicted with greater precision.

That goes for the two major X-ray observatories soon to be launched into space: the Japan-led X-ray Spectroscopy Imaging Mission (XRISM will launch in May 2023) and the European Space Agency’s Athena X-ray Observatory. (launched in early 2030s).