X-ray analysis without a doubt: the riddle of four decades of cosmic X-rays solved

An international team led by the Heidelberg MPl for Nuclear Physics has solved a decades-old problem in astrophysics with a high-precision experiment.

The intensity ratios of important iron radiation lines measured in the laboratory deviated previously from those calculated, and so there was uncertainty about the states of very hot gases derived from X-ray spectra, such as in the solar corona or near black. Holes.

With the new experimental data, agreement with the theory is now 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 that underlie them.

Very hot gases, such as those that exist in the solar corona or in the immediate vicinity of black holes, emit intense X-radiation. It reveals what physical conditions, such as temperature and density, are present there. But for decades, researchers have struggled with the problem of the mismatch between measured and calculated ratios, and thus 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 an extraordinarily accurate experiment.

Almost everything we know about distant stars, gas nebulae and galaxies is based on 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 a gas radiates most strongly 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 the atoms have lost one or more electrons and are in the form of positively charged ions. For example, extremely hot plasmas with temperatures of over a million degrees are present in the visible solar corona during a total solar eclipse. In addition, they are found in the vicinity of black holes or as intergalactic gas between galaxies.

The X-rays emitted by such plasmas show the fingerprints of the chemical elements they contain. The spectral lines (emission lines) of multiple ionization iron are very important, especially Fe XVII, which has lost 16 of its original 26 electrons. The reason: iron is common among heavy elements and Fe XVII is present over a wide temperature range.

When analyzing an X-ray spectrum, one compares not only the energies of the emission lines, but also the intensity ratios of the characteristic lines. In order to be able to draw conclusions about the properties of cosmic plasma, these intensity ratios must be well known. It is possible to do this by calculating them theoretically and verifying them experimentally in the laboratory.

And that was just the problem so far: quantum mechanical calculations and lab results of the intensity ratios of two strong lines called 3C and 3D deviated from each other by about 20 % and questioned our understanding of atomic structure as well as confidence in the models used.

This was not only a puzzle for astronomers, but also for physicists, because where was the error, in the theory or in the experiment? Two years ago, the team led by doctoral student Steffen Kühn at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg conducted the most precise experiment yet, and even then an inexplicable gap remained.

The MPIK theory team led by Natalia Oreshkina and Zoltan Harman, along with Marianna Safronova and Charles Cheung in the United States and Julian Berengut in Australia had been running supercomputers at full throttle to recalculate the 3C and 3D emission lines of Fe -XVII with the greatest precision: divergence so that the question remained: who was right?

“We were convinced that we had mastered all the systematic effects known at the time,” recalls Kühn. However, in a last attempt, he and the research team led by José Crespo sought to get to the bottom of things: instead of measuring the intensity ratio of the two lines, they tried to measure the absolute strength of the transitions individual, also called oscillator strength. But in order to measure these individual line strengths and identify the villain of the two lines in the theoretical observation, the quality of the measurement data needed to be significantly improved.

For this tricky measurement, as part of his doctoral thesis, Kühn used an Electron Beam Ion Trap (PolarX-EBIT) apparatus that had been built as part of a Sonja Bernitt postdoc project at MPIK. Inside, iron ions are produced by an electron beam and trapped in a magnetic field. Thus, the electron beam removes outer electrons from the iron ions until the desired Fe XVII is present. Then the trapped iron ions are irradiated with X-ray light of appropriate energy so that they fluoresce. For this purpose, the incident energy of the X photons must be varied until the desired lines are exactly reached.

Since commercially available sources cannot produce the required X-radiation, the PolarX-EBIT had to be transported to DESY in Hamburg. There, the PETRA III synchrotron generates an X-ray beam whose energy can be tuned to a specific energy range. In this way, the iron ions are excited to emit X-rays, which were then spectrally analyzed based on the energy of the incident photons.

Thanks to clever improvements in the device and the measurement scheme, Kühn and his colleagues Moto Togawa, René Steinbrügge and Chintan Shah managed during long days and short nights at the PETRAIII beamline to double the resolution of the spectra a times more than their previous measurement and to remove the interfering background noise, as it occurs in each measurement, by a factor of a thousand.

The enormous improvement in the quality of the data allowed a breakthrough: for the first time, the emission lines to be studied could be completely separated from the neighboring lines. Additionally, 3C and 3D lines could now be measured to their edge.

“In previous measurements, the wings of these lines were hidden in the background, leading to misinterpretation of the intensities,” Kühn explains. Maurice Leutenegger of NASA Goddard Space Flight Center, who participated in the experiment as an expert in X-ray astrophysics, is also very satisfied with the result: “The final result is now in excellent agreement with the theoretical predictions. theorists.

“This builds confidence in the quantum mechanical calculations used to analyze astrophysical spectra. This applies particularly to lines for which there are no experimental reference values,” Kühn stresses on the importance of the new result. In addition, spectra from space telescopes can now be assessed with greater accuracy.

This also applies to two major X-ray observatories soon to be launched into space: Japan’s X-Ray Imaging Spectroscopy Mission (XRISM, May 2023 launch) and the Athena X-Ray Observatory. European Space Agency ESA. (launch in the early 2030s).

The article is published in the journal Physical examination letters.