Persistent discrepancies: For decades, astrophysicists have been puzzling over why some spectral lines measured in X-ray spectra look different than they should in theory. Now, for the first time, an experiment has succeeded in generating the theoretically calculated spectral values in practice. This not only solves the mystery of the discrepancies in these highly excited iron lines. The new findings also help X-ray astronomy in researching cosmic plasmas.
When astronomers want to find out how hot cosmic gas clouds, the sun’s corona or the extremely fast accretion disks around black holes are, they look at the X-ray spectra of these plasma accumulations, some of which are millions of degrees hot. Their X-rays are released by high-energy, strongly excited atoms and therefore contain characteristic emission lines of the elements contained.
However, the spectral line pattern also reveals how hot such a plasma is. Because the wavelength of the emission lines shows the ionization state of the atoms. The hotter and more energetic a plasma is, the more electrons are lost from its atoms – and this is reflected in the X-ray spectrum. Astrophysicists can classify such strange plasmas by comparing them with theoretically calculated values for the ionization levels and excitation states.
Iron lines don’t fit the theory
But just some astrophysically important spectral lines are stepping out of line. These are two emission lines from iron XVII – iron atoms, from which 16 of their 26 electrons were snatched in the hot plasma. The intensity ratio of these two lines is a crucial indicator for the temperature of cosmic plasmas and the processes taking place in them. But for decades, the Fe-XVII lines observed in X-ray spectra have deviated from theoretical calculations by 20 percent.
Even more irritating, however: Even in laboratory experiments, it was not possible to reproduce the theoretical values; physicists last tried this in 2020. “We were convinced that we had all the systematic effects known at the time under control during the measurement,” reports Steffen Kühn from the Max- Planck Institute for Nuclear Physics (MPIK) in Heidelberg. But the discrepancies persisted. This raised the question of whether perhaps the nuclear physics models were wrong?
With the ion trap in the X-ray synchrotron
To get to the bottom of this question, Kühn and his colleagues have now carried out another experiment. Unlike in previous experiments, they did not measure the intensity ratio of the two iron spectral lines, but the absolute intensity of the individual lines, the so-called oscillator strength. To do this, they used a mobile ion trap newly developed at the institute. In this, the iron XVII ions are produced by an electron beam and trapped in a magnetic field.
In the next step, the team irradiated these trapped iron ions with the focused X-ray beam of the PETRA III synchrotron at the German Electron Synchrotron (DESY) in Hamburg, whose energy could be finely adjusted. By combining the new ion trap with this X-ray beam, the researchers were able to increase the resolution of the X-ray spectrum by a factor of two and a half compared to previous experiments. The signal-to-noise ratio improved a thousandfold.
Finally a match
This brought the breakthrough: For the first time, the physicists determined spectral intensities in their experiment that corresponded to the theoretical values for these two iron lines. “This finally clarifies the decades-old mystery of the Iron XVII line widths,” state Kühn and his colleagues. Observation and theory finally agree – and the correctness of the models is confirmed.
The experiment also revealed why the previous measurements so persistently deviated from the models. Because the high resolution of the X-ray spectra showed for the first time the two iron lines right into their wings – the wavelengths that lie on the outer edge of the respective lines. “In previous measurements, the wings of these lines were hidden underground, which led to an incorrect interpretation of the intensities,” explains Kühn. As a result, the oscillator strengths of the lines were underestimated.
Important for astronomy
Thanks to the new experimental data, X-ray data from space telescopes can now be evaluated with greater accuracy in the future – and with the confidence that the theoretical comparison values are based on correct models. This is important for X-ray observatories that are already active in space, but also for future X-ray satellites such as the Japanese XRISM mission starting in 2023 or the Athena X-Ray Observatory of the European Space Agency ESA, which is planned for the early 2030s.
“This work represents a remarkable achievement in experimental atomic physics,” comments non-study physicist Roberto Mancini of the University of Nevada at Reno. “It was made possible by technical breakthroughs, excellent data analysis and the identification of uncertainties.” (Physical Review Letters, 2022; doi: 10.1103/PhysRevLett.129.245001)
Source: Max Planck Institute for Nuclear Physics