Laboratory X-ray Astrophysics with Highly Charged Ions

Space observatories, like the satellites XMM-Newton and Chandra, observe the x-ray spectra of hot astrophysical plasmas. Such are present in stellar atmospheres, the accretion discs around black holes, intracluster media, and many other environments. The comparison of observed x-ray spectra with plasma models can reveal the state and dynamics of different components of those hot objects. The models heavily depend on the accurate knowledge of the underlying atomic and molecular processes. Recent observations of the Perseus galaxy cluster with the Hitomi Soft X-ray Spectrometer microcalorimeter provided a high-resolution spectrum in the photon energy range from 0.1 to 12 keV, with many well-resolved line features originating from highly charged ions of most astrophysically relevant elements, from silicon to nickel. However, its analysis has uncovered significant shortcomings of commonly used spectral modelling software packages. These include inaccurate transition energies, but also atomic-scale processes completely missing from the models. One component not included in spectral models was emission following charge exchange between bare sulfur ions and atomic hydrogen. We have studied this process with an electron beam ion trap (EBIT) and found it to be a likely explanation of a weak line feature around 3.5 keV found in galaxy cluster spectra. This feature had previously sparked enormous interest in the scientific community, when it was attributed to a possible dark matter decay process. This illustrates how incomplete knowledge of atomic-scale processes limits the amount of information that can be extracted from astrophysical x-ray spectra. We have combined EBITs with ultrabrilliant synchrotron and free-electron laser x-ray light sources to resonantly excite electronic transitions in trapped highly charged ions. These experiments have provided valuable atomic data and help to benchmark atomic structure theory. Furthermore, we have used a newly developed compact EBIT to provide an accurate calibration of molecular photoabsorption spectra relevant for current and future x-ray satellite missions.