Hot iron in the cosmos

A precision experiment has provided details of the X-ray emissions from highly ionised iron, advancing our understanding of black holes and supernovae

A forty-year old puzzle in astrophysics is one step closer to a solution. The puzzle involves astrophysical phenomena such as black holes, stellar coronae, and supernova explosions. All these objects consist of plasmas at several million degrees Celsius. Under these extreme conditions, atoms lose many of their electrons and become multiply charged positive ions. The gas now becomes hot plasma, a powerful emitter of X-rays. However theoretical astrophysical models have not yet been able to explain the observed intensities of these X-rays for the most prominent species, iron ions. Two reasons had been proposed for the discrepancy: insufficient quantum-mechanical descriptions of the highly ionised iron, or insufficiently accurate models of the collisions between electrons and ions that take place constantly in the ultra-hot gases. A group at the Max Planck Institute for Nuclear Physics in Heidelberg, working with an international team of researchers, has now been successful in excluding the latter. The team is the first to use an X-ray laser to perform spectroscopy on highly ionised ions, and thus obtained new insights not granted by previous methods.

Ions of various elements, such as iron and oxygen, are found in the plasmas of astrophysical objects such as stellar coronae, supernova remnants, and active galactic nuclei surrounding neutron stars or black holes. Every element produces characteristic X-rays, known as emission lines. These can be said to provide an X-ray fingerprint for each ion species. The total sum of all the X-ray emissions from an astrophysical object forms its X-ray spectrum, which can be detected by satellites in Earth orbit, such as Chandra (NASA) and the XMM-Newton (ESA). Models that describe such X-ray spectra are very important for astrophysicists. They contribute to the understanding of important astrophysical processes such as the transport of energy in stars, and, in general, the composition, temperature, density and velocity of cosmic plasmas.

Scientists are especially interested in the element iron in this context, as highly ionised iron creates some of the brightest of all X-ray emission lines seen in hot astrophysical objects. But the X-ray spectrum of iron is not satisfactorily described by theoretical models. This is clear, since even the intensity of one of the strongest (and best studied) emission lines, which comes from Fe^₁₆₊ ions (iron ions having a positive charge of 16) is predicted by all these models to be higher than observed, compared to those of a neighbouring emission line. "Through our experiment, we have contributed to ending the controversy about the causes for the inadequate predictive power of the models”, says José R. Crespo López-Urrutia from the Max Planck Institute for Nuclear Physics in Heidelberg. The physicist carefully analyzed the X-ray emissions of Fe¹⁶⁺ with a team that, along with Heidelberg-based researchers, included scientists from Lawrence Livermore National Laboratory in California headed by Peter Beiersdorfer and Gregory V. Brown, the Stanford Linear Accelerator Center National Accelerator Laboratory (SLAC), the Max Planck Advanced Study Group at Center for Free-Electron Laser Science (CFEL), the NASA Goddard Space Flight Center, the Canadian TRIUMF Research Centre, the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, and the Universities of Giessen, Heidelberg and Erlangen-Nuremberg.

The role of individual photon excitation processes has become clear for the first time

In order to understand the controversy between observation and theory, it must be realized that every model of iron plasmas is made up of several sub-models. One important sub-model is the quantum-mechanical description of the individual ions, developed using the methods of atomic physics. This sub-model also calculates a part of the radiation that the iron ions emit within a plasma – namely, that fraction released through direct excitation of the ions by light. Another critical sub-model describes the local plasma environment of each ion. It models collisions between ions and electrons, and their influence on the emitted radiation, for example. Essentially, there is one model for the components of the plasma, and a further model for their interactions. Many researchers have believed for a long while that the latter model was responsible for the discrepancy between the observed and the calculated intensity of the Fe_¹⁶⁺ emission line. Unfortunately, this assumption could not be tested using only the spectroscopic measurement methods available to date.

The principle underlying spectroscopy is to excite the ions to emit particles of light (photons) by adding energy, and to record the resulting spectrum. To end the controversy, however, researchers would have to be able to differentiate between photons which arise through the addition of energy by collisions of electrons with ions, and those which arise from the direct energetic excitation of the ions by light. In the case of excitation by collisions, the electron shells are initially excited in a random fashion and a photon is emitted during the intervening transitions. As the excitation occurs by heating, the physicists could not differentiate the cause. This is because, in these experiments, ions are also excited by photons that are created by the collision processes, as well as by the thermal collisions themselves.

Through excitation with X-ray pulses, the contribution by collisions can be calculated

As a result, the researcher team, headed by Sven Bernitt and José Crespo from the Max Planck Institute in Heidelberg, has used an X-ray laser for the first time to investigate the Fe¹⁵⁺ and Fe¹⁶⁺ ions spectroscopically. The X-ray laser they used was the Linac Coherent Light Source at the Stanford Linear Accelerator Center in Menlo Park, California.

The laser fires hundred times per second X-ray pulses of only a few femtoseconds in duration into the plasma. The X-ray photons were absorbed and re-emitted as fluorescence shortly after. “Since the iron ions that have been excited by an X-ray pulse re-emit the photons also in the form of a brief pulse, we can differentiate these photons from all the others that the plasma emits”, says Sven Bernitt. The reason for this is that the iron ions that have been excited by collisions emit their photons in the form of a uniform background unrelated to the X-ray laser firing. In this way, the researchers could measure the emission intensity resulting from photon excitation, and compare it with the quantum-mechanical models of the individual ions.

The Heidelberg-based team of researchers were surprised by the result: the intensity of the measured emission deviated significantly from all the various theoretical predictions. The Heidelberg theoreticians, led in this project by Zoltán Harman, had also performed state-of-the-art calculations in order to achieve the highest possible degree of convergence and accuracy using the best available methods, but the disagreement remained. “On this basis, we can say that the largest contribution to the discrepancy has to do with the inadequate quantum-mechanical description – and not primarily with the models of the collision processes”, says Greg Brown of Lawrence Livermore National Laboratory.

Theoretical physicists can now improve their models

The researchers are the first to have conducted spectroscopic investigations with photons having such short wavelengths in a plasma like this using X-ray laser technology that has only become available in the last few years. A transportable ion trap developed by José Crespo and Sascha Epp at the Max Planck Institute in Heidelberg was crucial for the experiment. The researchers transported the three-ton instrument to California by plane. With the help of electron irradiation, they create a five-centimetre-long cloud of highly ionised iron as thin as a human hair, and hold this in position with the assistance of electric and magnetic fields. The X-ray laser irradiates the cloud in the axial direction and thereby comes into contact with as many of the iron ions as possible, which in turn provides a strong signal for measurement.

The research team's results give theoretical physicists a guide for improving the quantum-mechanical model of ions in plasma. That would be a first step toward a better understanding of astrophysical X-ray sources. The measurement method of the researchers in Heidelberg could also solve earthly problems: “In nuclear fusion, for example, where hot plasmas are certainly employed as well”, believes José Crespo. He expects that the transportable ion trap has many more trips to look forward to.

CM/PH