The hot gas in galaxy clusters shines in X-rays, and a significant portion of that light comes from iron atoms that have lost many electrons. These atoms leave characteristic bright lines in the spectrum, like fingerprints. Astronomers use them to determine the temperature, density, and motion of the gas. But how accurately these lines match theoretical models has been unclear until now.

In the new study, scientists used data from the Japanese X-ray satellite Resolve and examined 17 galaxy clusters. Instead of fitting the entire spectrum to a ready-made model, they isolated individual iron lines, such as those from Fe25+ and Fe26+ ions, and measured their brightness directly. This approach allowed them to test how well the real lines agree with calculations.

It turned out that in some clusters with dense, cool cores, the brightness ratio of two lines (w and z) is about 20% lower than theory predicts. Meanwhile, the w line appears broader than the z line. This behavior can be explained by resonant scattering: photons of the w line are absorbed and re-emitted by iron atoms along the way to us, causing some light to be deflected. Another deviation was found in the ratio of lines y and x: it differs slightly from the model, hinting at contributions from recombination processes, where free electrons are captured by ions.

Of particular interest was the pair of lines Lyα1 and Lyα2. According to theory, their brightnesses should be in a ratio of 2 to 1, but in data with good statistics, the ratio is closer to 0.55 rather than 0.5. This slight excess can be explained by the contribution of a magnetic dipole transition that is not accounted for in one of the atomic databases. Interestingly, clusters with temperatures around 7 million degrees (7 keV) more often show ratios above 0.55. Although the statistical significance is still low, this hints that precise measurement of iron lines could become a sensitive tool for studying additional atomic processes — from dielectronic recombination to radiation polarization.