Kyoto, Japan -- The stability of the iron atom's nucleus has made it one of the most abundant heavy elements in the universe. When excited, iron atoms emit distinctive fluorescent X-ray lines which can be identified using the Fe Kα emission line, an approximately 6.4 keV fluorescent line produced when an electron transitions from the 2p orbital to the 1s orbital of the atom.
The Fe Kα emission line is widely used as a diagnostic tool for understanding the physical conditions of matter across a variety of astronomical objects. The energy of an emission line depends on the ionization state of iron: the degree to which its electrons have been stripped away. As ionization progresses and electrons are removed, the effective electric attraction between each remaining electron and the atomic nucleus becomes stronger.
From this, one might expect the energy of the Fe Kα emission line to increase as ionization increases. However, theoretical studies have demonstrated that, for iron, there exists a limited range of low-ionized states in which the energy of the Fe Kα line sees a slight decrease instead of an increase. This happens because during the removal of electrons from the 3d orbital, the repulsion between electrons within the 3d shell is reduced, and the 3d orbital contracts toward the nucleus. While the Fe Kα emission line corresponds to the 2p → 1s transition, the Fe Kβ line corresponds to the 3p → 1s transition, and this line increases almost uniformly with increasing ionization.
It is difficult to stably produce and maintain such low-ionized states of iron in ground experiments, and even in previous astronomical observations, the charge state could not be distinguished. This inspired a team of researchers at Kyoto University and ISAS/JAXA to employ the X-ray microcalorimeter Resolve onboard the XRISM satellite, which boasts world-leading energy resolution.
The XRISM satellite observed the accretion-powered X-ray pulsar Centaurus X-3, a binary system composed of a blue supergiant star and a neutron star, covering its full orbital period of approximately two days. Resolve detected the Fe Kα and Fe Kβ emission lines originating from this system, which the researchers then analyzed to quantitatively determine the ionization state of iron.
The results revealed that the iron Kα emission line at approximately 6.4 keV is in fact produced by low-ionized iron ions that have lost about five electrons. This is the first time such a low-ionized state has been conclusively identified as the origin of the 6.4 keV Fe Kα emission line.
"Through XRISM's high-precision spectroscopy, we were able to determine the ionization state of low-ionization iron in unprecedented detail," says first author Yutaro Nagai. "While this single result alone may not immediately lead to broad conclusions, the method we developed is highly versatile and is expected to be applicable to a wide range of astronomical objects."
One of XRISM's key goals is to obtain observational data that will contribute to advancing new frontiers in plasma physics. This study achieves this by presenting a new method for probing microscopic atomic processes by utilizing macroscopic cosmic phenomena.
