When two neutron stars collide, a kilonova is born — a cosmic explosion that shines not only from heat but also from the decay of freshly created heavy elements. Among them is neodymium, a rare-earth metal that hides in minerals deep within the Earth, but in space, for a brief time, becomes the main conductor of light.
Astrophysicists simulated such a flash on a computer using three different sets of data on how neodymium atoms absorb and emit light. The result was surprising: the brightness of the explosion in the infrared range changed by almost 1.5 times depending on the chosen tables. The spectrum, the rainbow stripe by which scientists determine composition, also became noticeably distorted, especially in the near and mid-infrared light.
The cause turned out to be in the description of neutral neodymium (Nd I)—an atom with all its electrons in place. Even small inaccuracies in calculating its energy levels lead to large changes in the spectrum. The scientists also tried calibrating the data against the NIST reference database, and again saw strong differences in the infrared part.
This means that for accurate modeling of kilonovae, including the famous AT2017gfo, special attention must be paid to the atomic physics of each rare element. Otherwise, we risk misinterpreting how much gold, platinum, or uranium was born in such an explosion.
as written by the authors arXiv:2604.04362
We examine the impact of input neodymium (Nd) atomic data on the light curves and spectra of kilonovae, probing the sensitivity of kilonova observables to the atomic physics of this important lanthanide element. We use the SuperNu Monte Carlo radiative transfer code, simulating a simple semi-analytic 1D kilonova with a pure Nd atmosphere, fixing the radiative transfer method while using input atomic data generated by three different codes: the LANL suite of atomic physics codes, HULLAC, and Autostructure. We see that the choice of atomic data significantly shapes the resulting light curves and spectra. Peak bolometric luminosities differ by a ratio of nearly 1.5 between HULLAC/Autostructure and LANL data sets. Moreover, we observe significant near- to mid-IR differences in the structure of the spectra. We specifically attribute these differences to the choice of atomic data for neutral Nd I. Many of the results here have been adapted from a presentation at "Radiative Transfer and Atomic Physics of Kilonovae" in Stockholm, 2023. We additionally present a LANL data set with energies calibrated to available values in the NIST Atomic Spectra Database, and demonstrate that this calibration also significantly affects IR spectral structure at late time. The substantial differences in kilonova observables that arise from tuning the atomic data of just one lanthanide element highlight the special attention that must be paid to atomic physics uncertainties when modeling kilonovae, from AT2017gfo to beyond.
Neodymium is used in powerful magnets that keep your refrigerator door closed, and in a kilonova, it briefly becomes one of the main sources of light.