When two neutron stars collide, they produce a flare that shines like a million Suns. In 2017, astronomers saw such a flare for the first time — event AT2017gfo. Now scientists have built computer models to understand exactly how this light arises.
In the new study, they simulated different collision scenarios: varying the stars' masses, their total mass, and the properties of the material. They focused on light escaping along the rotation axis — the light we see when looking at the flare from above. It turned out that the spectrum shows bright lines from the elements strontium, lanthanum, gadolinium, and cerium. These lines resemble those observed in AT2017gfo, though in the models they are shifted toward the blue.
Interestingly, the flare's brightness depends on the amount of ejected material, but the spectrum's shape barely changes with the details of the collision. This means that even just the dynamical ejecta, material thrown out at the moment of impact, can explain many features of AT2017gfo at early stages. But to fully understand it, future models with other ejecta components are needed.
Such studies help us understand where heavy elements like gold, platinum, and uranium come from in the universe. After all, they are born in these very collisions.
as written by the authors arXiv:2604.22671
We present 3D kilonova radiative transfer simulations for a series of binary neutron star merger models. The masses of the neutron stars are varied as well as the total mass of the system and two different equations of state were used (SFHO and DD2), producing a range in dynamical ejecta masses and elemental abundance patterns. In this paper, we focus on the bolometric light curves and spectra in the polar direction for comparison with observations of the kilonova AT2017gfo. We calculate line-by-line opacities and include new calibrated lanthanide atomic data. All of the simulated spectra show strong features from Sr II, La III, Gd III and Ce III, which appear to correspond to features identified in AT2017gfo, although the simulated features are generally more blueshifted. The models with the lowest lanthanide fraction in the polar direction also show a Y II feature. Ce III, Ce II, Nd III and Nd II play an important role in shaping the spectral continuum. While the bolometric luminosities in the polar direction vary with the ejecta mass of each model, we find only little sensitivity of the spectral properties to the merger configuration. Our study demonstrates that dynamical ejecta alone can reproduce (although at earlier times) many spectral properties of AT2017gfo, suggesting dynamical ejecta may have a strong impact on the early spectral evolution. However, future simulations are needed to also elucidate the role of other ejecta components for shaping the kilonova spectrum.
The kilonova spectrum revealed lines of cerium and neodymium — elements we use in lighters and magnets.