Neutron star mergers are cosmic factories of heavy elements and sources of gravitational waves with electromagnetic counterparts. The gravitational signal from such a pair is, to first order, determined only by the effective spin—a combination of the masses and spins of both stars. But how individual spins affect the ejecta, element synthesis, and the kilonova remained unknown: previous studies either changed the spins along with the effective spin or were limited to a single configuration. This work fills the gap by comparing models with the same effective spin but different individual spins, showing that the electromagnetic signal carries unique information not accessible to gravitational waves.
The authors used simulations of neutron star mergers with the SFHo equation of state, which accounts for finite temperature and composition, and with a neutrino emission and absorption model. They considered three total mass ranges (2.55, 3.05, and 4.10 solar masses) and several spin configurations: non-spinning, aligned, anti-aligned, mixed, and single-spin. For each model, they extracted ejecta at a radius of about 295 km, separating them into dynamical (tidal and shock-heated) and secular (slow outflows) components. Then, using the nuclear network SkyNet, they calculated element synthesis, and with the semi-analytic model xkn, they computed kilonova light curves in three bands (g, i, Ks) for different viewing angles. To isolate the effect of neutrino absorption, some models were run without it.
With a fixed effective spin of zero and a total mass of 2.55 solar masses, individual spins change the dynamical ejecta mass by a factor of 45: from 1.26 to 56.83 thousandths of a solar mass. The yield of elements with mass number A≥140 differs by more than two orders of magnitude, and the ratio of lanthanide mass to light r-process elements increases from 2 to 70. In models with rapid collapse to a black hole (4.10 solar masses), the difference in heavy element yield exceeds four orders of magnitude. The kilonova retains this imprint: peak brightness differs by up to 0.9 magnitudes, and color at a given epoch by up to 1.5 magnitudes. At a distance of 40 megaparsecs, all three pairs with zero effective spin remain simultaneously observable in optical and infrared bands at any viewing angle. Neutrino absorption systematically makes the kilonova brighter and shifts the color bluer.
This work shows that individual neutron star spins are not just a detail of the gravitational wave signal but a key factor determining the fate of ejecta and emission. It provides the first systematic evidence that pairs with the same effective spin can produce drastically different electromagnetic signals. Among neighboring studies that examine the effect of rotation on ejecta and kilonovae, this work stands out by fixing the effective spin and varying individual spins, as well as including neutrino absorption and analyzing all ejecta channels. This allows breaking the degeneracy between spins that limits the interpretation of gravitational wave observations.
The next step is longer high-resolution simulations that include magnetic fields, which become important at late post-merger stages. Magnetic fields can alter secular ejecta and hence light curves. It is also important to expand the range of equations of state and masses to understand how universal the found trends are. In the long run, this will enable more accurate kilonova models and their use to determine merger parameters from observations.
The results will impact the interpretation of kilonova observations, especially in connection with future sky surveys such as the Legacy Survey of Space and Time (LSST) at the Vera Rubin Observatory. They are also important for understanding the origin of heavy elements in the universe and for refining nucleosynthesis models under extreme conditions.
Run simulations with magnetic fields and longer evolution times to test the robustness of the individual spin imprint. Compare predictions with real kilonova observations such as AT2017gfo and with future events detected by next-generation gravitational wave detectors.
This work directly addresses the unresolved problem of the origin of r-process elements: which astrophysical objects produce gold, platinum, and uranium, and in what proportions. It shows that individual spins can drastically change the yield of heavy elements, which must be accounted for when estimating the contribution of neutron star mergers to galactic enrichment. It also refines the connection between merger parameters and observable kilonova properties, which is important for cosmology and dense matter physics.
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What the work does
The authors systematically investigated how the individual spins of neutron stars at a fixed effective spin affect the ejecta, element synthesis, and the kilonova, using numerical simulations with the SFHo equation of state and neutrino transport. They considered three total masses and several spin configurations, extracted the ejecta, and calculated nucleosynthesis and light curves in three bands. They found that for the same effective spin, the ejecta mass can differ by a factor of 45, the yield of heavy elements by more than two orders of magnitude, and the kilonova retains this imprint in brightness and color.
Where its strength is
This work is the first to isolate the effect of individual spins, showing that the electromagnetic signal carries information inaccessible to gravitational waves, and opens the possibility of distinguishing systems with the same effective spin through kilonova observations.
Where it could go next
Worth trying
Why this may matter
If electromagnetic observations can indeed distinguish individual spins, this would provide a new independent way to test models of double neutron star formation and refine the physics of mergers.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.58, against a usual 0.64. It looks like few have drilled nearby.
In one model with anti-aligned spins, fast ejecta (with speeds exceeding 0.6 times the speed of light) reach a mass of about one thousandth of a solar mass — tens of times more than in typical non-spinning mergers. Such ejecta could be the source of the fastest components of the kilonova.
In Arthur C. Clarke's novel "Rendezvous with Rama," an alien spacecraft uses a gravitational slingshot around the Sun to change its trajectory. Neutron star mergers are a natural version of such a maneuver: the spins of the stars, like invisible gyroscopes, steer the outflows and determine the fate of the system.