Imagine two spinning tops set on a table. If they spin in the same direction, their combined rotation is large; if they spin toward each other, the total rotation can be zero even though each top spins rapidly on its own. In a pair of neutron stars approaching merger, their individual rotations, spins, play the role of such tops. The gravitational waves emitted by the pair depend, to first approximation, only on the average total rotation, the so-called effective spin. But what if the spins of the stars differ while their sum is the same? New research shows that the ejecta, heavy-element synthesis, and the brightness of the kilonova, the flare after the merger, change dramatically.
The authors ran a series of numerical simulations of neutron star mergers with different masses and spin configurations, but they fixed the effective spin to zero. They found that the mass of dynamical ejecta, material thrown off within seconds of the merger, can vary by a factor of 45. The yield of elements heavier than barium varies by more than two orders of magnitude, and in models with rapid collapse to a black hole, by more than four orders of magnitude. It's as if two factories with the same total engine power produced tens of times different amounts of output.
The kilonova retains this imprint: peak brightness differs by up to 0.9 magnitudes, and color at one 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. This means that electromagnetic observations can distinguish pairs with the same effective spin but different individual spins, opening a path to refining the formation history of such systems.
The work shows that the individual spins of neutron stars are not just a detail of the gravitational-wave signal but a key factor determining the fate of the ejecta and the emission. This is the first systematic evidence that pairs with the same effective spin can produce drastically different electromagnetic signals. Among neighboring studies that examined the influence of rotation on ejecta and kilonovae, this work stands out because it fixes the effective spin and varies the individual spins for the first time, and also includes neutrino absorption and analysis of all ejecta channels. This allows breaking the degeneracy between spins that limits the interpretation of gravitational-wave observations.
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.