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A neutron star merger (or NS merger), the joining of two neutron stars, becomes likely when the orbit of a binary neutron star becomes sufficiently small that gravitational waves sap significant energy from the orbit, causing appreciable orbital decay. The term kilonova has been adopted for such an event, or the resulting transient. When they merge, a gravitational wave event may result, and the ground gravitational-wave detectors, LIGO and Virgo can detect them within a certain radius. The sixth accepted LIGO GW detection, GW170817, is ascribed to a neutron star merger, based upon the determined mass of the merging objects and the GW "sound" of the aftermath (ringdown). A slower inspiral than occurs with black holes (which generally have more mass) results in a substantially longer time within the detectable frequency range, and the ringdown is more complex due to the effects of the tidal forces on the material, moving the material around and ejecting some of it. The merger result can be a single neutron star, or a single black hole, possibly after a short life as a hypermassive neutron star (termed a delayed collapse), which can be as short as one second.
Like black hole mergers, it isn't obvious how the orbits become small enough for gravitational waves to pull them together, and the orbit decay is presumed to take a long time; one analysis of GW170817 suggested the decay took over 11 gigayears. Theories are being tested by simulation.
A neutron star's tidal deformability (TD, often symbolized by Λ, or tidal deformability parameter or just tidal parameter) is a calculated scalar value associated with the effects that a tidal force would have on it, a function of its mass, radius, and Love numbers. The binary tidal deformability (binary TD), which is a single scalar value derived from the tidal deformabilities of the pair, is associated with details of the merger. Data from the GW detection places constraints on the tidal deformability values, and in turn, on the neutron stars' characteristics used to calculate them.