Kilonova

A kilonova is an electromagnetic flare associated with the merger of compact objects, especially binary neutron stars. During a merger, some neutron-rich matter can be thrown into space. As this ejecta expands, radioactive decays of newly formed nuclei heat it and power a transient glow. The event is generally dimmer than a supernova but brighter than a classical nova, hence the name.

The ejecta cools and changes as it expands, so the kilonova’s observed brightness and color evolve with time. Early emission can be relatively blue, while material rich in heavier elements has many transitions that impede shorter-wavelength light and can make the emission redder and more infrared-dominated. The details depend on the amount, speed, composition, and geometry of the ejected matter; observations are interpreted with models rather than a single fixed light curve.

On 17 August 2017, LIGO and Virgo detected gravitational waves from GW170817, a binary neutron-star merger. A short gamma-ray burst and an optical transient in the galaxy NGC 4993 were subsequently observed, followed by measurements across the electromagnetic spectrum. The combined observations established a clear connection between a neutron-star merger and a kilonova, demonstrating how gravitational-wave and telescope observations can jointly investigate one cosmic event.

Spectral and light-curve observations of GW170817 provided evidence that neutron-star merger ejecta contain newly synthesized heavy elements. In neutron-rich ejecta, rapid neutron capture (the r-process) can build nuclei heavier than iron; radioactive decay of these products contributes to the kilonova’s light. Neutron-star mergers are an established site for producing some heavy elements, though their share of the cosmic inventory and the contributions of other sites remain active research questions.