kilonovae

Kilonovae are short-lived astronomical transients that arise when neutron-rich matter is ejected during the merger of two neutron stars or a neutron star with a black hole. The radioactive decay of freshly synthesized heavy elements (the rapid neutron capture, or r-process) powers the emission, producing a luminous but fading glow that peaks on timescales of days. The term kilonova was coined to indicate a luminosity between a nova and a supernova; some early literature used macronova.

The optical/near-infrared light from a kilonova is shaped by the opacity of the ejecta. Ejecta rich in lanthanide and actinide elements have high opacities, shifting emission toward the near-infrared and delaying the peak. If there is a lanthanide-poor, faster component, a brief blue optical emission may be seen early on. In many models, two components with different masses, velocities, and compositions reproduce observed light curves.

First direct detection: GW170817 in 2017 with electromagnetic counterpart AT2017gfo in NGC 4993. Observations showed an initial blue component fading within days, followed by redder emission. Inferred ejecta masses on the order of 0.01–0.05 solar masses for the blue component and up to ~0.05–0.1 for the red component, with velocities around 0.1–0.3 times the speed of light. Nucleosynthesis calculations indicate the production of heavy r-process elements, including lanthanides.

Significance: Kilonovae provide insight into the origin of many heavy elements and serve as electromagnetic counterparts to gravitational-wave events, aiding precise localization and independent distance measurements (standard sirens). They also constrain explode mechanics, ejecta geometry, and viewing-angle effects.

Observational challenges and outlook: The faint, rapidly evolving nature, and high optical/IR opacity make detections difficult; rapid, multi-wavelength follow-up is essential. Future facilities and wide-field surveys will enable larger samples, improving understanding of kilonova diversity and rates.

---