Astrophysics & Particle Physics-APPC 2026

Neutron-star mergers are astrophysical events whereby two neutron stars in a binary system spiral inward and collide because of the emission of gravitational waves. These mergers are among the most dynamic processes in the universe, generating gravitational waves, electromagnetic radiation, and heavy elements.LIGO and Virgo's historic detection of GW170817 in 2017 was the first-ever observation of a neutron-star merger with both gravitational waves and electromagnetic counterparts, opening a new era in multi-messenger astronomy. The impact created a kilonova, a short-lived astronomical event driven by radioactive decay of newly created heavy elements, such as gold, platinum, and other r-process nuclei.Neutron-star mergers are also associated with short gamma-ray bursts (GRBs). The strong gamma-ray emission is produced when relativistic jets created in the merger point toward the Earth. These outbursts, together with kilonova radiation, offer a full description of the physical events during and subsequent to the collision.These mergers are responsible for nucleosynthesis and create a majority of the universe's heavy elements heavier than iron. Astronomers can learn about r-process element creation and galaxy chemical evolution by examining the kilonova spectra.Neutron-star merger gravitational-wave observations permit very accurate determinations of neutron star masses, radii, and dense nuclear matter equation of state, and hence insights into physics at extreme conditions.

Observations from the electromagnetic spectrum, from gamma and X-rays to optical and radio waves, aid in following the development of ejecta and the interaction with the ambient setting these mergers are fundamental to cosmology, since the combination of the gravitational-wave signals and the electromagnetic redshift observations permits independent measurements of the Hubble constant.In conclusion, neutron-star mergers form the backbone of current astrophysics, bridging gravitational-wave astronomy, high-energy transients, and nucleosynthesis. They shed light on the physics of compact objects, heavy element formation, and the dynamic processes that sculpt the universe.