Kilonova

On August 17, 2017, scientists for the first time directly detected gravitational waves—“ripples” in spacetime—originating from the collision of two neutron stars. Until then, observed gravitational-wave events were primarily due to mergers of massive black holes. This cosmic cataclysm was also accompanied by the emission of light across multiple energies, which for the first time in history was systematically documented. Astronomers call this phenomenon a kilonova, because the total energy radiated is about a thousand times greater than that released by a classical nova.

The electromagnetic emission from the 2017 event was monitored by over 70 ground-based and space observatories. Neutron stars are, after black holes, the smallest and densest objects known. They form when massive stars collapse and explode as supernovae. Some neutron stars exist in binary systems, where their orbits gradually shrink, producing gravitational waves. The final ~100 seconds of this “dance” were captured by the LIGO and Virgo detectors.

During the collision, the neutron stars were disrupted, and a small fraction of their matter—a few percent of the Sun’s mass—was ejected into space at nearly 30% the speed of light. This produced a gamma-ray burst, detected in orbit roughly two seconds after the gravitational-wave signal. Over the following days, astronomers observed electromagnetic emission across multiple wavelengths, including X-ray, ultraviolet, optical, infrared, and radio radiation from the event.

Observations of neutron star mergers are a treasure trove of astronomical knowledge. The simultaneous detection of gravitational waves and the gamma-ray burst solved the mystery of the origin of such bursts and confirmed the theory proposed in 1986 by Prof. Bohdan Paczyński that they have an extragalactic origin. Paczyński, together with Prof. Li-Xin Li, had also predicted in 1998 the occurrence of optical flashes accompanying neutron star mergers (the so-called Li-Paczyński nova).Moreover, the simultaneous registration of gravitational waves and gamma rays allowed scientists to measure the speed of gravitational waves. Studies of the environment of the newly formed neutron star showed that such events are the primary sites in the Universe for producing heavy elements, including gold and platinum.