The late-time electromagnetic emission of the kilonova associated with the gravitational wave signal GW170817 from a binary neutron star merger is at odds with theoretical expectations. A binary system with an asymmetric mass ratio is one potential explanation. However, the known binary neutron star systems that are expected to merge within billions of years have similar component masses.
Robert Ferdman and colleagues have now identified the most asymmetric double neutron star system observed to date. They precisely measured the mass ratio of the pulsar PSR J1913+1102 and estimated its coalescence time to be around 470 million years. A population synthesis analysis of the nine merging binary neutron star systems with known masses showed that several per cent of all merging binaries have similarly asymmetric mass ratios. This implies the existence of a significant population of asymmetric binary neutron star systems, which could explain the anomalous properties of GW170817, and is expected to lead to future detections of bright kilonovae.