New insights from a merger between two neutron stars, which produced gravitational waves and electromagnetic radiation, are reported in seven papers published online in Nature and Nature Astronomy this week. The findings shed light on many aspects of astrophysics, including the origins of cosmic explosions known as gamma-ray bursts and of some heavy elements in the Universe.
On 17 August 2017, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Advanced Virgo Interferometer detected the merger of two neutron stars - the smallest, densest known stars - in a galaxy called NGC 4993, located 40 million parsecs (130 million light years) away. The event, named GW170817, produced both gravitational waves - ripples in the fabric of space-time predicted by Einstein - and electromagnetic radiation, and was followed two seconds later by a gamma-ray burst. Previous gravitational-wave signals have come from mergers of two black holes, which meant that only gravitational waves were expected.
GW170817 was detected in gamma-rays and, as reported in five Nature papers and one Nature Astronomy paper, in X-rays, optical light and infrared light. The papers reveal properties of the event, including its colour (blue at first, before it reddened) and geometry. The reported radiation signatures support long-held predictions that double neutron-star mergers eject radioactive material as part of a low-luminosity explosive event known as a kilonova. The studies also suggest that neutron-star mergers are a major source of some of the elements heavier than iron in the Universe, the origins of which have been uncertain. One paper reports that a fast jet of material was probably observed off-axis, which could help to explain why gamma-ray bursts often appear dim.
In a further Nature paper, the authors use properties of GW170817 to measure the Hubble Constant - a unit of measurement to describe the expansion of the universe. GW170817 is the first gravitational-wave event with a known host galaxy. The authors used the distance of the host galaxy to calculate a Hubble constant of about 70 kilometres per second per megaparsec, which is consistent with previous estimates.
In an accompanying News & Views article, M. Coleman Miller concludes, “GW170817 represents a remarkable opportunity to make major progress in multiple fields of physics and astrophysics, and it whets our appetite for the many expected observations of neutron-star mergers in future campaigns.