GW170817

GW170817 was a historic gravitational wave event observed on 17 August 2017.

This signal originated from a shell elliptical galaxy named NGC 4993. That galaxy is located approximately 140 million light years away from Earth. The event was caused by the final moments of a binary neutron star inspiral. An inspiral occurs when two dense objects orbit each other while spiraling closer. This process ended in a violent merger of the two stars. This discovery was a massive breakthrough for the field of multi-messenger astronomy. It marked the first time a gravitational wave detection was definitively linked to electromagnetic observations.

The mechanism of the event followed a specific sequence of physical actions. First, the two neutron stars spiraled inward toward one another. This movement produced gravitational waves that lasted for approximately 100 seconds. The signal started at a frequency of 24 hertz. It covered roughly 3,000 cycles as the stars moved closer. The frequency and amplitude increased in a pattern known as a chirp. The signal ended with the collision at 12:41:04.4 UTC. Following the merger, a short gamma-ray burst called GRB 170817A occurred. This burst lasted about 2 seconds and began 1.7 seconds after the gravitational waves.

Scientists used several different types of detectors to observe these stages. The gravitational waves were caught by the LIGO and Virgo interferometers. The LIGO detectors are located in Louisiana and Washington in the United States. The Virgo detector is located in Italy. The arrival times at these three locations helped scientists find the direction of the source. The gamma-ray burst was detected by the Fermi and INTEGRAL spacecraft. These spacecraft identified a large region of the sky that overlapped with the gravitational wave direction. This overlap confirmed that neutron star mergers are progenitors of short gamma-ray bursts.

The aftermath of the merger created a unique astronomical transient. This event was designated AT 2017gfo, but it is often called a kilonova. About 11 hours after the initial signal, the 1M2H team observed optical light. They used the Swope Telescope at the Las Campanas Observatory in Chile. This kilonova was a fast-moving, rapidly-cooling cloud of neutron-rich material. This material was the debris ejected from the collision. The cloud moved at roughly 10 percent of the speed of light. As the debris expanded and cooled, its color changed from blue to red.

Detailed analysis provided specific numbers regarding the mass of the stars. The total mass of the binary system was approximately 2.7 solar masses. The larger progenitor star had a 90% probability of being between 1.17 and 1.60 solar masses. The smaller star had a 90% probability of being between 1.14 and 1.62 solar masses. Scientists also measured the chirp mass, which is a specific parameter of the orbit. This mass was measured at 1.188 solar masses. Initially, researchers thought a black hole formed immediately. However, later analysis suggested a hypermassive magnetar might have existed for a few seconds.

Many different observatories participated in the global follow-up campaign. Over 70 observatories on seven continents used the electromagnetic spectrum to study the event. These observations included radio, optical, infrared, ultraviolet, and X-ray wavelengths. The Chandra X-ray Observatory detected the source 9 days after the merger. The Very Large Array in New Mexico detected radio waves 16 days later. The light from the radio and X-ray emissions peaked 150 days after the merger. Even 940 days later, the Chandra Observatory still observed X-ray emission. This massive coordination allowed for a very precise localization of the source.

GW170817 connects several major ideas in modern physics and astronomy. It proved that gravitational waves and light can come from the same event. This allows scientists to use two different "messengers" to study the universe. It also provided evidence for how heavy elements are produced in space. The merger of neutron stars creates a cloud of debris that carries new elements. This event was so significant that the journal Science named it the Breakthrough of the Year for 2017. It changed how we look at the violent deaths of stars.