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Scientists from the LIGO and Virgo collaborations along with their partner observatories around the world announced the first ever gravitational wave detection from a cosmic collision between neutron stars on August 17, 2017.

By Kaew Tinyanont

Unlike in previous gravitational wave detection from black holes collision, astronomers also detected light from this neutron star merger in all wavelengths ranging from the high energy gamma ray, X-ray, to the visible light and down to the radio frequencies. This wealth of information let astronomers study this event with unprecedented details.

Gravitational wave is the ripple of stretching and squeezing of the space-time caused by cosmic collisions between two very dense and massive objects such as black holes and neutron stars. The effect of gravitational waves is so slight; if you have a rod 2.5 miles long, a typical gravitational wave will change the length of the rod by smaller than the width of a proton, making it extremely difficult to directly detect. As a result, it took the Laser Interferometer Gravitational- Wave Observatory (LIGO) decades of intense research and development until it has enough sensitivity to detect the first black hole merger on September 14, 2015. The pioneers behind LIGO: Rainer Weiss (MIT), Barry Barish, and Kip Thorne (Caltech), were awarded the 2017 Nobel Prize in Physics.

All other gravitational wave detection so far come from collisions between very massive black holes, ranging from 7 to 35 times the mass of the Sun. While interesting in their own rights, these black hole mergers do not produce any light. Astronomers cannot scrutinize them using the familiar and much more developed techniques based on observations of light. What makes today’s discovery very exciting is that the neutron star merger produced not only gravitational wave, but also light in every wavelengths allowing for astronomers around the world to study its nature in exquisite details.

Neutron stars

Neutron stars are dense remains of massive stars’ core after they exhaust all their nuclear fuel, and collapse under their own gravity. A typical neutron star has a radius comparable to the size of Pasadena, however it has a mass around that of our Sun. This is an extremely dense object with very strong gravity. If two neutron stars orbit one another, their gravitational interaction emits enough energy through gravitational wave to cause the two stars to get closer and closer to each other. Observations of this orbital decay, exactly as predicted by general relativity, were the first indirect glimpse of gravitational wave that astronomers detected. The finding led to the 1993 Nobel Prize in Physics.

The final phase

The direct detection of gravitational waves from a collision between two neutron stars announced today is the final phase of this orbital decay when the two objects finally crash into each other. The initial gravitational wave detection was followed two seconds later by a short burst of gamma ray radiation seen by Fermi space observatory. This detection confirms a decade long theory that these elusive short duration gamma ray bursts are indeed caused by neutron star collisions. By this time, the gravitational wave detection triggers were sent to partner observatories around the globe to quickly catch it with optical and infrared telescope. And catch it we did! Multiple observatories, mainly in the Southern hemisphere, started to see a bright blue burst of light from a nearby galaxy NGC 4993, 130 million light years away. This transient event fades and turn red within the matter of days, as predicted by theoretical model. The most fascinating discovery in the optical and infrared light is that the spectrum of this event shows signs of multiple metals heavier than iron whose origin was uncertain until today. This is the first time we know for sure where precious metals like gold and platinum are made in the universe. X-ray and radio emissions from the interaction between the neutron star merger and the surrounding medium were also reported a few days after the initial explosion.

A new era in observational astronomy has truly begun

For centuries, all methods we have to study the heavens rely on only one messenger: light. While incredibly rich with information, light alone cannot tell the full story. The observation announced today, which synergizes traditional observations of light with a novel observation of gravitational wave, has resolved two outstanding problems in astrophysics: the origin of short duration gamma ray bursts and the origin of half of the heavy elements in the periodic table. These are big problems that need innovative observations to answer. With new gravitational wave detectors coming online along with a bigger, faster survey telescopes, the multi-messenger era of astronomy promises to reveal a trove of exciting discoveries in the years to come.

Kaew Tinyanont is a graduate student in astronomy at California Institute of Technology. His research focuses on brown dwarfs, which are objects with more mass than planets but less than stars.