Multi-messenger astronomy: detection of gravitational waves and electromagnetic counterparts in neutron star merger

The most recent news in astronomy consists of the report that scientists have managed to record a collision of neutron stars in both gravitational waves and electromagnetic waves. This is a remarkable feat for many reasons. First of all, this is the first time that astronomers detected the gravitational waves that are produced when these neutron stars orbit each other in a deadly inspiral. Equally impressive was the fact that this event was also “seen”: it has been detected and monitored all across the electromagnetic spectrum. There were observations in gamma rays, X-rays, ultraviolet, visible light, infrared and radio. Moreover, the short gamma-ray burst detected shortly after the gravitational waves provided sound evidence that these bursts are associated with the collision of neutron stars. Finally, this merger has given us very sound evidence that such destructive events are responsible for the creation of heavy chemical elements in the periodic table, such as gold and platinum.

The gravitational waves, which are ripples in spacetime that occur when these massive objects orbit each other, were picked up by the LIGO twin detectors in the United States, as well as by Virgo, a gravitational wave detector in Italy. These are the same detectors that have been picking up gravitational waves from the mergers of black holes – the first detection gave 3 LIGO scientists the 2017 Nobel prize in Physics. However, this was the first time that scientists were able to detect gravitational waves from a merger of neutron stars, rather than black holes.

Just like black holes, neutron stars are one of the possible outcomes for a star when it runs out of fuel and collapses due to its own gravity. In order to become a neutron star, a star has to be very massive – much more than the Sun – but not extremely massive, otherwise it will collapse into a black hole. Mergers of black holes are thought to produce very little – if any – light, so they are difficult to “see” with traditional (electromagnetic) astronomy, although we are now able to “hear” them due to the gravitational waves produced in these collisions. Mergers of neutron stars, however, are a different story.

Less than two seconds after the gravitational waves were detected by LIGO and Virgo, the space telescope Fermi detected gamma rays coming from the same region where the gravitational waves came from. Alarms were sounded and telescopes all over the world, as well as in space, started to sweep the sky in order to pinpoint the location of this event. The Swope telescope, in Chile, was the first to report the location as the galaxy NGC 4993, in the constellation of Hydra, about 130 million light-years distant from the Earth.

Above, we see the detections of gravitational waves as well as observations across the entire electromagnetic spectrum. Source: LIGO.

The gamma rays detected by Fermi are what we call a short gamma-ray burst (sGRB). Before this detection, there were strong suggestions that these sGRB were produced when two neutron stars collide, but this was the first conclusive evidence that this is indeed what happens.

Once the telescopes were pointed to the sky, astronomers noticed a blob at the very place where this collision occurred. This blob is what we call a kilonova: a violent explosion that occurs when two neutron stars merge. Upon measuring the electromagnetic spectrum of this kilonova, astronomers detected the presence of heavy chemical elements, such as gold and platinum. This confirmed theoretical predictions that stated that these heavy elements are produced in such collisions, which are so strong that neutrons are literally forced into the nuclei of atoms, making them heavier and therefore creating heavier elements. This implies that the idea that we are all made of stardust also applies to our jewelry.

This detection of gravitational waves and the subsequent observation of this collision across all types of electromagnetic radiation marks the beginning of multi-messenger astronomy: we are now able to study the same event occurring in the universe using two very different types of information. It was a massive effort done by thousands of scientists scattered across the globe and is a small sample of the true power of scientific collaborations.