Last October one of the most exciting gravitational wave observations yet was announced. Astronomers witnessed the collision of two neutron stars, the first observation that combined standard astronomical observations with gravitational waves. The event is known as GW170817.

At the time, astronomers weren’t sure what this cosmic collision created. Neutron star mergers can result in either an extremely heavy neutron star or a small black hole. New observations might have possibly cleared that up. According to X-ray observations of the region, the end product of GW170817 was a black hole with a mass around 2.7 times the mass of our Sun. The results are published in the Astrophysical Journal Letters.

On August 17 last year, researchers at LIGO and VIRGO, the three gravitational waves observatories, detected the neutron stars merging, found the location the signal was coming from and notified astronomical observatories on the ground and space telescopes the world over so they too could observe it. It was a historic world first.

The Chandra Observatory, NASA’s X-ray instrument, began its observations immediately but couldn't detect a source until nine days after the event, and then managed to follow this up 15 and 16 days after the event. At that point, the source moved behind the Sun, but then was observed again 110 days and then 160 days after the event. These late observations gave astronomers enough information to work out the true nature of the merger remnant.

If the two neutron stars had created a single heavier neutron star, they would have observed a bright X-ray emission. Neutron stars spin rapidly and have very strong magnetic fields, which would accelerate the expanding bubble of particles to create a high-energy emitting X-ray. But the observations showed something different.

The X-ray intensity observed at 110 days and at 160 days are both comparable to each other, a factor of 100 times lower than if they were produced by a neutron star. The team thinks the X-ray emission observed is from a shock wave instead; the energy from the merger is smashing through surrounding material like a sonic boom, heating up the gas and making it shine. 

This, they say, is likely evidence that the result of GW170817 is a black hole and not a neutron star.

The combination of gravitational wave observations and “traditional” approaches, is called multi-messenger astronomy. Although in its infancy, it holds the promise of bringing about a completely different outlook on the universe. LIGO and VIRGO are expected to resume their gravitational wave detection later this year after upgrade work is completed.