In A Historic Discovery, A Neutron Star Merger Has Been Observed For The Very First Time

The fifth observation of gravitational waves (GW) marks the beginning of a new era in astronomy. On August 17, 2017, the LIGO and VIRGO collaborations detected neutron stars merging for the first time and immediately alerted observatories around the world. In a matter of hours the event had been located, another first for GW astronomy, and telescopes around the world begun studying it almost immediately.

The event observed, called GW170817, was produced in galaxy NGC 4993, located 130 million light-years from Earth. The gravitational signal was the strongest ever observed, lasting over 100 seconds, and it emitted a gamma-ray burst (GRBs), providing the first piece of evidence that GRBs are produced by neutron star collisions. It also provided the strongest evidence yet that neutron star mergers are responsible for the creation of the heaviest elements in the universe, like gold and platinum.

The importance of this observation cannot be overstated. We are witnessing Galileo pointing the telescope up, or Henrietta Swann Leavitt working out the relation that will be used to measure cosmic distances. This observation brings a completely new dimension to astronomy. The dozens of papers published in Nature, Nature AstronomyAstrophysical Journal Letter, Science, and Physical Review Letters, are also record-breakers. They have over 45,000 authors – around 35 percent of all active astronomers in the world – who worked at the over 70 observatories that helped to make this discovery.

“Now we have the detection of not just the merger but the in-spiral motion of two neutron stars,” Dr Vicky Kalogera, the most senior astrophysicist in the LIGO Scientific Collaboration from Northwestern University, told IFLScience. “The signal we heard on August 17th is the strongest gravitational waves signal we have ever received and it is the longest. We have more than a hundred seconds. We detect the in-spiral motion very clearly until they merge. And this allowed us to measure the masses quite well.”

The merging neutron stars’ masses are between one and two times the mass of our Sun, and the object they formed has a mass of between two and three solar masses. Theoretical predictions suggest black holes should form when neutron stars collide but researchers currently can’t confidently say if the remnant is a black hole or a neutron star.

The gravitational detection alone is enough to be incredibly excited about this discovery, but knowing that astronomers were able to detect the source using light telescopes makes this a pivotal moment in astronomy. One with very far-reaching consequences.

“We got the alert on August 17th that they had detected at very high confidence a neutron star merger and we discovered very quickly it was going to be possible to try to find an optical counterpart to it using our observatories in Chile,” Dr Maria Drout, Hubble and Carnegie-Dunlap Fellow at the Carnegie Observatories, told IFLScience.

With her colleagues and many astronomers around the world, they looked at the candidate sources within the area of the sky that the LIGO and VIRGO collaboration suggested as the likely origin for GW170817. The region was not enormous but it still contained a large number of galaxies.  

“Our colleagues to put together a list of a hundred galaxies that we were going to search that night. And as it happens the new source was found in the ninth galaxy that was on our list,” Dr Drout added. 

“Using all our telescopes on the object, you can see all the different colors and it really tells you a lot about the physics. It looked like something that was very young; a very young, very hot new object. So it was either the source or it was something else that had just exploded recently.”

The masses of known stellar remnants, and in the middle…we’re not sure yet. Astronomers don’t know if this neutron star merger created a black hole or not. LIGO-Virgo/Frank Elavsky/Northwestern

The object was confirmed as the source of the gravitational waves and observatories across the world and in space were able to learn a lot about the event.

“Such small localization means that we could understand from which galaxy and where in that galaxy it happened. And once we had a counterpart in one wavelength we can do a search across the spectrum all the way from radio to the X-rays,” Dr Raffaella Margutti from Northwestern University told IFLScience.

The source changed very quickly compared to other cosmic explosions. For example, it cooled off in just a couple of days, while supernovae might take weeks or months. The light observations were also able to show that merger created a lot of heavy elements. The mechanism for their production, called the r-process, require so much energy that only catastrophic events like a neutron star being destroyed could create them. And this seems to be the case.

If all this wasn’t enough pivotal science, researchers were also able to work out more about the final interaction. The merger produced a very narrow high-energy jet, which wouldn’t normally be visible as it wasn’t pointing at us, but after about 10 days the jet opened up and was observed in both X-ray and radio. We have never seen this moment before and it told the team a lot.

“It’s an amazing amount of information on the merger itself,” Dr. Margutti confirmed.

The source has also shown that neutron star mergers can also be responsible for the emission of gamma-ray bursts. The Fermi telescope caught a gamma-ray emission at the same time as the GW signal, and the Swift observatory saw ultraviolet and optical light emission which was 1,000 times brighter than your typical nova. For decades, astronomers have hypothesized that neutron stars interacting were the cause behind such events and although this one is not exactly the classic GRB (it’s on the short-side) it tells us a lot about these events.

“Now for the first time, we’re basically solving the mystery with gravitational waves. It’s a weird short gamma-ray burst. it’s a bit faint and it’s the closest we have seen by far. Is it typical? We don’t know. So, there are more questions marks there,” Dr Kalogera explained.

Of the five gravitational waves observed so far, GW170817 is the strongest observed yet. 

The two LIGO observatories in Louisiana and Washington and the VIRGO one in Italy are currently switched off and undergoing some tuning before being turned back on in late 2018. By then, events like this might soon become the norm for astrophysical observations. The age of gravitational and electromagnetic astronomy has now begun in full.

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