Neutron Star Merger Gravitational Waves and Gamma Rays

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Due to the crazy technological challenges that had to be overcome in order to detect gravitational waves, some people were skeptical that scientists had actually done it, that they’d actually seen gravitational waves from black holes – after all, there was no corroboration. But, after the discovery that’s being announced today, there can be no more doubt, because scientists have detected the merging of two neutron stars a hundred and thirty million light years away. This is the first ever detection of gravitational waves from in-spiraling neutron stars, and what’s really exciting about this detection is that the same event has been observed with telescopes in all areas of the electromagnetic spectrum. It all began on August 17th at 8:41 a.m.

Eastern Time when LIGO interferometers identified a clear gravitational wave signal that lasted about a hundred seconds, which is way longer than any previous detection, and it’s consistent with theoretical predictions for the signal from two merging neutron stars. [“hear” the sound of two stars colliding ♪] Around 1.7seconds later NASA’s Fermi gamma-ray telescope identified a burst of gamma rays. For decades gamma-ray bursts have been thought to come from neutron star mergers, but the evidence has been lacking to know for sure that these gravitational waves and the gamma ray burst came from the same event. The key was to locate where in the sky this neutron star merger occurred. Unlike a merger of black holes neutron stars emit light when they smash together and continue emitting electromagnetic radiation afterwards. The Fermi gamma-ray Space Telescope identified a large patch of the sky roughly the size of six thousand full moons. Using the European Space Agency’s integral gamma-ray satellite, they were able to narrow down that range. Now, the gravitational waves detected by LIGO allowed them to identify two long strips in the sky, one of which overlapped with the existing search area. Now, interestingly, Virgo, which is the newest gravitational wave detector, which is in Italy – it was online at the time, and it should have easily been able to detect these gravitational waves, and yet it saw almost nothing, and that was kind of a key clue, because it indicated that the gravitational waves must be coming from one of that detector’s blind spots. Every interferometer has some blind spots where, if the waves are coming at that angle, it’s symmetric with respect to the two arms and so it just can’t be detected. So this helped further narrow the search area down to the size of about 144 full moons. Now, within that area, around fifty galaxies were identified to be studied with optical telescopes, and just 11 hours after the initial detection astronomers located a bright spot in the galaxy NGC 4993. You are seeing here pictures of the light from two neutron stars that merged 130 million years ago.

Watch how the color and brightness changes in the aftermath of the collision. So what are neutron stars? Well, they’re the leftover cores of big stars that have exploded – they’ve gone supernova. Now, those remaining cores are squeezed down by gravity and if they’re too big, say larger than two or three solar masses, well, they will keep on getting crushed until they collapse in on themselves forever and become a black hole. But if those cores are a little smaller, say 1.1 and 1.6 times the mass of our Sun, as they were in this case, well, then they get squeezed still and so electrons merge with protons to form neutrons and neutrinos and the neutrinos take off and the neutrons are left in a really, really densely packed star. And the only reason the neutrons don’t combine with each other is because of a quantum principle, the Pauli exclusion principle, that basically says you can’t put two of these particles right on top of each other, and that’s actually the only thing holding that neutron star up. So, if you have two of these neutron stars and they are orbiting each other, well, then they emit some of their energy as gravitational waves, and as they do that they lose energy, meaning they spiral in closer to each other and when they get really close, say a few hundred kilometers apart, the gravitational waves become intense, allowing us to detect them hundreds of millions of light years away. The collision of neutron stars creates a kilonova which spews debris out into space. This is debris that glows, allowing us to observe what’s been created – and, in fact, the new observations with light telescopes have shown that heavy elements like gold, lead and platinum were made in this event, and that helps us understand where a lot of the heavy elements in our universe come from. In my view, this event really shows us that we’re in a new age of astronomy. We can detect gravitational waves, not just from black holes, but now from neutron stars, we can use that information to locate places in the sky where that occurred and we can validate that with our other telescopes looking in all parts of the electromagnetic spectrum. So, now we really have more tools to understand our universe and I just can’t wait for the questions that we’re going to address next and all of the different things we’re going to be able to study as the gravitational wave observatories get better and better. It’s a phenomenal time to be studying the universe. Outro signal.

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