Neutron Stars Collission (Script) (Patreon)

Last year LIGO announced the detection of gravitational waves from the merger of two black holes. The science world went a little crazy. Only a few weeks ago a new rumour emerged: that LIGO had, for the first time, spotted gravitational waves from the collision of a pair of neutron stars. If it’s true, some long-standing astrophysical mysteries are about to be unlocked.

When the LASER Interferometer Gravitational Wave Observatory – LIGO – detected gravitational waves from a pair of merging black holes, an entirely new realm of the universe suddenly opened up to science. We now have an observatory that can explore the most extreme gravitational phenomena in the universe. Check out our episodes on the LIGO detections for more info. Since the first, LIGO has announced the detection of two more black hole mergers. As the data comes in we’re learning a ton about black holes, how they grow, and the stars that produce them.

But the merger of binary black holes isn’t the only game in town. LIGO was supposed to also detect some other crazy stuff – like certain supernova explosions and the merger of binary neutron stars. And it may have just done so. Rumours are abound that LIGO has finally spotted the long-expected neutron star-neutron star merger, AND that the event was accompanied by a bright flash of gamma rays. The rumour has already been hyped all over the press, so let’s dissect it with a skeptical eye.

Before we figure out whether the rumour is true. In fact before we talk about the supposed signal at all, let’s refresh our memory on neutron stars. When a massive star ends its life in a supernova explosion, it leaves behind an ultra-dense core. For the most massive stars that core will collapse into a black hole, but there’s an intermediate range – a remnant core between 1.4 and around 3 times the mass of our Sun instead ends up as a neutron star. These insane objects carry the mass of a star within a sphere the size of a city – around 18 km in diameter. They are mostly composed of neutrons at the density of an atomic nucleus, and are held up only by a quantum-mechanical force called degeneracy pressure. We talk about the bizarre  physics of these quantum and gravitational monsters in this video (https://youtu.be/xx4562gesw0).

Neutron stars can rotate up to thousands of times per second, and have enormous magnetic fields that result in jets of near-light-speed particles that sweep through space like a lighthouse. When those jets sweep past the Earth we see the regular flashes of a pulsar. In fact, the very first real evidence of the existence of gravitational waves came from a pulsar. This was the Hulse–Taylor binary – two neutron stars in orbit around each other, one of which is visible to us as a pulsar. This binary pair stirs up spacetime in its vicinity, creating ripples that travel outwards as gravitational waves. That gravitational radiation sucks energy from the orbiting system, causing the neutron stars to spiral inwards. By monitoring the pulses from one of those stars, this inspiral was measured. The rate of loss of orbital energy exactly matched the expected rate of emission of gravitational radiation.

Any neutron stars or black holes in close orbit with each other will eventually collide as they leak gravitational radiation. We now know of plenty of neutron star pairs in binary orbits. In fact we expect them to be much more common than black hole binaries. Why? Because the universe makes far more neutron stars than black holes.  See, black holes only form in the deaths of the most massive stars – those over approximately 20 times the sun’s mass. But these are also the rarest stars. Neutron stars form from the not-quite-as-rare stars of around 8 to 20 solar masses. That means neutron stars should be more common than black holes and neutron star binary systems should merge more often than black holes. 

So why isn’t LIGO seeing lots of them? Well again, it’s because of mass. The remnant core of a dead star must be less than 3 solar masses to make a neutron star. But that's a factor of 10 smaller than the 30 solar mass black holes that merged in the first LIGO detection. Smaller mass means weaker gravitational waves. In fact a typical neutron star merger needs to be around 10 times closer to us than a typical black hole merger for LIGO to see it. If we can see neutron star mergers out to only one-tenth the distance then that translates to being sensitive to one-one-thousandth the volume. We can see black hole mergers across 1000 times more universe compared to neutron star mergers. So even though the latter are common, we have to wait longer for one to happen close enough to us to be detectable.

Neutron star mergers do have one advantage over black hole mergers; they last a lot longer, at least from LIGO’s point of view. LIGO is sensitive to a specific frequency range. Inspiralling black holes only hit that range in the final second before merger, while neutron stars ring at audible gravitational wave frequencies for at least several seconds. If we did spot a neutron star merger as rumoured we’ll have a lot more juicy data to analyze compared to a black hole merger.

Let’s talk about the rumour. It was started with a tweet from astronomer J. Craig Wheeler about a LIGO detection with an optical counterpart. “Optical counterpart” means there’s a source of visible light associated the gravitational wave, and in this case it’s the suspected galaxy that the wave came from. How did we locate the galaxy? After all, LIGO can only constrain the origin of its signals to a wide band across the sky. There’s also a rumour that the Italian gravitational wave observatory VIRGO also spotted this signal, which helps triangulater the location, but not enough to get an exact origin.

Here’s how we know: the day before the fateful tweet, August 17th, the Fermi satellite had spotted a flash of gamma radiation – the highest energy light – from a galaxy 130 million light years away. That in itself wasn’t unusual. These gamma ray bursts are relatively common. Most are believed to result from supernova explosions, but around 30% of them – the short-lived ones, which last for less than 2 seconds – are believed to come from merging neutron stars. The observed GRB was of that type. Logs from the follow-up observation by the Chandra x-ray satellite confirm this.

Still not especially convincing. But check this out: a Hubble Space Telescope observation was triggered a few days later to look at the location of this gamma ray burst. And the particular observing program that was triggered is one specifically intended for following up on gravitational wave detections. Now it’s suddenly compelling. Astronomers don’t trigger Hubble observations lightly. Someone in the know decided that this gamma ray burst was very likely associated with a gravitational wave.

Here’s that Hubble observation. That blob is the origin of the gamma ray burst. It’s NGC4993, a known lenticular galaxy. We rarely see supernovae from this galaxy type them because their most massive stars have long since exploded to leave neutron stars and black holes. That makes them the perfect environment for neutron star mergers. NGC4993 is 130 million light years away, which is about at the limit for LIGO’s sensitivity to neutron star mergers. Compare that to the around one billion light year distance of the earlier black hole mergers.

OK, so assuming the rumour is true, why do we care? Well beyond raw curiosity, it may be that neutron star collisions produce many of the heavier elements of the periodic table. Most heavy elements like gold, lead, uranium etc. are produced when the nuclei of lighter elements capture fast-moving neutrons. This is the R-process. It definitely happens in supernova explosions, which for a long time were thought to be the primary source of heavy elements.

But it turns out that merging neutron stars can do this too. As these stars coalesce, most of their material goes into forming a new black hole. But the neutron stars’ thin iron atmosphere is likely bombarded with neutrons and blasted outwards, spraying a ton of R-process elements into the galaxy. Depending on how much of the outer layer is ejected, neutron star mergers could produce most of the heavy R-process elements that exist. Seeing a gravitational wave signal from merging neutron stars would allow us to determine pretty exactly how much mass is lost in the merger. That will be an extremely important piece of evidence in either killing or helping confirm the idea.

Besides their importance to nucleosynthesis, the simple fact that we can SEE neutron star mergers in regular light is extremely powerful. Black hole mergers are dark, so we have to infer almost everything from the gravitational waves alone. But colliding neutron stars are bright across the electromagnetic spectrum. Comparing the EM and gravitational wave signatures will teach us a lot.

If it’s real, why hasn’t LIGO announced it? The LIGO team has always shown admirable caution before making big announcements in the past. They want to analyze the data fully to ensure the signal meets their very strict statistical standards for claiming a detection. The last LIGO observing run ended on August 25th, at which point they announced that there were “promising candidates”. Probably that means more black hole-black hole mergers in addition to this rumoured neutron star merger. But to go from “promising candidate” to “confirmed detection” requires meticulous statistical analysis. Public announcements will happen when the team is sure of the significance of the signal.

Fingers crossed on this one. We may have spotted for the first time a long-theorized astrophysical catastrophe; one that birthed a new black hole and which created half of the Earth’s mass in gold. And we will have learned of it from over a hundred million light years distance, by collecting only a handful of gamma rays and by sensing the faint ripples it made in the very fabric of Space Time.