How Gravitational Waves Led Us to Neutron Star Gold

Artist impression of a violent neutron star collision (Dana Berry, SkyWorks Digital, Inc.)

One hundred and thirty million years ago in a galaxy 130 million light-years away, two neutron stars met their fate, merging as one. Trapped in a gravitational embrace, these two stellar husks spiraled closer and closer until they violently ripped into one another, causing a detonation that reverberated throughout the cosmos.

On August 17, the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and Italian Virgo gravitational wave detector felt the faint ripples in spacetime from that ancient neutron star collision washing through our planet. Until now, LIGO and Virgo have only confirmed the collisions and mergers of black holes, so the fact that a nearby (a relative term in this case) neutron star merger had been detected was already historic.

But the implications for this particular neutron star signal, which is comparatively weak in comparison with the black hole mergers that have come before it, are so profound that I’ve been finding it hard to put this grand discovery into words (though I have tried).

Why It Matters

With regards to gravitational waves, I feel I’ve described each gravitational wave discovery as “historic” and “a new era for astronomy” since their first detection on Sept. 15, 2015, but the detection of GW170817 may well trump all that have come before it, even though the signal was generated by neutron stars and not black hole heavyweights.

The thing with black holes is that when they collide and merge, they don’t necessarily produce electromagnetic radiation (i.e. visible light, X-rays or infrared radiation). They can go “bump” in the cosmic night and no intelligent being with a conventional telescope would see it happen. But in the the gravitational domain, black hole mergers echo throughout the universe; their gravitational waves travel at the speed of light, warping spacetime as they propagate. To detect these “invisible” waves, we must build instruments that can “see” the infinitesimal wobbles in the fabric of spacetime itself, and this is where laser interferometry comes in.

Very precise lasers are fired down miles-long tunnels in “L” shaped buildings in the two LIGO detectors (in Washington and Louisiana) and the Virgo detector near Pisa. When gravitational waves travel through us, these laser interferometers can measure the tiny spacetime warps. The more detectors measuring the same signal means a more precise observation and scientists can then work out where (and when) the black hole merger occurred.

There are many more details that can be gleaned from the gravitational wave signal from black hole mergers, of course — including the progenitor black holes’ masses, the merged mass, black hole spin etc. — but for the most part, black hole mergers are purely a gravitational affair.

Neutron stars, however, are a different beast and, on Aug. 17, it wasn’t only gravitational wave detectors that measured a signal from 130 million light-years away; space telescopes on the lookout for gamma-ray bursts (GRBs) also detected a powerful burst of electromagnetic radiation in the galaxy of NGC 4993, thereby pinpointing the single event that generated the gravitational waves and the GRB.

And this is the “holy shit” moment.

As Caltech’s David H. Reitze puts it: “This detection opens the window of a long-awaited ‘multi-messenger’ astronomy.”

What Reitze is referring to is that, for the first time, both gravitational waves and electromagnetic waves (across the EM spectrum) have been observed coming from the same astrophysical event. The gravitational waves arrived at Earth slightly before the GRB was detected by NASA’s Fermi and ESA’s INTEGRAL space telescopes. Both space observatories recorded a short gamma-ray burst, a type of high-energy burst that was theorized (before Aug. 17) to be produced by colliding neutron stars.

The growing family of merging black holes and neutron stars observed with gravitational waves (LIGO-Virgo/Frank Elavsky/Northwestern University)

Now scientists have observational evidence that these types of GRBs are produced by colliding neutron stars as the gravitational wave fingerprint unquestionably demonstrates the in-spiraling and merger of two neutron stars. This is a perfect demonstration of multi-messenger astronomy; where an energetic event can be observed simultaneously in EM and gravitational waves to reveal untold mysteries of the universe’s most energetic events.

Another Nod to Einstein

The fact that the gravitational waves and gamma-rays arrived at approximately the same time is yet another nod to Einstein’s general relativity. The century-old theory predicts that gravitational waves should travel at the speed of light and, via this brand spanking new way of doing multi-messenger astronomy, physicists and astronomers have again bolstered relativity with observational evidence.

But why did the gravitational waves arrive slightly before the GRB? Well, NASA’s Fermi team explains: “Fermi’s [Gamma-ray Burst Monitor instrument] saw the gamma-ray burst after the [gravitational wave] detection because the merger happened before the explosion,” they said in a tweet.

In other words, when the two neutron stars collided and merged, the event immediately dissipated energy as gravitational waves that were launched through spacetime at the speed of light — that’s the source of GW170817 — but the GRB was generated shortly after.

Enter the Kilonova

As the neutron stars smashed together, huge quantities of neutron star matter were inevitably blasted into space, creating a superheated, dense volume of free neutrons. Neutrons are subatomic particles that form the building blocks of atoms and if the conditions are right, the neutron star debris will undergo rapid neutron capture process (known as “r-process”) where neutrons combine with one another faster than the newly-formed radioactive particles can decay. This mechanism is responsible for synthesizing elements heavier than iron (elements lighter than iron are formed through stellar nucleosynthesis in the cores of stars).

Artist impression of colliding neutron stars generating gravitational waves and a “kilonova” (NSF/LIGO/Sonoma State University/A. Simonnet)

For decades astronomers have been searching for observational evidence of the r-process in action and now they have it. Soon after the merger, massive amounts of debris erupted in a frenzy of heavy element creation, triggering an energetic eruption known as a “kilonova” that was seen as a short GRB. The GRB was cataloged as “SSS17a.”

The Golden Ticket

Follow-up observations by the Hubble Space Telescope, Gemini Observatory and the ESO’s Very Large Telescope have all detected spectroscopic signatures in the afterglow consistent with the r-process taking place at the site of the kilonova, meaning heavy elements are being formed and, yes, it’s a goldmine. As in: there’s newly-synthesized gold there. And platinum. And all the other elements heavier than iron that aren’t quite so sexy.

And there’s lots of it. Researchers estimate that that single neutron star collision produced hundreds of Earth-masses of gold and platinum and they think that neutron star mergers could be the energetic process that seed the galaxies with heavy elements (with supernovas coming second).

So, yeah, it’s a big, big, BIG discovery that will reverberate for the decades to come.

The best thing is that we now know that our current generation of advanced gravitational wave detectors are sensitive enough to not only detect black holes merging billions of light-years away, but also detect the nearby neutron stars that are busy merging and producing gold. As more detectors are added and as the technology and techniques mature, we’ll be inundated with merging events big and small, each one teaching us something new about our universe.

Warning, Over-Hyped Title Alert: But It’s A Frackin’ SUPERNOVA!

"SuperNova" by Shadow-Trance (DeviantArt)
"SuperNova" by Shadow-Trance (DeviantArt)

I’m not kidding, last week was a huge mess of a supernova doomsday circus. It was like whispering “there’s a bomb under your chair” to the person next to you in a crowded theater and then watching the resulting flood of people slam into the fire escape. It was internet chaos. And there was no stopping it.

I am of course talking about the first, great doomsday scare of 2010: T Pyxidis.

Luckily for me, the first headline I saw was in the UK’s Telegraph that read “Earth ‘to be wiped out’ by supernova explosion.” Uh oh, that title sounds rather definite. Immediately, the bullshit sensor in my brain was tripped so I stopped flicking through the embarrassing excuse for a UK newspaper and had a read.

According to the article, some star (that I can’t pronounce) was “set to self-destruct” (as a big hairy supernova), a little over 3,000 light years away. Global chaos will therefore ensue. The ozone layer will be stripped away… and the Earth will be “wiped out.” (I still can’t work out how the Earth will be “wiped out.”)

I’m only picking on the as my skepticism knives were already sharpened after a series of idiotic woo-fueled articles (here, here and here) the website has played host to in recent months, but they weren’t the only news outlet to go batshit crazy with the “WE’RE ALL GONNA DIE” angle.

But who was really to blame for this mess? After all, the media was just the messenger, they must have gotten their lead from somewhere. Ah yes, the scientists… what did those guys really say?

You can find out how I got to the bottom of the science behind the hype in my Discovery News article “Will Earth ‘Be Wiped Out’ by a Supernova?” but cutting to the chase, it turns out that the scientists may have been a little hasty in their attempt to make international headlines.

As my mate Phil Plait mentions in his excellent write up (about my write up) of the T Pyxidis debacle on Bad Astronomy, this isn’t just a simple case of media hype, a lot of the blame should lay with Edward Sion et al. from Villanova University in Philadelphia.

Sure, some of the numbers didn’t add up (mistakes happen), but issuing a press release with a huge wad of inaccurate doom wrapped inside is pretty irresponsible. Have a read for yourself:

An interesting, if a bit scary, speculative sidelight is that if a Type Ia supernova explosion occurs within [that distance] of Earth, then the gamma radiation emitted by the supernova would fry the Earth, dumping as much gamma radiation (~100,000 erg/square centimeter) into our planet [sic], which is equivalent to the gamma ray input of 1000 solar flares simultaneously. –Excerpt from the Villanova press release, “THE LONG OVERDUE RECURRENT NOVA T PYXIDIS: SOON TO BE A TYPE Ia SUPERNOVA?”

“…fry the Earth”? Come on, that’s not even an accurate scientific term about what would happen if we were hit by a surge of gamma-rays. What’s wrong with saying “…the Earth would be at the receiving end of a Death Ray”? If you’re going to do the job of the tabloid press, hyping up your own research before the tabloid press has even read the release, you may as well be accurate.

And speaking of accuracy, my colleague Ray Villard was at the AAS and confirmed that Sion’s numbers were out by a factor of 10. “A supernova would have to be 10 times closer [to Earth] to do the damage described,” Ray said.

Although I was tough on the Telegraph in my Discovery News article (let’s face it, with an inaccurate and inflammatory title like that, they had it coming), in this case I think the main issue lies with Sion and co.

Why over-hype your research to get attention, when the research was interesting enough without declaring doomsday? By me even writing about the subject again, I think I just answered my own question.

But on a plus point, at least everyone knows what T Pyxidis is now…