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Starts With A Bang

Merging Neutron Stars Made An Unstoppable Jet, And It Moves At Nearly The Speed Of Light

In 2017, we saw gravitational waves a neutron star merger for the first and only time. And it keeps getting more interesting.

On August 17, 2017, a cosmic signal arrived at Earth that would forever change how we viewed the Universe. Over 100 million years prior, two neutron stars bound together in the distant galaxy NGC 4993 finished inspiraling and merged together, creating a stupendous cosmic explosion when they did. The event is now known as a kilonova, and is thought to be responsible for the creation of the heaviest elements present throughout the Universe.

The inspiral and merger created two signals that we were able to detect practically simultaneously: gravitational waves, detectable with LIGO and Virgo, and electromagnetic radiation, or light, across the full suite of wavelengths we’re able to observe. But there’s something else emitted too: matter. Today, in a new paper published in Science, scientists determined that an enormous jet was produced, and it’s still moving at nearly the speed of light.

Artist’s illustration of two merging neutron stars. The rippling spacetime grid represents gravitational waves emitted from the collision, while the narrow beams are the jets of gamma rays that shoot out just seconds after the gravitational waves (detected as a gamma-ray burst by astronomers). The jet seen by astronomers must be distinct from this one. (NSF / LIGO / SONOMA STATE UNIVERSITY / A. SIMONNET)

It’s not a surprise that an event like this would produce something so energetic. Neutron stars themselves are some of the most extreme objects you can imagine. Imagine taking an object as massive as the Sun or even greater, and compressing it down into a ball about the size of a major city like Chicago. It would be like one enormous atomic nucleus, where the inner 90% of it is simply a ball of solid neutrons, hence the name: neutron star.

On their own, neutron stars can spin so fast — up to about two-thirds the speed of light — that they create the largest known magnetic fields in the Universe: hundreds of millions of times as strong as any magnet on Earth, and a quadrillion times stronger than Earth’s magnetic field. As far as we know, if you were to make a neutron star any denser, it would collapse down into a black hole.

A neutron star, despite being mostly made of neutral particles, produces the strongest magnetic fields in the Universe, a quadrillion times stronger than the fields at the surface of Earth. When neutron stars merge, they should produce both gravitational waves and also electromagnetic signatures, and when they cross a threshold of about 2.5 to 3 solar masses (depending on spin), they can become black holes in under a second. (NASA / CASEY REED — PENN STATE UNIVERSITY)

What we observed in 2017 was even more spectacular than a neutron star on its own: we observed the inspiral and merger of two of these objects. Before the merger took place, we know that two neutron stars, each a little more massive than our Sun, were locked in a binary orbit. As they moved about their mutual center of mass, they emitted gravitational waves, radiating energy away as their orbits became tighter and faster.

The inspiral and merger of two neutron stars, as illustrated here, produced a very specific gravitational wave signal. Additionally, the moment and aftermath of the merger also produced electromagnetic radiation that’s unique and identifiable as belonging to such a cataclysm. (NASA/CXC/GSFC/T.STROHMAYER)

In the final instants, this radiation increased in both amplitude and frequency, and then they reached the most crucial moment of all: their surfaces touched. In a tiny fraction of a second, their densities increased past a critical threshold, and a runaway nuclear reaction took place where they contacted one another. All at once, an event known as a kilonova occurred.

Less than two seconds after the gravitational waves reached their strongest, a spike was seen in the electromagnetic spectrum: by NASA’s Fermi gamma-ray observatory. This event, known as a gamma-ray burst, was the first one ever correlated with a neutron star-neutron star merger.

The galaxy NGC 4993, located 130 million light years away, had been imaged many times before. But just after the August 17, 2017 detection of gravitational waves, a new transient source of light was seen: the optical counterpart of a neutron star-neutron star merger. (P.K. BLANCHARD / E. BERGER / PAN-STARRS / DECAM)

The burst may have been short-lived, both in gravitational waves and in gamma-rays, but the signals we received were spectacularly informative. Almost immediately, we learned:

  • what the masses (about 1.3 Suns) and distances (about 130 million light-years) of the neutron stars were,
  • what they became after the merger (a rapidly spinning neutron star that collapsed to a black hole in less than a second),
  • how much of the mass became a black hole (about 95%),
  • and what happened to the rest of the mass (it became the heaviest elements in the periodic table, including gold, platinum, uranium and plutonium).
When two neutron stars merge, as simulated here, they should create gamma-ray burst jets, as well as other electromagnetic phenomena that, if close enough to Earth, might be visible with some of our greatest observatories. (NASA / ALBERT EINSTEIN INSTITUTE / ZUSE INSTITUTE BERLIN / M. KOPPITZ AND L. REZZOLLA)

But we weren’t done yet. There was still the afterglow, which became visible to telescopes of all different wavelengths all across the world. X-ray, ultraviolet, optical, infrared and radio telescopes all viewed this first-of-its-kind event, monitoring it continuously for weeks on end. The afterglow, as we went to longer and longer wavelengths, brightened as time went on, then faded in most of the frequencies where we could look.

We were able to quantify the production of the different elements. For example, about 10⁴⁶ atoms of gold were created, or ten quadrillion times as much as we’ve mined in all of human history. We learned that the two neutron stars had their origin some 11+ billion years ago, and were inspiraling ever since, right up until the moment they merged. We learned that the majority of the heaviest elements in the Universe are made in neutron star collisions like this one.

Two merging neutron stars, as illustrated here, do spiral in and emit gravitational waves, but create a much lower-amplitude signal than black holes. Hence, they can only be seen if they’re very close by, and only over very long integration times. The ejecta, thrown off from the outer layers of the merger, remained a rich source of electromagnetic signal for many months. (DANA BERRY / SKYWORKS DIGITAL, INC.)

But we still weren’t done. Even though the signals were fading all across the electromagnetic spectrum, there was still more science to be done. The majority of the light was coming from radioactive decays of the material that was injected into the interstellar medium surrounding the collision point, and — as you’d expect from anything with a half-life — the majority of the decays occurred early on, and dropped off rapidly.

But then, weeks after the collision, there was a re-emergence of both X-rays and radio waves, and this enhanced new signal lasted for months. It was initially theorized that there was material ejected from the collision, and it was smashing into gas that already existed in the interstellar medium. That interaction provided an energy injection, the line of thought went, and that was responsible for the re-emergence of a glow that was previously fading away.

During an inspiral and merger of two neutron stars, a tremendous amount of energy should be released, along with heavy elements, gravitational waves, and an electromagnetic signal, as illustrated here. But what was a great surprise was a second, later burst of two relativistic jets that emerged from the aftermath of the merger. (NASA / JPL)

In the best instances of science, though, we don’t simply put forth a likely explanation and consider the case closed. We look for follow-up information to test our ideas out, and determine whether they hold water or not. As powerful and advanced as our best theories may be, we absolutely have to confront them with experimental or observational data, or we aren’t truly doing science at all.

The most impressive part about the new research that was just published is that it contains a fantastic suite of data. Using an array of 32 individual radio telescopes, spread over 5 continents and making simultaneous observations of the same objects, scientists were able to observe the radio afterglow as never before. By implementing the technique of very long-baseline interferometry (VLBI) with a bright source like this, they achieved unprecedented resolution.

An array of 32 radio telescopes across five separate continents were used to directly image the aftermath of the merging neutron stars in NGC 4993, enabling astronomers to resolve the structured jets emerging from the interaction point, even though they were less than a light-year across. (PAUL BOVEN)

Resolution is what you need if you want to determine the shape or configuration of a distant source in the Universe. Typically, you get better resolution by building a bigger telescope, as the number of wavelengths of light that fit across it determine the angular size of what you can resolve.

But using the VLBI technique, you can do even better if your source is bright enough. Sure, you’ll only get the light-gathering power of the size of your individual dishes, but you can get the resolution of the distance between the various telescopes. This is the technique that the Event Horizon Telescope is using to construct their first image of a black hole’s event horizon, and this is the technique that enabled astronomers to determine the shape of what resulted after this neutron star-neutron star merger.

Artist’s impression of a jet that breaks out of the material ejected by the neutron stars merger. The jet is produced by the black hole, surrounded by a hot disc, which was formed after the merger. (O.S. SALAFIA, G. GHIRLANDA, NASA/CXC/GSFC/B. WILLIAMS ET AL.)

Led by Giancarlo Ghirlanda, a whopping 207 days of data was combined, allowing astronomers to see what was created over time.

The result was spectacular: the merger produced a structured jet of material, that sped away from the collision point in two anti-parallel lines. While many scientists expected that there would be some sort of cocoon-shape, or something constraining any jets that were produced, the data indicated otherwise. Instead, this structured jet punched through all the material ejected in the merger and continued to rapidly escape into interstellar space at nearly the speed of light. It was as though nothing at all could slow it down.

The second-largest black hole as seen from Earth, the one at the center of the galaxy M87, is around 1000 times larger than the Milky Way’s black hole, but is over 2000 times farther away. The relativistic jet emanating from its central core is one of the largest, most collimated ones ever observed. (ESA/HUBBLE AND NASA)

How can you make a jet like this? We’ve only ever seen them from one other source: from black holes that are feeding on matter. That must be the clue that solves the puzzle! It isn’t that the merger itself created a jet, but that the completed merger produced a black hole, and this spinning black hole accelerated the matter around it, producing the jets that we saw afterwards. It explains why there was a dimming followed by a second round of brightening, and it explains the collimated structure and the fantastically large energies and speeds. Without a central black hole, there’s no known way to do it.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

This is, perhaps, the long-awaited proof that these merging neutron stars, observed in 2017, must have produced a black hole. Based on our current understanding of the Universe, we could not be more certain.

In the final moments of merging, two neutron stars don’t merely emit gravitational waves, but a catastrophic explosion that echoes across the electromagnetic spectrum. Simultaneously, it generates a slew of heavy elements towards the very high end of the periodic table. In the aftermath of this merger, they must have settled down to form a black hole, which later produced collimated, relativistic jets that broke through the surrounding matter. (UNIVERSITY OF WARWICK / MARK GARLICK)

In science, sometimes the best results are the ones you weren’t expecting. We may have anticipated that merging neutron stars would create the heaviest elements of all, but no one saw a structured jet emerging from a black hole afterwards as something that should occur. Yet here we are, reaping the gifts of the Universe. It’s a reminder from the cosmos to us: the day we stop our scientific inquiries, we stop uncovering the mysteries that underlie our existence.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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