A Neutron Star Collision: Gamma Rays & Gravitational Waves

Gravitational waves have been on our radar non-stop lately, from LIGO’s fourth reported detection—enhanced by data from Italy’s Virgo project—to this year’s physics Nobel going to three of LIGO’s cofounders. But here we are again and, far from getting old, the news is more exciting than ever: we’ve picked up a new kind of signal, from merging neutron stars rather than black holes. That’s not all, though—while black hole mergers are expected to be difficult or impossible to see, this collision produced electromagnetic waves across a broad portion of the spectrum, allowing multiple telescopes to pick up the signal and giving us our first confirmed glimpse of a binary neutron star system coalescing into a single object.

The LIGO and Virgo collaborations released their findings today in a flurry of publications across several journals, including one in the American Physical Society’s Physical Review Letters. In their report, the teams describe details of the signal they received, as well as some of the highlights of their analysis. While this was the most intense gravitational wave signal yet detected, its “loudness” came from the merging masses’ proximity, not their size. While previously detected gravitational waves seem to have come from black holes, some 30-50 times the mass of our sun, these neutron stars were much closer to a single solar mass. It’s difficult to put definite numbers on their size, because factors like their rotational speeds can influence the dynamics of the inspiral, which in turn affects the shape of the waveform—and it’s by comparing this waveform to predictions from general relativity that we determine their masses. The work puts some bounds on these masses, though, reporting that they’re somewhere between 0.86 and 2.26 times the mass of the sun. While gravitational wave signals from previously detected merger events were extremely short, this one occurred over the course of roughly 100 seconds, indicating that the merging objects are much less massive than previously detected ones.

To catch up our unfamiliar readers, a neutron star is an astronomical object that’s created when the pressure inside a star becomes so great that the protons and electrons inside it are crushed together, essentially joining to form neutrons. This is believed to happen most frequently in the supernova explosions that sometimes mark the end of a giant star’s lifetime.

You might know that the nucleus of an atom, though it only occupies about one ten-thousandth the atom’s actual size, contains the vast majority of its mass; electrons are very light compared to the protons and neutrons that make up the atom’s core. But it’s the electrostatic repulsion of electrons that keeps atoms at their normal size, and the mutual repulsion of protons similarly gives atomic nuclei some of their structure. Once those charges cancel out, neutron stars can become unbelievably dense—packing more mass than our sun into a ball that’s only about ten miles across. It wouldn’t be inaccurate to compare them to giant atomic nuclei, which means that the event we’ve just detected is something akin to nuclear fusion happening on a galactic scale.

It’s long been suspected that binary neutron star mergers like this one are responsible for the short bursts of gamma rays—extremely high-energy photons—that we’ve seen streaming in from space since the 1960s, when we first launched a space-based detector that could pick them up. We’ve also known since the ’70s that an orbiting pair of neutron stars ought to radiate energy as gravitational waves, causing them to spiral in toward one another and eventually merge. It’s only with the recent advent of gravitational wave astronomy, however, that we’ve gained the ability to confirm these theoretical predictions.

Just before the announcement that this year’s Nobel prize in physics would go to the pioneers of gravitational wave astronomy, combined data from LIGO and Virgo’s detectors proved to the scientific world that it’s possible to locate the source of a gravitational wave signal based on the delay between when the signal arrives at various detectors across the globe. Some heralded this as the dawn of a new era in astronomy, and now—only a few short weeks later—it seems as though that claim is being borne out. As soon as this latest signal rolled in, the scientists manning the gravitational wave detectors reached out to 70 telescopes around the world, asking for images all across the electromagnetic spectrum from any telescope that happened to be looking in the direction of the constellation Hydra, where the signal appeared to originate.

The satellite-based Fermi gamma ray telescope, it turned out, had seen something: a brief blast of high-energy photons, apparently emanating from the cloudy, elliptical galaxy NGC 4993. Because gravitational waves travel at or near the speed of light, any signal LIGO picks up is telling us about events that happened in the distant past; the event that led to the first-ever gravitational wave detection happened billions light years away and, thus, billions of years ago, some time around when life first emerged on Earth. By comparison, NGC 4993 is practically right on our doorstep—it looks like these neutron stars merged sometime during the reign of the dinosaurs. Fermi wasn’t the only instrument to pick up an electromagnetic counterpart to the gravitational wave; examination of data from other telescopes revealed a component of the signal in other high-energy regions of the spectrum as well—meaning we now have images from Hubble showing what the merger’s flash would have looked like to the (nearly) naked eye!

One of the most interesting takeaways from this new work is that the light from this event arrived 1.7 seconds after the “chirp” of the gravitational waves. A lead of 1.7 seconds after a journey of several million years isn’t all that much, but it still hints that there’s intriguing physics at work here. At first, it might sound like we’ve finally detected something traveling faster than light speed, but—while that would be groundbreaking—it doesn’t seem likely. Consider that light’s velocity is only the “universal speed limit” in a perfect vacuum—any time photons travel through a medium like air, it’s moving slower than the figure typically quoted as c. Although the paper mentions the possibility that this delay is related to the Shapiro effect, Dr. Amber Stuver of the LIGO collaboration has suggested that “…the change in the speed of light when it passes through the little matter along the way could do that (and that won’t affect gravitational waves).”

The press conference detailing their results is live now—check it out, and keep checking back here for updates as the story continues to unfold!

—Stephen Skolnick