The finding suggests that LIGO’s neutron-star merger was a typical gamma-ray burst after all.
Millions of years ago two neutron stars, the city-size cinders left behind when massive stars explode, smashed together at a significant fraction of the speed of light. The event was so violent that it shook the universe — producing perturbations in the fabric of space-time known as gravitational waves. The merger also shot off fireworks, emanating electromagnetic radiation across the cosmos.
Those two signals finally washed across Earth in August 2017. That’s when LIGO in the United States and Virgo in Italy detected gravitational waves, and some 70 observatories spotted light streaming from the same region in space. But perhaps the most exciting find was a burst of gamma rays — the most energetic explosions in the universe whose origins remain unknown.
The strong correlation was smoking-gun evidence that gamma-ray bursts are caused by neutron star collisions, says Jonathan Zrake (Columbia University). Needless to say, the discovery took the astronomical community by storm.
But a Nature study published last December called the finding into question. Kunal Mooley (California Institute of Technology) and his colleagues argued that the fast-moving jet of high-energy radiation, which funnels gamma rays toward Earth, failed to punch through the shell of material kicked out in the neutron-star smashup, meaning it was anything but a typical gamma-ray burst.
Mooley’s team continued to observe the crime scene over the past year and more recent findings suggest just the opposite: The jet did break through the billowing debris. The latest observation, also published in Nature, finally pins down the decades-old mystery behind some of the brightest explosions in the universe.
Did the Jet Choke?
Although gamma-ray bursts were first observed in the late 1960s, their source long remained a mystery. Only in the last decade or so have astronomers begun to suspect that they arise when a pair of neutron stars collide and blast a fast-moving jet toward Earth, “like a cannon that's firing off high-energy photons in one direction,” Zrake says.
Viewed face-on, these events can be extremely bright. But viewed off-axis, without relativistic effects enhancing their emission, they can appear extremely dim — nearby aliens might miss the event entirely if they weren’t looking down the barrel of the gamma-ray gun, so to speak.
So when astronomers realized that the gamma-ray signals produced by the neutron-star smashup were 10,000 times dimmer than those seen in other short gamma-ray bursts, they assumed that the jet was facing slightly away from Earth but not so far away that it would be invisible.
Still, astronomers expected the jet to light up as it slammed into the gas and dust around the merger, which would cause it to slow down and suddenly start emitting light in all directions. When Mooley and his colleagues saw no such thing and instead witnessed a slow and steady brightening, they determined that the jet had most likely failed to burrow its way through that billowing cocoon of neutron-rich material.
The objections the team raised were fair, says Brian Metzger (Columbia University), who was not involved in either study. But it turns out you should never underestimate the power of a jet.
A Mighty Punch
Mooley’s team continued to observe the aftermath of the merger using several radio telescopes across North America, including the Very Long Baseline Array (VLBA), the Karl G. Jansky Very Large Array (VLA) and the Robert C. Byrd Green Bank Telescope (GBT). By combining the data, the team was able to peer at the region with such high-resolution it was like using a continent-wide telescope, Mooley says.
That allowed them to see a dramatic difference between two images, one shot 75 days after the merger and one 230 days after. Within that timeframe, a compact radio source jumped — so much that the team determined the only explanation was that the radio source was actually the edge of a jet, which had shocked the interstellar medium. There was no question — a jet had punched through the debris.
Such a sighting meant that the neutron-star crash produced a typical short gamma-ray burst, after all. “The combined effort of the community on this event has really strongly increased our confidence on that connection — and it is a Holy Grail,” Metzger says, given that astronomers have long-searched for the origins behind these powerful explosions.
With that connection firmly in place, astronomers can start to better understand the details behind these systems, like their geometry. Mooley and his colleagues, for example, argue that this jet is very narrow, at most 5 degrees wide, and was pointed only 20 degrees away from the Earth’s direction.
That means that very few alien observers would have imaged a true short gamma-ray burst from the event. If other mergers host the same geometry, it could mean that the number of short gamma-ray bursts detected on Earth represent only a small fraction of neutron star mergers.
That information is crucial if we want to understand our own origins, because scientists now think these collisions (as opposed to supernova explosions) produce several important elements, such as gold, platinum and uranium. It’s still unclear if such mergers can account for all of these elements, or if another mechanism also comes into play. Only a better understanding of how often these events occur will help astronomers answer that question.
Astronomers will likely have to answer further questions with the help of future neutron-star mergers, as this crash site is fading fast. The final radio observations are likely occurring now. “It’s a bit of a loss,” Zrake says. “It’s kind of like saying goodbye to a very dear friend.”
K. P. Mooley et al. “Superluminal motion of a relativistic jet in the neutron-star merger GW170817.” Nature, 6 September 2018.