Clemson astrophysicist works to unravel mystery of what happens after neutron stars collide

Clemson University astrophysicist Jonathan Zrake is a detective of sorts.

But Zrake is not a part of a team of investigators reconstructing a crime scene to identify a suspect. Instead, he is among international researchers working to determine what caused a new X-ray glow three and a half years after two neutron stars merged 130 million light-years away from the Earth, creating a new black hole and a bright “kilonova” explosion.

GW170817 is the first, and so far only, neutron star merger observed in both gravitational waves — minute but recently detectable vibrations of space-time — and electromagnetic radiation, or light. X-rays are a type of light.

“The event was a cataclysmic thing that happened in seconds. But the dynamics that took place in that very short window of time are still playing out,” Zrake said.

Astrophysicists have two leading explanations for the new X-ray source: either a “kilonova afterglow” — a plasma shock wave that could have been re-strengthened by outflowing gas — or matter heating up and shining as it falls back into the black hole left behind after the neutron stars merged. In either case, it would be the first time such a phenomenon has been observed.

This will keep theorists busy for some time. We may argue about the most viable explanation, but no matter what, the enhancement of the X-ray emission is a big discovery.

jonathan zrake, assistant professor in the Department of Physics and Astronomy

On Aug. 17, 2017, astronomers detected gravitational waves from the merger of two neutron stars — the very dense cores of collapsed giant stars — using the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and Virgo, a detector in Italy. Scientists detected visible and infrared light several hours later. 

“It highlights the importance of multi-messenger astrophysics. We would have never detected this event at all if the gravitational wave observatories hadn’t seen it first. We saw the light, but only because the gravitational wave detectors told us where and when to look,” Zrake said.

NASA’s Chandra X-ray Observatory detected X-rays from GW170817 nine days later. Researchers believe the merger produced a narrow jet of high-energy particles that wasn’t pointed directly towards Earth.

The X-ray emission caused by the jet had been steadily getting fainter since early 2018. But researchers noticed that the decline stopped in March 2020 and the X-rays steadied.

One explanation is that the expanding debris from the merger generated a shock, like a sonic boom from a supersonic plane. The emission produced by material heated by the shock is called a “kilonova afterglow.”

An alternative explanation is that some of the material launched by the merger fell back into the black hole left behind after the neutrons stars merged, producing radiation because it got very hot. That phenomenon is called fallback accretion.

Scientists are continuing to monitor GW170817 in X-rays and radio waves. If it is a kilonova afterglow, it should lead to increasingly bright radio emissions, said Zrake, whose research group at Clemson studies the dynamics of black holes and astrophysical explosions. If it is fallback accretion, the X-ray emission should stay steady or steadily decline. Either way, scientists are learning something new about what happened.

“It’s inevitable that something like this will happen again. It’s just a question of when. When it does, this team of astronomers and astrophysicists, including scientists at Clemson, will be poised to investigate it,” Zrake said.

The Astrophysical Journal Letters published the findings.

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