Neutron-star merger creates new mystery

MILWAUKEE _ The neutron-star merger announced in October solved one mystery – where gold comes from – but created another, an international team reports today in the journal Nature Astronomy.

As its name suggests, GW170817 was detected in on Aug. 17, 2017, by the Laser Interferometer Gravitational-Wave Observatory (LIGO), shortly before it was shut down for upgrades. The signature of the gravitational waves and the concurrent observation of a gamma ray burst strongly suggested that the event would be visible in the electromagnetic spectrum.

The location was somewhat fortuitously pinpointed to the galaxy NGC 4993, 120 million light years away, allowing worldwide astronomical observations to begin within hours of the first detection. The collision’s glowing wreckage generated radio waves, detected by several international teams, including one led by David Kaplan, an associate professor at the University of Wisconsin-Milwaukee, and Tara Murphy, an associate professor at the University of Sydney and ARC Centre for All-Sky Astrophysics.

“We expected to find evidence that merging neutron stars create something we’ve never found the cause of – short gamma-ray bursts,” Murphy said.

“Theorists made a good argument that gamma ray bursts were caused by colliding neutron stars,” said Kaplan.

But there was a puzzle:  The models required a near light-speed, or “ultrarelativistic,” jet of radio-emitting particles to make a short gamma-ray burst. Such a jet was not observed.

“We thought we knew how short gamma-ray bursts worked, and seeing that would have been pretty cool,” Kaplan said. “What we saw is even cooler.”

What the astronomers saw instead of a jet suggested a slower, broader outflow of radio-emitting material – a “cocoon.” This could be matter thrown out by the explosion that’s been energized by a jet hidden inside it.

“The cocoon scenario can explain the radio light curve of GW170817 as well as the gamma-rays and X-rays. It’s the one most consistent with the data,” Murphy said.

“The jet might still break out of the cocoon, which would preserve that aspect of the model,” she added. “So we’ll be watching for that.”

Astronomers from University of Wisconsin-Milwaukee, the University of Sydney, Caltech, CSIRO and other institutions around the world have monitored the radio source for months with three radio telescopes – CSIRO’s Australia Telescope Compact Array radio telescope, the Very Large Array (VLA) in the USA and the Giant Metrewave Radio Telescope (GMRT) in India.

“Since we haven’t seen a jet, from the radio data we can’t say there’s a definite link between merging neutron star mergers and short gamma-ray bursts,” said team member Gregg Hallinan, assistant professor of astronomy at Caltech. “The jury is still out.”

While they continue to monitor the source, the astronomers are puzzling over the “cocoon” they believe they’ve found.

“This event continues to deliver the unexpected, and we’ll keep on it as long as possible,” Kaplan said. “As the surprises mount, it makes us wonder exactly what we will see when LIGO turns back on next year.”