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Smothered jet may explain weird light from neutron star crash

The glow from the stellar collision has lingered much longer than expected

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1:06pm, December 20, 2017
neutron star illustration

NO WAY OUT  Astronomers thought a merged neutron star’s odd light was explained by a jet facing away from Earth (left in this illustration). But a new study says the jet may have been smothered in a cocoon of debris from the merger, causing the cocoon to glow (right).   

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The neutron star collision heard and seen around the world has failed to fade. That lingering glow could mean that a jet of bright matter created in the crash has diffused into a glowing, billowy cocoon that surrounds the merged star, researchers report online December 20 in Nature.

Gravitational waves from the collision between two ultradense stellar corpses was picked up in August by the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, and its sister experiment in Italy, Advanced Virgo (SN: 11/11/17, p. 6). Using telescopes on the ground and in space, physicists raced to conduct follow-up observations, and found that the collision released light across the electromagnetic spectrum.

Right away, the event looked unusual, says astrophysicist Kunal Mooley, who conducted the research while at the University of Oxford. Physicists think that a jet of fast-moving, bright material blasts out of the center of neutron star collisions. If that jet is aimed directly at Earth, telescopes can see it as an ephemeral flash of light called a short gamma-ray burst, or GRB.

Exceeding expectations

The radio afterglow of the neutron star collision (GW170817) in galaxy NGC 4993 was spotted with the Very Large Array radio observatory in New Mexico on September 2, 16 days after the gravitational waves from the collision were detected. The glow continued to brighten for at least 93 days, until November 18, this animation shows.  

But the gamma-ray signals produced by the August collision were 10,000 times less bright than those seen in other detected short gamma-ray bursts. Even stranger, X-rays and radio waves from the event didn’t appear until about 16 days after the collision. Most short gamma-ray bursts are visible in X-rays and radio waves right away and fade over time.

Astronomers thought those oddities meant the jet was facing slightly away from Earth and expected the light to fade quickly. But Mooley and colleagues continued tracking the glow with three radio telescope arrays on three continents for more than 100 days after the collision. Radio wave emissions continued to brighten for at least 93 days, and are still visible now, the team found. (X-rays were temporarily blocked when the neutron star moved behind the sun from Earth’s perspective.)

“This thing continues to rise, instead of fading into oblivion as we expected,” says astrophysicist Wen-fai Fong of Northwestern University in Evanston, Ill., who was not involved in the new study.

The finding may mean that astronomers are seeing a new kind of gamma-ray burst. Mooley and colleagues suggest that the rise in radio wave emissions could be explained if the jet slammed into a shell of neutron-rich material kicked out in the neutron star crash, transferring most of its energy to that debris and smothering the jet. That extra energy could create a glowing cocoon that keeps radiating far longer than the original blast.

The new result is “really challenging our understanding of what physics is happening from this merger,” Fong says. But, she adds, “the jury is still out on whether this is the same as the short GRBs we’ve seen over the past decade, or whether it’s something completely different,” such as a luminescent cocoon. She and her colleagues also took radio wave observations of the merged stars in the first 100 days after the collision. The team is preparing a paper with a different interpretation that includes a jet emerging from the wreckage later, she says.

Other explanations for the lingering light are possible, Mooley acknowledges. Future detections “will give us an opportunity to really study … what fraction of neutron star mergers give rise to [short] GRBs and what fraction give rise to other phenomena and explosions that we haven’t seen so far in our universe,” he says.

MUSHROOM CLOUD A computer simulation shows how matter from a neutron star collision detected in August could have billowed after the crash. That cocoon of radiation may explain why light from the crash didn't fade quickly, researchers suggest. Ehud Nakar/Tel Aviv, Ore Gottlieb/Tel Aviv, Leo Singer/NASA, Mansi Kasliwal/Caltech and the GROWTH collaboration

Citations

K.P. Mooley et al. A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817. Nature. Published online December 20, 2017. doi: 10.1038/nature25452.

Further Reading

E. Conover. Neutron star collision showers the universe with a wealth of discoveries. Science News. Vol. 192, November 11, 2017, p. 6.

E. Conover. Collision illuminates the mysterious makeup of neutron stars. Science News. Vol. 192, December 23, 2017, p. 7.

E. Conover. This year’s neutron star collision unlocks cosmic mysteries. Science News. Vol. 192, December 23, 2017, p. 19.

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