“Unlike Anything We’ve Seen Before” — Strange Afterglow of a Neutron-Star Merger




The 2017 discovery of a binary neutron star merger opened a new era in astronomy. It marked the first time that scientists have been able to observe a cosmic event with both light waves — the basis of traditional astronomy — and gravitational waves, the ripples in space-time predicted a century ago by Albert Einstein’s general theory of relativity. Mergers of neutron stars, among the densest objects in the universe, are thought to be responsible for showering the Universe with heavy elements such as gold, platinum, and silver.

The Strange Afterglow

Observations from NASA’s orbiting Chandra X-ray Observatory indicated that the gamma-ray burst unleashed by the collision is more complex than scientists initially imagined. The afterglow from the distant neutron star merger, first detected in August 2017, persisted well into 2018 – much to the surprise of astrophysicists.

The massive collision took place about 138 million light years away and sent gravitational waves rippling through the Universe. Previous short gamma-ray bursts have all been detected at much longer distances, typically billions of light years away, which is too far to detect gravitational waves from the inspiral of the progenitor binary neutron star. The relative proximity of the August 2017 event provides astronomers a unique perspective into the afterglow of short gamma-ray bursts. 


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“An Entirely New Level of Knowledge”

“Usually when we see a short gamma-ray burst, the jet emission generated gets bright for a short time as it smashes into the surrounding medium – then fades as the system stops injecting energy into the outflow,” said McGill University astrophysicist Daryl Haggard, whose research group led the 2018 study. “This one is different; it’s definitely not a simple, plain-Jane narrow jet.”

The new discovery “allows us to link this gravitational wave source up to all the rest of astrophysics: stars, galaxies, explosions, massive black holes and, of course, neutron-star mergers,” says Haggard, who led one of many teams of affiliated scientists around the world who examined the source of the latest gravitational-wave signal. “It’s an entirely new level of knowledge.”


Was a Hot Cocoon Created?

The new data could be explained using more complicated models for the remnants of the neutron star merger. One possibility: the merger launched a jet that shock-heated the surrounding gaseous debris, creating a hot ‘cocoon’ around the jet that has glowed in X-rays and radio light for many months. 

The X-ray observations jibed with the radio-wave data reported by another team of scientists, which found that those emissions from the collision also continued to brighten over time.

While radio telescopes were able to monitor the afterglow throughout the fall of 2018, X-ray and optical observatories were unable to watch it for around three months, because that point in the sky was too close to the Sun during that period.

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Discovery Opened a New Era in Astronomy

“When the source emerged from that blind spot in the sky in early December, our Chandra team jumped at the chance to see what was going on,” said John Ruan, a postdoctoral researcher at the McGill Space Institute and lead author of the paper. “Sure enough, the afterglow turned out to be brighter in the X-ray wavelengths, just as it was in the radio.”

That unexpected pattern has set off a scramble among astronomers to understand what physics was driving the emission. “This neutron-star merger is unlike anything we’ve seen before,” said Melania Nynka, another McGill researcher currently at MIT’s Kavli Institute. “For astrophysicists, it’s a gift that seems to keep on giving.” Nynka co-authored the new paper, along with astronomers from Northwestern University and the University of Leicester.

The binary neutron star merger was first detected by the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO). The European Virgo detector and some 70 ground- and space-based observatories helped confirm the discovery.

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via McGill University




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