Afterglow from Neutron-Star Merger Continues to Brighten

The afterglow from the faraway neutron-star merger spotted last August has continued to brighten – much to the surprise of astrophysicists studying the aftermath of the huge collision that took place almost138 million light years away and sent gravitational waves rippling through the universe.

New observations from NASA’s orbiting Chandra X-ray Observatory, reported in Astrophysical Journal Letters, specify that the gamma ray burst released by the collision is increasingly complex than scientists originally imagined.

“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,” says McGill University astrophysicist Daryl Haggard, whose research group led the new study. “This one is different; it’s definitely not a simple, plain-Jane narrow jet.”

Cocoon Theory

It could be possible to explain the new data by using more complex models for the remnants of the neutron star merger. One possibility: the merger launched a jet that shock-heated the gaseous debris in the surrounding environment, producing a hot ‘cocoon’ around the jet that has glowed in radio light and X-rays for many months.

The X-ray observations jibe with radio-wave data reported in Dec 2017 by another team of scientists, which discovered that those emissions from the collision also sustained to brighten over time.

Radio telescopes succeeded in monitoring the afterglow throughout the fall, however, X-ray and optical observatories were incapable of watching it for around three months, since that point in the sky was extremely close to the Sun during that period.

“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,” says John Ruan, a postdoctoral researcher at the McGill Space Institute and lead author of the new paper.  “Sure enough, the afterglow turned out to be brighter in the X-ray wavelengths, just as it was in the radio.”

Physics Puzzle

That unforeseen pattern has set off a scramble among astronomers in order to understand what physics is driving the emission. “This neutron-star merger is unlike anything we’ve seen before,” says Melania Nynka, another McGill postdoctoral researcher. “For astrophysicists, it’s a gift that seems to keep on giving.” Nynka also co-authored the new paper, together with astronomers from Northwestern University and the University of Leicester.

The neutron-star merger was initially detected on Aug. 17 by the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO). The discovery was confirmed by the European Virgo detector and some 70 ground- and space-based observatories.

The discovery launched a new era in astronomy. It marked the very first time that scientists succeeded observing a cosmic event with both light waves -- the basis of standard astronomy -- and gravitational waves, the ripples in space-time predicted by Albert Einstein’s general theory of relativity a century ago.  Mergers of neutron stars, taking place in the midst of the densest objects in the universe, are believed to be responsible for developing heavy elements such as platinum, silver, and gold.

The National Sciences and Engineering Research Council of Canada, the Fonds de recherche du Québec – Nature et technologies, the McGill Trottier Chair in Astrophysics and Cosmology, and the Canadian Institute for Advanced Research funded the research.