A collision that occurred between two neutron stars in a galaxy 138 million light years from Earth was observed in August of 2017, generating both gravitational waves that were detected by the LIGO and Virgo gravity-wave observatories, and a gamma-ray burst that was recorded by the Fermi and INTEGRAL satellites. This is the first time that both gravity and electromagnetic waves have been detected from an astronomical event, as all previous detections of gravity waves have come from collisions between black holes. But in the observations made by the Chandra X-Ray observatory since the event, the afterglow from the initially-brilliant gamma ray burst hasn’t faded in the way scientists would expect: it has instead continued to brighten.

"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," explains astrophysicist Daryl Haggard, with Canada’s McGill University. "This one is different; it’s definitely not a simple, plain-Jane narrow jet."

In the months after the event, the leftover glow from the neutron star merger, dubbed a "kilonova" due to the energy released being over one thousand times greater than that of a typical stellar nova, continued to brighten in the X-ray and radio wave wavelengths. Astronomers were unable to see the source for three months, as the elliptical galaxy that it originated from, NGC 4993, was obscured behind the Sun. But when it finally reemerged, the signal had gained even more strength.

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

The leading theory behind this unexpected brightening is that the merger of the two neutron stars produced an energetic jet that illuminated the gaseous debris surrounding the system, creating a heated "cocoon" that continued to increase in its radiance. Regardless, this mysterious event has provided a tantalizing puzzle for physicists.

"This neutron-star merger is unlike anything we’ve seen before," says Melania Nynka, an X-ray astronomer and postdoctoral researcher at McGill. “For astrophysicists, it’s a gift that seems to keep on giving."