More than a thousand days have passed since the historic Kilonova observation, and yet the region continues to emit x-rays long after models predicted they should have faded. What's happening?
August 2017 was a pretty significant month in astronomy. The gravitational wave detectors LIGO and VIRGO discovered the unique signature of two colliding neutron stars and named the event GW170817. Moments later, the orbiting Fermi LAT satellite experienced a huge burst of gamma rays from the same region in the sky. Within hours, telescopes around the world were focusing on the source, watching a cosmic catastrophe unfold across the electromagnetic spectrum.
It was the first time that the same event (this event was called "Kilonova" because it is about a thousand times brighter than a nova) was observed in both traditional and gravitational astronomy. The combined observations confirmed what astronomers had long suspected: when neutron stars merge, they go out with a bang.
In addition to the original flash of the fusion itself, the area around the explosion continued to glow for some time, powered by a beam released by the surrounding gas and dust during the collision.
X-rays continue to stream from the region around the Kilonova event, recorded in 2017. Photo credit: E. Troy
Models of such collisions predicted that the energetic afterglow should fade after a day or two. Once the collision is over, there should be nothing left to propel emissions over the long term.
But here we are three years later, and the region around GW170817 continues to release X-rays, as reported recently in the Royal Astronomical Society's monthly bulletin.
What is causing all of this copious x-ray emission? The Kilonova itself has long since disappeared, but something still has to be an energy source. Perhaps there is a shock wave traveling through a denser area of gas. Maybe there is some residue left from the collision. Maybe it's something else entirely.
Be that as it may, the new observations are forcing astronomers to update their models of how Kilonova works and uncover the true richness of cosmic explosions.