It's a cosmic collision that has astronomers rethinking one of the universe's most colossal events: the collision of massive stars. In a new paper published in the journal Monthly Notices of the Royal Astronomical Society, astronomers reveal the finding of a kilonova produced by the collision of two massive stellar objects called neutron stars. The collision is roughly 1,000 times brighter than the death of a massive star called a supernova. And they say it produced several hundred planets' worth of gold and platinum.
But astronomers almost missed it.
In 2016, telescopes detected an extremely energetic but brief flash called a gamma-ray burst. Astronomers were quick to determine the source, which they concluded was the collision of two extremely dense stars called neutron stars. But what they expected to see from a kilonova — an increased brightening in the infrared spectrum over several days — failed to materialize. Disappointed, they moved on. Fast forward to 2017, when the Laser Interferometer Gravitational-wave Observatory (LIGO) detected a gravitational wave, a ripple through space-time, followed by optical observations of a pair of neutron stars colliding.
That gave astronomer Eleonora Troja pause. "It was like, wait a minute. Maybe we were wrong. Maybe what we thought a kilonova looked like is not what a kilonova looked like," said Troja, lead author of the study and an astronomer at both the University of Maryland and NASA's Goddard Space Flight Centre. "Or maybe there were different types of kilonovae." She decided to go back to the 2016 data. And what she found thrilled her. "I went back and collected all the data that we had on the event of 2016," Troja said. "I simply took what the LIGO event looked like in the infrared, I matched the two data, and it was like, 'Oh yeah, it fits. That's it.'"
Remnant could be magnetar
Troja said that she's encouraged by the finding, as now it makes two kilonova events. When neutron stars collide, just like a supernova, something else is created. With a supernova, sometimes a black hole is the result. But with neutron stars, it's not exactly clear. "The remnant could be a highly magnetized, hypermassive neutron star known as a magnetar, which survived the collision and then collapsed into a black hole," said Geoffrey Ryan, a Joint Space-Science Institute prize postdoctoral fellow in the University of Maryland's Department of Astronomy and a co-author of the research paper. If it is, this presents another puzzle since the theory is that a magnetar should slow down or even stop producing heavy metals. But, based on the brightening astronomers witnessed, it suggests that somehow whatever was formed is still able to produce them.
Troja said that she's anxious to discover more of these short-lived events astronomers refer to as "transients." "We've been studying supernovas for 50 years or more, and we're still learning. We've been studying kilonova for two years," said Troja. "I'd say this field of study is in its infancy. And I think it will develop and become more complex with more observations. "It gives us the opportunity to understand the chemistry of the universe," Troja said.