In August 2016 astronomers spotted an intensely bright explosion in the sky, which faded after about 10 days, but it’s only now that scientists have identified this event as a collision between two neutron stars. Known as a kilonova, these are events that may be the source of all the gold and platinum in the universe.
The event, known as GRB160821B, was originally assumed to be a gamma ray burst, which are extremely energetic explosions that are fairly common throughout the cosmos. The initial flash can last anywhere from milliseconds to hours, and that’s followed by an afterglow in longer wavelengths that can last weeks.
“The 2016 event was very exciting at first,” says Eleonora Troja, an author of the study describing the event. “It was nearby and visible with every major telescope, including NASA’s Hubble Space Telescope. But it didn’t match our predictions – we expected to see the infrared emission become brighter and brighter over several weeks. Ten days after the event, barely any signal remained. We were all so disappointed.”
The event would turn out to be far more exciting than it first seemed – but astronomers wouldn’t realize that for another year. In August 2017, astronomers around the world directly witnessed a collision between two neutron stars for the first time. This groundbreaking moment was detected by dozens of observatories as gravitational waves, X-rays, gamma rays, light and radio waves.
With so much new data to work through, the event taught astronomers a lot about how kilonovae look from Earth. And armed with that new knowledge, researchers from the University of Maryland began looking over old observations in a new light. And sure enough, the 2016 event suddenly looked very familiar.
“We looked at our old data with new eyes and realized we had indeed caught a kilonova in 2016,” says Troja. “It was a nearly perfect match. The infrared data for both events have similar luminosities and exactly the same time scale.”
This infrared signature is given off by the huge amounts of heavy metals, such as gold and platinum, forged in the explosion.
Interestingly, the two events work well together, patching up holes in each other’s observations. The 2016 kilonova was tracked right from the start, within minutes of detection, but didn’t have as many eyes on it as the 2017 event. For the latter, observations missed the first 12 hours.
One thing the earlier event taught them about was the aftermath of the explosion. Exactly what kind of object is created is still unknown, but the 2016 kilonova helped provide new clues.
“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,” says Geoffrey Ryan, co-author of the study. “This is interesting, because theory suggests that a magnetar should slow or even stop the production of heavy metals, which is the ultimate source of a kilonova’s infrared light signature. Our analysis suggests that heavy metals are somehow able to escape the quenching influence of the remnant object.”
With our improved understanding of what a kilonova looks like, more observations could help unlock more of these secrets.
The research was published in the Monthly Notices of the Royal Astronomical Society.
Source: University of Maryland