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Chandra investigates an X-ray mystery from epic kilonova

For the first time, astronomers are studying the aftermath of a kilonova — an epic burst of energy that occurs when two neutron stars collide and merge. NASA has used it Chandra X-ray Observatory to observe the kilonova GW170817, and the observations have raised some unexpected questions.

Neutron stars are some of the densest objects in the universe, and when they collide it is with such force that they create ripples in spacetime, called gravitational waves. These gravitational waves were detected for the first time in 2017, in a signal called GW170817. Even four years later, astronomers were able to see the aftereffects of this event in the X-ray wavelength. “We have entered uncharted territory here in studying the aftermath of a neutron star merger,” said Aprajita Hajela, lead author of the new research, in a statement.

An artist’s conception illustrates the aftermath of a kilonova.
An artist’s conception illustrates the aftermath of a “kilonova,” a powerful event that happens when two neutron stars merge. X-ray: NASA/CXC/Northwestern Univ./A. Hajela et al.; Illustration: NASA/CXC/M.Weiss

There was something strange about the X-ray readings from this event though. When the merger was first detected, Chandra quickly moved to observe the pair but, despite the epic burst of both visible and infrared light from the collision, there were no X-rays observed. But when Chandra looked again, nine days later, it did find X-rays.

Researchers think this might have happened because the collision caused jets of X-rays that shot out at different angles, which is why Chandra didn’t see them at first because they were pointed away from Earth. Over time, the jets slowed down and widened out until they became visible. Another odd finding was that the X-rays had been getting fainter since 2018, but in March 2020 that stopped and the X-rays remained at the same brightness. That makes the researchers think that the jets are not the only source of X-rays from the merger.

“The fact that the X-rays stopped fading quickly was our best evidence yet that something in addition to a jet is being detected in X-rays in this source,” said co-author Raffaella Margutti of the University of California at Berkeley. “A completely different source of X-rays appears to be needed to explain what we’re seeing.”

There are two possible explanations for this. Either debris from the merger had expanded to the point that it has created a shockwave, like a sonic boom. Or it could be that the merger has created a new black hole, and the emissions come from material falling into this black hole. To learn which of these is correct, the researchers will keep looking at both X-rays and radio waves from the source.

“This would either be the first time we’ve seen a kilonova afterglow or the first time we’ve seen material falling onto a black hole after a neutron star merger,” said co-author Joe Bright, also from the University of California at Berkeley. “Either outcome would be extremely exciting.”

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