On March 22, 2015, NASA's Chandra X-ray observatory recorded a blip on its data. Not far from the southern constellation of Fornax, something lit up, and then slowly disappeared.
Thanks to a new technique, we now know that blip was two neutron stars colliding, 6.6 billion light years from Earth.
We also know that when neutron stars collide, they produce two powerful jets, firing in opposite directions, firing bursts of gamma rays – but if those jets are not pointing in our direction we will not be able to detect them.
But in 2013, astronomer Bing Zhang of the University of Nevada predicted that a fusion of neutron stars could produce a powerful X-ray if the result of the fusion were a highly magnetized and fast-rotating neutron star – a magnetar.
Then, in August 2017, gravitational wave astronomy gave the world a marvel. For the first time, we have seen colliding neutron stars in real time, not just through gravitational wave detectors, but through optical, infrared, ultraviolet and X-ray instruments around the world.
So a research team analyzed Chandra's archived data for events matching the new information in GW170817 – and found an event that also matched Zhang's predictions.
"We found a completely new way of identifying a fusion of neutron stars," said astronomer Yongquan Xue of China's University of Science and Technology. "The behavior of this x-ray source matches what one of our team members predicted for those events."
They called the XT2 event and located it suddenly when it appeared in the data, and then slowly disappeared over about seven hours. They carefully studied how the X-ray emission changed over time and compared with Zhang's predictions.
They also considered other possibilities as if the event could have been caused by the collapse of the nucleus of a dying star. The position of the event at the periphery of the host galaxy is more consistent with neutron stars expelled from the galactic center, and the low star formation rate means that the event was less likely to be caused by a massive young star supernova.
Analyzing specifically the XT2, the team found that the emission was consistent with a magnetar spinning hundreds of times per second and with a magnetic field around a quadrillion times stronger than Earth's.
The X-ray emission of the magnetar remained constant for about 30 minutes, after which it faded by a factor of over 300 during the next 6.5 hours, eventually disappearing. The team believes it was losing power through an X-ray wind that gradually slowed down.
This means that the two neutron stars probably produced a larger neutron star, not a black hole. Astronomers think that at least three times the mass of the Sun is needed to produce a black hole; anything less massive becomes a neutron star. So this places restrictions on the size of the neutron stars involved in the collision.
But it also tells us something about the interior of the neutron stars, which is incredibly difficult to study because of the insane density.
"We can not throw neutron stars together in a lab to see what happens, so we have to wait until the Universe does it for us," Zhang said. "If two neutron stars can collide and a heavy neutron star survives, it tells us that its structure is relatively rigid and resilient."
The team is now working hard to analyze other Chandra data to, among other things, see if they can begin to get some concrete statistics about how often events like these can occur.
"As with this source, the data contained in archives may contain some unexpected treasures," said Xuechen Cheng of the China University of Science and Technology.
The research was published in Nature.