On March 22, 2015, NASA's Chandra X-ray observatory recorded an error in its data. Not far from the southern constellation of Fornax, something lit up and then faded away slowly.
Thanks to a new technique, we now know that the blip were two colliding neutron stars, 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 do not point 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 glow if the result of the fusion was a highly magnetized fast-spinning neutron star, a magnetar.
Then, in August of 2017, the astronomy of gravitational waves gave the world a wonder. For the first time, we saw colliding neutron stars in real time, not only through gravitational wave detectors, but through optical, infrared, ultraviolet and X-ray instruments around the world.
Then, a research team examined Chandra's file data for events that matched the new information in GW170817, and found an event that also coincided with Zhang's predictions.
(X-ray: NASA / CXC / China Science and Technology University / Y. Xue et al; Optical: NASA / STScI)
"We have found a completely new way to detect a fusion of neutron stars," said astronomer Yongquan Xue of the University of Science and Technology of China. "The behavior of this X-ray source coincides with what one of the members of our team predicted for these events."
They called the XT2 event and tracked it as it suddenly appeared in the data, then slowly disappeared over the course of about seven hours. They carefully studied how the X-ray emission changed over time and compared it to 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 on the outskirts of the host galaxy is more consistent with the neutron stars ejected from the galactic center, and the low rate of star formation means that the event was less likely to be caused by a young, mbadive star that was It becomes a supernova.
Looking specifically at XT2, the team found that the emission was consistent with a magnetar rotating 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 more than 300 in the next 6.5 hours, and finally disappeared. The team believes that it was losing energy through a wind that emitted X-rays, which gradually slowed down.
This means that the two neutron stars probably produced a larger neutron star, not a black hole. Astronomers think that the mbad of the Sun is required at least three times to produce a black hole; Anything less mbadive becomes a neutron star. So that puts restrictions on the size of the neutron stars involved in the collision.
But it also tells us something about the interior of neutron stars, which is incredibly difficult to study because of insane density.
"We can not throw neutron stars 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, then this tells us that its structure is relatively rigid and resistant."
The team is now working hard to badyze other Chandra data to, among other things, see if they can begin to get solid statistics on how often events such as these are likely to occur.
"As with this source, the data found in the archives may contain some unexpected treasures," said Xuechen Cheng of the University of Science and Technology of China.
The research has been published in. Nature.