[ad_1]
On March 22, 2015, NASA's Chandra X-ray observatory recorded a blip in its data. Not far from the southern constellation of Fornax, something is lit up, then slowly fainted.
Thanks to a new technique, we now know that the blip was a collision of two neutron stars, 6.6 billion light years from Earth.
We also know that when neutron stars collide, they produce two powerful jets, shooting in opposite directions, emitting gamma-ray bursts. But if these 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 remanence if the result of the fusion was a highly magnetized, fast-spinning neutron star – a magnetar.
Then, in August 2017, gravitational wave astronomy left the world a marvel. For the first time, we witnessed the collision of neutron stars in real time, not only with the help of gravitational wave detectors, but also with the help of optical, infrared, ultraviolet and x-ray instruments worldwide.
A research team therefore scanned Chandra's archive data for events matching GW170817's new information – and discovered an event that also matched Zhang's predictions.
(X-rays: NASA / CXC / University of Science and Technology of China / Y. Xue et al; Optical: NASA / STScI)
"We have found a totally 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 corresponds to what one of our team members had predicted for these events."
They called the event XT2 and traced it as it suddenly appears in the data, and then faded in about seven hours. They carefully studied the evolution of X-ray emission over time and compared it to Zhang's predictions.
They also considered other possibilities, especially 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 the neutron stars expelled from the center of the galaxy and the low rate of star formation means that the event was less likely to be caused by a massive young star becoming supernova.
By looking specifically at the XT2, the team discovered that the show was compatible with a magnetar rotating several hundred times a second and with a magnetic field about four times more powerful than that of the Earth.
Magnetar X-ray emissions remained constant for about 30 minutes, after which they were attenuated by a factor of more than 300 in 6.5 hours, eventually disappearing. The team expects to lose energy through an X-ray wind, which has progressively slowed its rotation.
This means that the two neutron stars probably produced a larger neutron star, not a black hole. Astronomers think that it takes at least three times the mass of the Sun to produce a black hole; nothing less massive turns into a neutron star. This therefore imposes constraints 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 senseless density.
"We can not throw the neutron stars together in a laboratory to see what happens, so we have to wait for the Universe to do it for us," Zhang said. "If two neutron stars can collide and a heavy neutron star survives, it tells us that their structure is relatively rigid and resilient."
The team is now working to analyze other Chandra data, including whether it can begin to obtain reliable statistics on the likely frequency of such events.
"Just like with this source, the data stored in the archives could contain unexpected treasures," said Xuechen Cheng of the China University of Science and Technology.
The search was published in Nature.
[ad_2]
Source link