6.6 billion light-years away: neutron stars



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On April 11, 2019, an international team led by Professor XUE Yongquan of the University of Science and Technology (USTC) announced that he had observed a single, highly magnetar X-ray signal as a result of a binary neutron . star 6.6 billion light-years away. Such a signal was captured in the deepest and most sensitive X-ray survey – the 7 Ms Chandra Deep Field-South. This finding was published in Nature.

The neutron star is one of the most miraculous objects in the Universe, for a typical neutron star with a mass equal to our Sun has only a radius of about 10 kilometers, in contrast to a radius of 700,000 kilometers for our Sun. However, our understanding of the physics of neutron stars is still unclear. For years, astronomers were wondering about the question: What is the end of a binary fusion system of neutron stars?

Many researchers bet their money on the interpretation of the black hole, suggesting that the molten celestial body would collapse under its extreme gravity and would become a black hole. While some believe that a magnetar, a rapidly rotating neutron star with an extremely strong magnetic field, which is one hundred million times what humans can reach in the laboratory, will be formed after the fusion process and can survive its own gravity with help of its powerful centrifugal force. After years of discussion, this time the Universe finally gives its answer – evidence of the existence of a magnetar has been revealed!

When two neutron stars "embrace" each other, there is the emission of gravitational waves – ripples in space and time, along with a massive cloud of colliding debris traveling outward. Under the gravitational force of the molten magnetar, part of the colliding debris falls and causes internal shocks, resulting in jets of high energy particles and intense radiation. If the jet is pointed in our direction, energetic explosions known as short gamma ray bursts with a typical duration of less than 2 seconds will be observed, which has been taken as evidence of a binary neutron star fusion for decades.

Meanwhile, our main role – an isotropic X-ray transient begins. If a magnetar is formed after the fusion process, this bright X-ray transient will last for several hours and can be seen off-axis of the jet by us. Therefore, even without any detected gamma ray blast, an X-ray transient can also provide us with valuable information about the neutron star fusion system. On the other hand, if the X-ray transient is fed by a resulting black hole, it may shine for only a few seconds.

Such transient X-rays have been desired by theorists and have not been discovered so far. The signal is located in a galaxy 6.6 billion light years from Earth and lasted up to 7 hours. It indicates that a neutron / magnetar star may form as a result of a binary fusion of neutron stars.

Interestingly, the observed X-ray transient is at the periphery of its host galaxy, as neutron stellar systems usually do, since they are usually expelled from the center after supernova explosions. This can be taken as evidence that the X-ray transient is in fact powered by the binary fusion of neutron stars.

"The discovery of this new X-ray transient is highly intriguing. Particularly exciting, due to Chandra's discovery of its excellent spatial resolution, is the ability to identify the host galaxy," said one critic of this article.

Meanwhile, the researchers also calculated the density of the event rate of similar X-ray transients. The result is consistent with the density of the neutron star fusion event rate, strongly inferred from the detection of gravitational waves from a binary neutron star fusion in 2017.

This finding has profound implications for the extremely dense nuclear matter state equation: the neutron star's state equation can be sufficiently rigid, that is, the pressure increases dramatically as the density of matter increases such that a neutron star supra-solid or even stable can survive the binary fusion of the neutron star. It helps to rule out a number of theoretical nuclear matter models that are inconsistent with the observations, providing new insights into the physics of neutron stars.

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