In the globular star cluster NGC 1851 in the constellation Dove in the southern sky, a newly discovered celestial body poses a puzzle for astronomers. It has 2.35 times the mass of our sun, but does not shine itself, so it is not an ordinary star, researchers reportn their publication in Science. Normally something like this is a clear candidate for either a neutron star or a black hole. But in this case, the data actually fits neither one nor the other: the object is too heavy to be a neutron star and significantly too light to be a black hole. Rather, the object lies in the so-called mass gap between these two types of compact objects.
This makes it unclear what the unusual celestial body is. It could even be that it is something previously unknown.
“Each of these possibilities for the nature of the object is exciting,” said Benjamin Stappers from the University of Manchester, one of the project leaders of the observations carried out at the MeerKAT radio telescope facility in South Africa. “If it is a black hole, we can use it to test the theory of gravity. If it is a neutron star, it can give us new insights into nuclear physics at very high densities.”
Fate of great stars
When a large star dies in a supernova explosion at the end of its life, its interior collapses, creating either a neutron star, in which matter is as densely packed as in atomic nuclei, or a black hole, in which gravity is as strong , that not even light can escape. According to the theory, neutron stars cannot contain more than 2.2 times the mass of the sun – otherwise gravity would gain the upper hand and a black hole would have to form. But in fact, black holes are only found in the cosmos at around five solar masses.
There is a gap in between that has so far been a mystery to astronomers. Only measurements of gravitational waves indicate that there are isolated celestial bodies in this mass gap – although their nature and formation is still unclear. Therefore, the discovery of such a celestial object represents a major advance for astronomers.
The researchers came across the strange object while observing the pulsar PSR J0514-4002E. A pulsar is a neutron star with a strong magnetic field that, through its own rotation, sends regular radio pulses to Earth – in this case 170 times per second.
The precise measurement of these pulses showed the scientists that the pulsar forms a close double system with another, compact and therefore invisible object – the simulation in the video above shows how this constellation could have come about. The orbit data gives the compact object a mass of between 2.09 and 2.71 solar masses, with the most likely value being 2.35 solar masses.
But how could such an object have formed in the mass gap? Since PSR J0514-4002E is located in a globular cluster, Stappers and his colleagues suspect a complicated formation history. Because NGC 1851 contains around half a million stars that are very close together. That's why – on an astronomical scale – there are often close encounters in which double stars are formed or even swap partners.
It is possible, the authors suggest, that the pulsar's exotic companion was formed by the merger of two smaller neutron stars and only entered orbit around the pulsar later in a close encounter similar to the one visualized in the simulation above.
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