Mysterious discovery: A supernova leaves behind a neutron star or black hole

Scientists have discovered a mysterious dark object in the Milky Way Galaxy, about 40,000 light-years from Earth. The celestial body, which has a mass of 2.35 times the mass of our Sun, in the spherical star cluster NGC 1851, does not shine on its own, so it is not an ordinary star, according to the international research team in the specialized magazine “Science”. But for the remains of an exploded star — a neutron star or a black hole — its mass is unusual.

Therefore, the mass lies in the so-called mass gap between these strange bodies. This makes it unclear whether this unusual celestial body is an exceptionally heavy neutron star, an exceptionally light black hole, or something previously unknown. A light year is the distance that light travels in one year, which is 9.46 trillion kilometers.

“All these possibilities regarding the nature of the object are exciting,” explained Benjamin Stubbers of the University of Manchester, one of the project leaders for the observations made at the Meerkat radio telescope facility in South Africa. “If it's a black hole, we can use it to test the theory of gravity. If it's a neutron star, it could give us new insights into nuclear physics at very large densities.”

If a massive star dies in a supernova explosion at the end of its life, its interior collapses and forms either a neutron star, where matter is densely packed as in atomic nuclei, or a black hole, where gravity is so strong that even light cannot escape. According to theory, neutron stars cannot contain more than 2.2 times the mass of the Sun, otherwise gravity would have the upper hand and a black hole would form. But black holes only exist in the universe at about five solar masses.

There is a gap between them that has until now remained a mystery to astronomers. Gravitational wave measurements only indicate the presence of isolated celestial bodies in this mass gap – although their nature and composition remain unclear. Therefore, the discovery of such a celestial body represents a major advance for astronomers.

Researchers found the strange object while observing the pulsar PSR J0514-4002E. A pulsar is a neutron star with a strong magnetic field that, due to its own rotation, sends regular radio pulses to Earth – in this case 170 times per second. Careful measurement of these pulsations showed scientists that the pulsar forms a close binary system with another object. Data for this object give a mass between 2.09 and 2.71 solar masses, with the most likely value being 2.35 solar masses.

But how could such an object form in a mass gap? Since PSR J0514-2002E is located in a globular cluster, Stabbers and his colleagues suspect a complex formation history. Because NGC 1851 contains about half a million stars that are very close together. This is why, on astronomical scales, there are often close encounters in which binary stars are newly formed or even partners are exchanged.

The team suspects that the pulsar's strange companion may have been formed by the merger of two smaller neutron stars and only entered orbit around the pulsar later during a close encounter.