Astronomers have discovered a mysterious object in the Milky Way that is more massive than the heaviest neutron star but lighter than the smallest black hole.
The mysterious object could help scientists better determine where to draw the dividing line between neutron stars and black holes, both of which are born when a massive star dies.
“Both possibilities are exciting in terms of the nature of the companion,” team leader and University of Manchester astrophysics professor Ben Stappers said in a statement. “A pulsar-black hole system will be an important target for testing theories of gravity, and a heavy neutron star will provide new insights into nuclear physics at very high densities.”
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The object was discovered using the MeerKAT Radio Telescope, which consists of 64 antennas located in the Northern Cape of South Africa. The dense stellar remnant orbits a rapidly spinning neutron star, or “millisecond pulsar,” located about 40,000 light-years away within a dense cluster of stars in the Milky Way called a “globular cluster.”
While a system with two neutron stars is fascinating, if the mysterious object is a black hole, this would make the system an incredibly coveted radio pulsar-black hole binary system. Thanks to the pulsar’s extremely periodic bursts, which can be used as a timing mechanism, and the black hole’s intense gravitational influence, such a system could be crucial for testing the limits of Einstein’s 1915 theory of gravity, known as general relativity. .
The pulsar, known as PSR J0514-4002E, was detected through the weak pulses of radio waves it sent as it orbited Earth.
Because the neutron star rotates 170 times per second like a cosmic lighthouse, small variations in the highly regular pulses allowed researchers to determine that PSR J0514-4002E had an incredibly dense object orbiting it; This means it could just be the remnant of a larger mass. collapsed star.
The team discovered that the pulsar and the mysterious object are 5 million miles (8 million kilometers) apart, about 0.05 times the distance between Earth and the sun, and orbit each other every seven Earth days.
The orbiting object has more mass than any known neutron star but less mass than any known black hole, landing it exactly in the mass gap of the black hole.
Black holes and neutron stars: Spot the difference
Both neutron stars and black holes are born when massive stars run out of fuel for nuclear fusion and can no longer support themselves against the internal pressure of their own gravity. As the star’s core collapses, the outer layers of this dying star are blown away in a supernova explosion.
At the lower end of the mass scale, the collapse of the stellar core is stopped by the quantum properties of the neutron sea from which it now forms, becoming a neutron star with a stellar remnant between 1 and 2 times its mass. The Sun is about the width of a city here on Earth, about 12 miles (20 kilometers).
However, above a certain mass, the quantum pressure that keeps the neutrons apart is exceeded and the nucleus collapses completely and turns into a black hole. A neutron star can also exceed this limit and collapse into a black hole if it has a companion star that can steal material from it to increase its own mass.
Astronomers think that if a star’s core still has a mass above 2.2 times the mass of the Sun after losing its outer layers and most of its mass, it is heavy enough to form a black hole.
The problem here is that the lightest black holes we see still have about 5 times the mass of the Sun. The absence of black holes between 5 solar masses and 2.2 solar masses has become known as the “black hole mass gap” and casts doubt on the 2.2 solar mass limit for neutron stars.
Closing the black hole mass gap
While using MeerKat to study the globular cluster NGC 1851 in the southern constellation of Columbus, Stappers and his colleagues discovered the object that could be key to solving this mystery and closing the mass gap.
The stars in this ancient star cluster are more tightly bound together than stars in the rest of the Milky Way. The stars in NGC 1851 are so crowded that they interact with each other, disrupting each other’s orbits and, in extreme cases, even colliding.
The team thinks that such a collision between two neutron stars may have created the mysterious object they detected orbiting the pulsar PSR J0514-4002E.
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The team cannot yet determine whether PSR J0514-4002E’s companion is a neutron star, a black hole, or even a hitherto unknown dense cosmic object, but they do know that this system could be a unique cosmic laboratory for studying behavior. matter and physics under extreme conditions.
“We are not done with this system yet,” Dutta concluded. “Uncovering the true nature of the companion will be a turning point in our understanding of neutron stars, black holes, and everything else lurking in black hole mass space.”
The team’s research was published Thursday, January 18, in the journal Science.