Astronomers have discovered a neutron star orbiting a mysterious object that, by all indications, should not exist. Appearing invisible in light and too small to be a black hole, this object defies explanation.

The mystery began when astronomers using the MeerKAT radio telescope in South Africa detected a pulsar in a globular cluster called NGC 1851 in the constellation Columbus. Pulsars are a type of neutron stars with strong magnetic fields that create rotating cones of radio waves like the beams of a lighthouse. When these cones look at the Earth, they appear to pulse regularly at us, hence the name.

Because these signals are so stable and predictable, astronomers can study their timing and calculate a surprising amount of information about the environment. In this case, they discovered that the pulsar was orbiting next to the second object, and that’s when things started to get weird.

“When we looked at the Hubble image of NGC 1851, we didn’t see anything at that location,” said Prajwal Woraganti Padmanabh, a co-author of the study. “So an object orbiting a pulsar is not an ordinary star, but an extremely dense remnant of a collapsed star.”

These remnants of collapsed stars are known to take two forms: either another neutron star or a black hole. But there is a problem; It turned out that the object was too massive to be a neutron star, but not too massive to be a black hole.

According to models, neutron stars are always less than about two solar masses, while black holes are never less than about five solar masses. This is also confirmed by space observations – compact objects always fall into one or another category. Until now. This newly discovered object was found to have a mass of approximately 2.1 to 2.7 times the mass of the Sun and falls right into the designated “mass gap.” This means it could be either the heaviest known neutron star, the lightest known black hole, or something else entirely.

“Whatever this object is, this is exciting news,” said Paulo Freire, a co-author of the study. “If it is a black hole, it would be the first known pulsar/black hole system, which has been the Holy Grail of pulsar astronomy for decades! “If it is a neutron star, it will have fundamental consequences for our understanding of the unknown state of matter at such an incredible density!”

The study was published in the journal Science. You can see the animation of the proposed system formation in the video below.

The researchers suggest that the strange system actually consists of two previous binaries. One of them contained two neutron stars that collided and merged into a single, smaller-than-average black hole. Meanwhile, another system contained a neutron star in close orbit of another star, with the former sucking material from the latter. This process, common throughout the universe, transfers angular momentum to the neutron star, turning it into a rapidly rotating pulsar. The other star remained as a dead shell called a white dwarf.

Eventually the black hole entered the binary system, where the complex motions of the three objects caused the white dwarf to be ejected. What remains is the pulsar/black hole system seen today.

This isn’t the first object discovered during a mass eruption. In 2019, gravitational wave detectors detected signals from a 23 solar-mass black hole eating something weighing 2.6 solar masses. However, since the object was found after it was destroyed, we can learn a lot from it. Fortunately, there is a live one in NGC 1851 that we can continue to study.

“We’re not done with this system yet,” said study co-author Arunima Dutta. “Uncovering the true nature of the companion will be a turning point in our understanding of neutron stars, black holes, and anything else that might be hidden in a black hole’s mass rift!”