Astronomers have discovered a neutron star orbiting a mysterious object that, by all accounts, shouldn’t be able to exist. Seemingly invisible in light, and too small to be a black hole, the object defies explanation.
Using the MeerKAT radio telescope in South Africa, the mystery began when astronomers spotted a pulsar in a globular cluster called NGC 1851, in the constellation of Columba. Pulsars are a type of neutron star that have strong magnetic fields, which creates cones of radio waves that sweep around like lighthouse beams. When these cones happen to be facing Earth, they appear to us to be pulsing regularly, hence the name.
Because those signals are so steady and predictable, astronomers can study their timing and calculate a surprising amount of information about the environment around it. In this case, they found that the pulsar was orbiting alongside a second object – and that’s when things started to get weird.
“When we looked at Hubble images of NGC 1851, we saw nothing at that position,” said Prajwal Voraganti Padmanabh, co-author of the study. “Hence, the object in orbit with the pulsar is not a normal star, but an extremely dense remnant of a collapsed star.”
These collapsed star remnants are known to take one of two forms: either it’s another neutron star, or it’s a black hole. But there’s a problem – the object was found to be too massive to be a neutron star, but not massive enough to be a black hole.
According to models, neutron stars are always smaller than about two solar masses, while black holes never get lighter than about five solar masses. This is backed up by observations of the cosmos too – compact objects always fall into one category or the other. Until now, anyway. This newly discovered object was found to have a mass of between about 2.1 and 2.7 times that of the Sun, falling squarely into that established “mass gap.” That means it could be either the heaviest known neutron star, or the lightest known black hole – or perhaps, something else entirely.
“Whatever this object is, it is exciting news,” said Paulo Freire, co-author of the study. “If it is a black hole, it will be the first pulsar/black hole system known, which has been a Holy Grail of pulsar astronomy for decades! If it is a neutron star, this will have fundamental implications for our understanding of the unknown state of matter at these incredible densities!”
The researchers propose that the strange system actually formed from two previous binary systems. One contained two neutron stars, which collided and merged into a single, smaller-than-average black hole. The other system, meanwhile, contained a neutron star in close orbit with another star, with the former slurping material off the latter. This process, common across the cosmos, transfers angular momentum to the neutron star, turning it into a fast-spinning pulsar. The other star is left as a dead husk called a white dwarf.
Eventually, the black hole wandered into the binary system, where the complex motions of the three bodies caused the white dwarf to be ejected. That left the pulsar/black hole system seen today.
This isn’t the first object discovered in the mass gap. In 2019, gravitational wave detectors picked up signals from a 23-solar mass black hole chowing down on something weighing 2.6 solar masses. Since that was only found after the object was destroyed, there’s only so much we can learn from it. Thankfully, NGC 1851 has a live one we can continue to study.
“We're not done with this system yet,“ said Arunima Dutta, co-author of the study. “Uncovering the true nature of the companion will a turning point in our understanding of neutron stars, black holes, and whatever else might be lurking in the black hole mass gap!”
The research was published in the journal Science. An animation of the proposed formation of the system can be seen in the video below.
Source: Max Planck Institute
