A strange object about 40,000 light years away is either the heaviest neutron star or the lightest black hole we have ever seen, sitting in a mysterious void of objects that astronomers have never directly observed.
A neutron star forms when a star has run out of fuel and collapses under its gravity, creating a shockwave called a supernova and leaving an ultra-dense core behind. According to astrophysical calculations, these cores must remain below a certain mass, around 2.2 times the mass of the sun, or they will collapse even further, creating a black hole.
However, black holes have only been observed with a mass more than five times that of the sun, leaving a gap in scale between neutron stars and black holes. There have been some dense objects observed in this gap by gravitational wave observatories, but astronomers have never spotted them with conventional telescopes.
Now, Ewan Barr at the Max Planck Institute for Radio Astronomy in Germany and his colleagues have spotted an object of 2.5 solar masses by observing a pulsar that orbits it. A pulsar is a neutron star that spits out pulses of light at regular millisecond intervals due to an intense magnetic field.
Pulsars emit light with extreme regularity, but very massive nearby objects can warp these rhythms, as predicted by Albert Einstein’s theory of relativity. By observing the pulsar’s pulses for more than a year using the MeerKAT radio telescope in South Africa, Barr and his team were able to calculate the mass of the pulsar’s partner.
“What we found in this binary system looks to be above that [upper limit for neutron star mass], which would suggest that either some new physics is going on here and this is a new kind of star, or it’s just simply a black hole and it’s the lightest stellar mass black hole found like this,” says Barr.
The pulsar is located in a globular cluster, a region of tightly packed stars and more exotic objects that can pass close to each other. These unusual interactions could explain the mysterious object, says Barr.
If it is a black hole, it will let researchers test theories of gravity that they couldn’t previously. “The pulsar is just this ludicrously precise measurement device that you’ve got in orbit around the black hole and it’s not going anywhere; it’s going to be there for the next billion years,” says Barr. “So it’s this incredibly stable, natural testbed for looking at black hole physics.”
“If it’s a neutron star, then it’s heavier than any neutron star we’ve seen,” says Christine Done at the University of Durham, UK. “That actually tells us about the ultimate densities that stars can support before they collapse under their own gravity and become black holes. We don’t know the physics of matter at these extreme densities; we don’t know what that limit is.”
Barr and his team plan to observe the pulsar with other telescopes over the coming years to look for clues that could reveal what the object is. If it is a black hole, then they should see the pulsar’s orbit change over time as the black hole drags spacetime around it, similar to how a ship drags smaller boats in its wake. Or, if it is a neutron star, they might be able to detect light with more sensitive instruments.