Is this the Lightest Black Hole or Heaviest Neutron Star?

About 40,000 light-years away, a rapidly spinning object has a companion that’s confounding astronomers. It’s heavier than the heaviest neutron stars, yet at the same time, it’s lighter than the lightest black holes. Measurements place it in the so-called black hole mass gap, an observed gap in the stellar population between two to five solar masses. There appear to be no neutron stars larger than two solar masses and no black holes smaller than five solar masses.

Astronomers working in the Transients and Pulsars with MeerKAT (TRAPUM) collaboration found the object named PSR J0514-4002E in a globular cluster named NGC 1851. It’s an “eccentric binary millisecond pulsar,” according to the authors of a new research article in Science. The total mass of the pulsar’s companion object is 3.887 ± 0.004 solar masses, placing it right in the black hole mass gap.

What is it?

The new research article is titled “A Pulsar in a Binary with a Compact Object in the Mass Gap Between Neutron Stars and Black Holes.” The lead author is Ewan Barr from the Max Planck Institute for Radio Astronomy. It’s published in the journal Science.

Barr and his colleagues found the object orbiting a rapidly spinning millisecond pulsar. A pulsar is a rotating neutron star resulting from a supernova explosion. Pulsars emit beams of electromagnetic energy from their poles as they spin. If the orientation between Earth and the pulsar is right, we see the pulsar’s flashes. That’s why they’re referred to as cosmic lighthouses.

A millisecond pulsar has a rotational period between 1 and 10 milliseconds. That means it revolves from 60,000 to 6,000 times per minute.

Pulsars are powerful tools because of their rapid and predictable spinning. The pulsar timing technique measures the pulses with precision, and any changes are noted. Those changes indicate the presence of another body, its mass, and its distance from the pulsar.

“Think of it like being able to drop an almost perfect stopwatch into orbit around a star almost 40,000 light years away and then being able to time those orbits with microsecond precision,” said lead author Barr.

In this research, the astronomers used the pulsar’s timing to detect the object in binary relationship with it. But it couldn’t tell them what it is. Could it be a binary system containing a pulsar and a black hole? Or could it be a pulsar and a neutron star? Could it be something else?

Astronomers have never found a system containing a pulsar and a black hole, but they’d really like to. These pairings present a new way to study black holes and could also serve as a new test for Einstein’s general relativity. It the companion isn’t a small black hole but instead is a heavy neutron star, that’s scientifically valuable for a different reason.

“Either possibility for the nature of the companion is exciting,” said Ben Stappers, Professor of Astrophysics at Manchester University and one of the co-authors. “A pulsar–black hole system will be an important target for testing theories of gravity, and a heavy neutron star will provide new insights in nuclear physics at very high densities.”

Neutron stars are extremely dense compact objects that remain after a massive star collapses and explodes as a supernova. Neutron stars can collapse even further if they gain mass by interacting with another stellar object. But astrophysicists don’t know what these neutron stars become after they collapse. They could become black holes.

This is where the black hole mass gap comes into play.

Scientists think that for a neutron star to collapse, it needs to have about 2.2 times the mass of the Sun. That’s the threshold needed for a collapse to occur. But theory and observation both show that these collapsed neutron stars could create black holes that are five times more massive than the Sun. This gives rise to the black hole mass gap.

Astrophysicists are uncertain about the nature of objects that lie in the mass gap. There’s something there, as these observations show, but the nature of the object is difficult to discern. Whatever the companion is, the authors think it resulted from a merger of two neutron stars. “We propose that the companion formed in a merger between two earlier NSs,” they write.

If the companion is a massive neutron star, then it could be a pulsar. But the authors couldn’t detect any pulsations. “We searched for radio pulsations from the companion, assuming the full allowed range of mass ratios, but did not detect any,” they explain.

The binary object’s origins could explain what the object is, and astrophysicists have detailed models of binary evolution. Those models indicate that mass transfer was involved somehow.

“The combination of the location in a dense globular cluster (where stellar exchange encounters often occur), the highly eccentric orbit, the fast spin of the pulsar and the large companion mass
indicates that the PSR J0514?4002E system is the product of a secondary exchange encounter,” the researchers explain in their article.

The authors think that an earlier companion object of lower mass transferred mass to the pulsar. Those types of interactions are more likely in a globular cluster like the one the binary object is located in, where stars are tightly packed. The pulsar also rotates very rapidly, another indication that it gained mass from a companion. If this was the case, then, somehow, the current companion object replaced the previous companion.

“However, a more complicated evolution with multiple exchange encounters is also possible,” the researchers explain. “We, therefore, cannot infer the nature of the companion from binary evolution models.”

For now, the nature of the object is up in the air. “We, therefore, cannot determine whether the companion is a massive NS or a low-mass BH,” the authors write. But they might one day.

“We’re not done with this system yet,” said co-author Arunima Dutta from MPIA. “Uncovering the true nature of the companion will be a turning point in our understanding of neutron stars, black holes, and whatever else might be lurking in the black hole mass gap.”