Astronomers have spotted a rapidly spinning neutron star gravitationally bound to a helium-rich companion star for the first time. The discovery of this unusual binary system helps confirm a long-theorized but rarely observed cosmic process called co-shell evolution.
Binary star systems, or pairs of stars that orbit each other, are very common. In fact, an estimated 85 percent of stars in the universe have at least one companion. But this newly discovered pair is unlike any seen before.
In this case, the helium star is bound to a millisecond pulsar, a rapidly spinning neutron star that emits beams of radiation at regular intervals. These stars achieve their extreme rotation speeds by sucking matter from their nearest stellar companions.
In May 2020, a team of researchers led by Jin Ling Han, a radio astronomer at the National Astronomical Observatory and the Chinese Academy of Sciences in Beijing, used China’s FAST radio telescope to detect faint signals from a point deep within the Milky Way galaxy.
A few months later, the researchers confirmed that the signals were radiation from a pulsar. They then tracked the bursts for four and a half years, and their measurements showed that this star is not alone. It is actually part of a binary system that orbits its companion every 3.6 hours. But for one-sixth of that orbit, the pulsar’s radiation is blocked — or obscured — by its companion.
“That’s a big part of the orbit,” Han told Gizmodo. “It’s amazing, only a bigger companion could do that.”
In binary systems, a millisecond pulsar is usually accompanied by a white dwarf: the hot, dense core that remains after a star like our Sun has run out of fuel. But the data Han and his colleagues collected suggest that this companion should be somewhere in between a compact object and a regular star, he said.
Further study of this strange companion showed that it’s about as massive as our Sun, but it can’t be a regular star because it’s undetectable at all wavelengths beyond radio. This led the researchers to conclude that it’s a star that’s been stripped of hydrogen, leaving behind a core made mostly of helium. They published their findings today in the journal Science.
According to Han, this type of binary system “has never been seen before.” But it has long been theorized that such a pair could form through the evolution of a shared envelope, and he and his colleagues believe that is exactly what happened in this case.
“The process of shared envelope evolution is a bit different from how stars like pulsars are often thought to interact in binary systems,” Duncan Lorimer, a professor of physics and astronomy at West Virginia University who was not involved in the study, told Gizmodo in an email.
Normally, the intense gravitational field of a neutron star pulls in material from a companion star that has expanded, allowing its gaseous outer layers to be “eaten up” by the neutron star, he explained. This process, called accretion, causes the neutron star to “spin off” and become a pulsar.
But in shared envelope evolution, “the companion star is so massive that its outer layers absorb and neutron star,” Lorimer says. “It acts as a brake on the entire binary system.”
In the outer layers of the companion star—the envelope—friction causes the pulsar and the companion core to spiral toward each other, forming a very compact binary system, similar to the one Hahn and his colleagues have now observed. With an orbital period of just 3.6 hours, this pulsar and its companion orbit very close together.
Eventually, the outer layers of the companion star are forced out, Lorimer says, which explains why the helium companion star of this millisecond pulsar was stripped of its envelope.
“The evolutionary path that the authors have outlined is not unexpected,” Victoria Caspi, a physics professor at McGill University who was not involved in the study, told Gizmodo. “It has been recognized, identified, and discussed in detail for many years. years.”
“The interesting question is: If you find 1,000 millisecond pulsars, what fraction of them are going to be like this one? About one in 1,000, somewhere like that. And they found it,” she said.
Han and his colleagues believe there are more than a dozen other systems like this one in our galaxy, making them extremely rare. The fact that these researchers have found one of them is “a big breakthrough,” Lorimer said.
“The more millisecond pulsars we find, the more likely we are to find examples of rare outcomes of evolution. This system is a great example of that,” he said.
