Astronomers Discover Rare ‘Black Widow’ Binary, With Shortest Orbit Yet

The blink of a nearby star has drawn astronomers at MIT to a mysterious new system 3,000 light-years from Earth. The stellar weirdness appears to be a new “black widow binary” – a rapidly spinning neutron star, or pulsar, that spins in circles and slowly consumes a smaller companion star, as its arachnid namesake does to its companion.

Astronomers know about two dozen black widow binaries in the Milky Way. This new candidate, named ZTF J1406+1222, has the shortest orbital period ever identified, with the pulsar and companion star rotating every 62 minutes. The system is unique in that it appears to host a distant third star that orbits the two inner stars every 10,000 years.

This probable Triple Black Widow raises questions about how such a system could have formed. Based on their observations, the MIT team offers an origin story: As with most Black Widow binaries, the triple system likely originated from a dense constellation of old stars known as globular cluster. This particular cluster may have drifted toward the center of the Milky Way, where the central black hole’s gravity was sufficient to pull the cluster apart while leaving the Triple Black Widow intact.

“It’s a complicated birth scenario,” says Kevin Burdge, a Pappalardo postdoctoral fellow in MIT’s Department of Physics. “This system has probably been floating around in the Milky Way for longer than the Sun has existed.”

Burge is the author of a study published in Nature which details the team’s discovery. The researchers used a new approach to detect the triple system. While most Black Widow binaries are found through the gamma and X-ray radiation emitted by the central pulsar, the team used visible light, and specifically the blinking of the binary’s companion star, to detect ZTF J1406 +1222.

“This system is really unique as far as black widows are concerned, because we found it with visible light, and because of its large companion, and the fact that it comes from the galactic center,” says Burdge. “There’s still a lot we don’t understand about this. But we have a new way to search the skies for these systems.”

The study’s co-authors are collaborators at several institutions, including the University of Warwick, Caltech, the University of Washington, McGill University and the University of Maryland.

Day and night

Black Widow binaries are powered by pulsars – rapidly rotating neutron stars that are the collapsed cores of massive stars. Pulsars have a dizzying rotation period, spinning every few milliseconds and emitting flashes of high-energy gamma and X-rays in the process.

Normally, pulsars slow down and die quickly because they burn up a huge amount of energy. But once in a while, a passing star can breathe new life into a pulsar. As a star approaches, the pulsar’s gravity pulls material from the star, which provides new energy to pull the pulsar back up. The “recycled” pulsar then begins to re-emit energy which further strips the star and eventually destroys it.

“These systems are called black widows because of the way the pulsar kind of consumes the thing that recycled it, just like the spider eats its mate,” Burdge explains.

Every black widow binary to date has been detected by gamma and X-ray flashes from the pulsar. At first, Burdge came across ZTF J1406+1222 through the companion star’s optical blink.

It turns out that the day side of the companion star—the side that perpetually faces the pulsar—can be many times hotter than its night side, due to the constant high-energy radiation it receives from the pulsar.

“I thought instead of looking directly for the pulsar, try looking for the star it’s cooking,” Burdge says.

He reasoned that if astronomers observed a star that periodically changed in brightness enormously, that would be a strong signal that it was in binary with a pulsar.

star movement

To test this theory, Burdge and his colleagues looked at optical data taken by the Zwicky Transient Facility, a California-based observatory that takes wide-field images of the night sky. The team studied the brightness of stars to see if any changed dramatically by a factor of 10 or more, on a time scale of around an hour or less – signs that indicate the presence of a companion star in orbit around a pulsar.

The team was able to select the dozen or so known black widow binaries, validating the accuracy of the new method. They then spotted a star that changed in brightness by a factor of 13 every 62 minutes, indicating that it was likely part of a new black widow binary, which they labeled ZTF J1406+1222.

They searched for the star in observations taken by Gaia, a space telescope operated by the European Space Agency that keeps precise measurements of the position and movement of stars in the sky. Looking back through decades of measurements of the star from the Sloan Digital Sky Survey, the team discovered that the binary was followed by another distant star. Judging by their calculations, this third star seemed to orbit the inner binary every 10,000 years.

Curiously, the astronomers did not directly detect gamma or X-ray emissions from the pulsar in the binary, which is the typical way black widows are confirmed. ZTF J1406+1222 is therefore considered a black widow binary candidate, which the team hopes to confirm with future observations.

“The only thing we know for sure is that we’re seeing a star with a much hotter dayside than the nightside, orbiting something every 62 minutes,” Burdge said. “Everything seems to point to this being a black widow binary. But there are a few weird things about it, so it’s possible this is something entirely new.”

The team plans to continue observing the new system, as well as applying the optical technique to illuminate more neutron stars and black widows in the sky.