Three thousand light-years from Earth, MIT astronomers have discovered something weird. Out in the void lies a flashing oddity that appears to be a new “black widow binary,” of which only two dozen are known to exist in the Milky Way. The rapidly spinning neutron star, or pulsar, is slowly consuming its smaller companion star, hence the name.
This newly found binary system, named ZTF J1406+1222, has the shortest orbital period yet detected, with the pulsar and companion star circling each other every 62 minutes. What makes the system really rare is that it appears to host a third, far-flung star that completes an orbit around the two inner stars every 10,000 years.
As may imagine, astronomers are very curious about how a potential triple black widow could come into being.
“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 been around.”
As with most black widow binaries, the MIT team believes that the triple system likely arose from a dense constellation of old stars known as a globular cluster. This specific one may have wandered into the Milky Way’s center, where the gravity of the central black hole was enough to tear the cluster apart while leaving the triple black widow intact.
Black widow binaries are powered by pulsars which have a dizzying rotational period, spinning around every few milliseconds, and emitting flashes of high-energy gamma and X-rays in the process. Generally these pulsars spin down and die quickly as they burn off a huge amount of energy. However, once in a while, a passing star can give a pulsar new life. As a star closes in on the pulsar, the pulsar’s gravity yanks material off the star, which provides new energy that gets the pulsar spinning again. This “recycled” pulsar then starts reradiating energy that further strips the star, and eventually consumes it.
“These systems are called black widows because of how the pulsar sort of consumes the thing that recycled it, just as the spider eats its mate,” Burdge said.
While most black widow binaries are found through the gamma and X-ray radiation emitted by the central pulsar, the team from MIT used visible light, specifically the flashing from the binary’s companion star, to spot ZTF J1406+1222.
Burdge and his team found that it turns out the companion star’s dayside — the side continually facing the pulsar — can actually be many times hotter than the night side due to the continual high-energy radiation it receives from the pulsar.
“I thought, instead of looking directly for the pulsar, try looking for the star that it’s cooking,” Burdge explains.
His team verified their visible-light method to identify some already-known black-widow binaries. After going through data collected by the California-based Zwicky Transient Facility, which takes wide-field images of the cosmos, they were able to study the brightness of stars to see whether any were changing dramatically by a factor of 10 or more on a timescale of an hour or less. The new black widow fit into that model with a brightness that changed by a factor of 13, on a timescale of about an hour.
Technically, the new find is just a “candidate” since they were not able to detect gamma or X-rays emitting from the entity. Further observations will have to be made to make it an official black widow. This technicality could mean the team has found a whole new object.
“This system is really unique as far as black widows go, because we found it with visible light, and because of its wide companion, and the fact it came from the galactic center,” said Burdge. “There’s still a lot we don’t understand about it… The one thing we know for sure is that we see a star with a day side that’s much hotter than the night side, orbiting around something every 62 minutes. Everything seems to point to it being a black widow binary. But there are a few weird things about it, so it’s possible it’s something entirely new.”
The discovery was published in Nature.