Scientists have discovered the most extreme example of a star system known as a “black widow” lurking in our galaxy, a find that challenges models of how these so-called “spider binaries” form and evolve, reports a new study.
The black widow is not a literal space arachnid, but rather belongs to a class of systems that contain a rapidly spinning dead star, known as a pulsar, that feeds on the gassy material of an object orbiting it, just as its namesake animal feeds on her mates.
Now, researchers led by Kevin Burdge, a Pappalardo Postdoctoral Fellow at MIT, have used a novel technique to spot the first ever black widow triple, meaning a spider system that contains three objects, which they have named ZTF J1406+1222 in part after the Zwicky Transient Facility (ZTF), the astronomical survey that captured it.
In addition to its unprecedented composition, this cosmic black widow, located about 3,000 light years from Earth, has the shortest orbital period ever recorded, a discovery that “pushes the boundaries of evolutionary models” and distinguishes the system “from any known spider binary,” according to a study published on Wednesday in Nature.
“This thing is weird,” said Burdge in a call. “It behaves exactly like a black widow in many, many ways, but it also does a few new things that we've never seen before in any known black widow.”
In fact, ZTF J1406+1222 is so strange that it is still classified as a black widow candidate, to leave room for alternate explanations of its unusual properties. Though the new study presents strong evidence that the system is a black widow, further observations will be necessary to officially confirm it belongs to this spidery class of star systems.
“It's important, as a scientist, not to get sucked into the mode of thinking you figured it out and to keep an open mind, because if the new data shows something else, that we may need to change our interpretation,” Burdge said. “There's still enough things that we don't understand about this thing that I think it's worth keeping the interpretation open.”
ZTF J1406+1222 stands out so much from the two dozen other known black widows in part because of the novel detection method that the team devised to discover it. Whereas previous black widows were spotted by the energetic X-ray and gamma ray light they emit from their poles, this new system was first detected through visible wavelengths emitted from its orbital companion, which is a type of failed star known as a brown dwarf. This light is produced by the intense radiation the brown dwarf receives from the pulsar, which causes optical flashes on its day side.
Burdge came up with this approach while examining the light curves of known black widow systems, which are measures of a system’s brightness over time. He noticed this highly specific pattern in this data that could point the way to hidden black widows in the galactic dark.
“If you looked at the light curves of the known black widow systems, what you saw is they would get really, really bright and then fade,” he explained. “And when I say get really, really bright, I mean they brighten by more than a factor of ten, every few hours, and then fade away again.”
“If you look across the entire sky, there are a billion or more sources of light that we can see with surveys like ZTF, which is what I was using, and there are really not many other things that get brighter and fainter by a factor of ten every couple hours,” he continued. “That's a unique thing to these black widows. I thought, well, if I see that the known black widows that we found with all these gamma rays are doing this, what if I just look for this behavior and see if I can find more of them using the visible light instead? So I did it and, basically, this thing popped out.”
No sooner than ZTF J1406+1222 was detected than it started flauting expectations. First, Burdge marveled at its 62-minute period—the time it takes the brown dwarf and pulsar to orbit each other—which is by far the shortest time-scale observed in these systems. The team was also surprised by a mysterious amplification of the system’s shorter wavelengths, especially ultraviolet frequencies, that ultimately pointed to a third star attached to the system at a distance 600 times that between Earth and the Sun, which orbits the inner binary at a slow pace every 10,000 years.
“It was crazy, because every step of the way, it just kept getting weirder,” Burdge said. “We kept finding out new facts about it, like it is in a wide triple. We were like: ‘Really, in a triple? No known black widows are in a triple.’”
The existence of this farflung third star challenges formation models for these systems, prompting some creative brainstorming from the team to try to explain it. The issue is that most of these extreme pulsars, which are a subcategory of collapsed stellar remnants called neutron stars, experience what’s known as a “natal kick” when they are created, meaning that they are forcefully catapulted across space at high speeds by the asymmetric force of their births.
“We've seen evidence for these kicks in many neutron stars; we see them flying through space at really high velocities, including many systems in black widows,” Burdge said. “That sounds great, except that this thing has a really wide companion in a triple.”
“What that means is, if a neutron star gets a 100 kilometer per second kick when it's born, if that happened with such a wide companion, the black widow would just go flying off and leave it behind, and unbind it,” he added. “The fact that this thing is still orbiting in such a wide orbit challenges the idea that this could have formed as it is with a kick. You need some other explanation to form it.”
The team suggests that the black widow formed first and that the distant star was captured by the binary’s gravitational pull during a close encounter. But it will take more observations to untangle the many outstanding mysteries of this system and its unique backstory. To that end, Burdge plans to follow-up on the discovery—and use his novel technique to look for similar systems—with sophisticated telescopes such as NASA’s Chandra X-ray Observatory and the next-generation Vera C. Rubin Observatory.
“I actually think we're just scratching the surface right now, with ZTF, for these types of objects and that we're going to find a ton of them once [Vera C. Rubin Observatory] turns on,” he concluded.