A black widow binary is a rare oddity — a rapidly spinning dead star slowly devouring a smaller companion star, as its namesake spider does to its mate. In a new study published Wednesday in the journal Nature, scientists may have found the most tightly coupled black widow binary yet, one whose partners are so close that the duo could fit inside our Sun. Much about it remains a mystery, hinting this discovery may be a case of mistaken identity, and that the researchers might have found something entirely new to science.
HERE'S THE BACKGROUND — Black widow binaries are powered by pulsars — rapidly spinning neutron stars, which are the extraordinarily dense cores of dead stars that each pack a mass up to 2.17 times the mass of the Sun into the size of a city. Pulsars can spin at dizzying rates, whirling hundreds of times per second, emitting flashes of gamma rays and X-rays like high-powered lighthouses.
Normally, pulsars spin down and settle into life as ordinary neutron stars as they burn off an enormous amount of energy. However, in a black widow binary, the pulsar's companion gives the dead star new life. The pulsar's gravity — strong enough to crush protons together with electrons to form neutrons — rips material off its companion, a flood of plasma whose force helps spin the pulsar back up. The pulsar subsequently blasts out energy that further strips away its companion, eventually destroying it.
Astronomers discovered the first black widow binary in the 1980s. Since then, astronomers have found roughly two dozen black widow binaries in the Milky Way, along with another spidery kind of star called a redback, which also consists of a pulsar and its doomed partner. (Black widows have companions less than a tenth the mass of our Sun, whereas redbacks have larger companions.)
WHAT DID THE SCIENTISTS DO? — Every black widow binary astronomers have detected to date appeared via gamma and X-ray flashes from its pulsar. In the new study, researchers instead sought to see black widow binaries from visible flashes of light from the companions of the pulsars.
The dayside of a black widow pulsar's companion — the side perpetually facing its vampiric captor — can prove many times hotter than its night side because of the constant torrent of radiation it receives from the pulsar. The astronomers sought to find a star whose brightness was regularly changing by a considerable amount, suggesting it might orbit tightly around a pulsar.
In the new study, the scientists investigated data from the Zwicky Transient Facility (ZTF) instrument at the California Institute of Technology's Palomar Observatory near San Diego. They examined images of the entire night sky, analyzing 20 million stars to see if the brightness of any of them changed dramatically by a factor of 10 or more on a timescale of roughly an hour or less.
WHAT DID THEY FIND? — This new technique identified known black widow binaries, validating its accuracy. It also detected what appeared to be a star whose brightness changed by a factor of 13 every 62 minutes. This suggests it was likely part of a new black widow binary roughly 3,700 light-years from Earth, which the scientists named ZTF J1406+1222.
"All known black widow pulsars have been found because they emit X-rays, gamma rays, or radio waves, but this is the first time we've used visible light, the kind our eyes can see, to find something like this," study lead author Kevin Burdge, an astrophysicist at the Massachusetts Institute of Technology (MIT), tells Inverse.
ZTF J1406+1222 likely consists of a pulsar and a cool failed star known as a brown dwarf. These objects are about 13 to 90 times the mass of Jupiter — too big to be planets, but not quite big enough to be true stars.
Telescopes cannot distinguish between ZTF J1406+1222's pulsar and the brown dwarf, but the pulsar is likely between 1.4 and two times the Sun's mass, whereas the brown dwarf is likely several dozen times Jupiter's mass, Burdge says. The scientists estimated they are separated by less than roughly 450,000 kilometers. Given this short distance and the relatively tiny sizes of each object, "you could fit the entire thing inside of the Sun," Burdge says.
ZTF J1406+1222's pulsar and its companion orbit around each other every 62 minutes, the shortest orbit yet observed for a black widow binary. The previous record-holder, PSR J1653-0158, had stars orbiting around each other every 75 minutes.
Intriguingly, this black widow binary is unique in that it has a third companion about 10 to 20 percent of the Sun's mass orbiting much farther out, at a distance of roughly 600 times the average distance between Earth and the Sun. This distant partner orbits the close pair every 10,000 years.
It remains uncertain how ZTF J1406+1222's pulsar and its companion could have entered into such a tight orbit, as the way in which a pulsar erodes its companion should drive them apart.
"The third companion may have helped them get there," Burdge suggests. "Basically, anytime you have two things orbiting each other, and also have a wider third thing orbiting, that wider third thing can gravitationally interact with the inner binary, and actually change the orbit of the inner binary to nudge things a bit closer."
More technically, the third companion can make the orbit of its two partners more elongated or eccentric. When the brown dwarf gets close to the pulsar in the eccentric orbit, it feels a powerful gravitational tug, "and that dissipates energy, which quickly causes the orbit to settle into a really tight circular orbit," Burdge says.
WHAT'S NEXT? — Curiously, the astronomers have so far not detected gamma rays or X-rays from ZTF J1406+1222. Although many details about it so far suggest it is a black widow binary, until future observations — say, from NASA's Chandra X-ray Observatory — confirm the presence of a pulsar, "it is entirely possible that this object could also be something we've never seen before and even more exotic," Burdge says. "The one thing I know for sure is we really have never seen anything quite like this object, and that there is probably a lot more to learn from it and other similar objects that I am finding right now, and that's what has me so excited about these."
Burdge notes the Vera Rubin Observatory in Chile may find many more of these objects, as it "will be around 40 times more powerful than ZTF in terms of how far away it can see."
