Astronomers recently glimpsed a “new and mysterious star system 3,000 light-years from Earth that appears to be a so-called ‘black widow‘ binary.” The interesting part is how they found it. We already have a catalog of rapidly spinning neutron stars which slowly suck the life from their smaller companion star, like the female black widow spider does to her mate.
Black widow binary
The concept behind black widow binary systems begins with a pulsar. That’s what spinning neutron stars are called. When a pulsar has another star nearby, it pulls material from the star as long as the supply lasts.
According to MIT’s Kevin Burdge, who happens to be the lead author of the study about the new discovery, his team found the newest candidate with the “shortest orbital period of any identified so far.” The companion star buzzes around it’s pulsar “every 62 minutes.”
They call this particular black widow binary system ZTF J1406+1222 and soon learned it has another odd aspect “because it appears to host a third, far-flung star which orbits around the two inner stars every 10,000 years.”
It joins a club of about two dozen inhabiting the Milky Way.
Pulsars themselves are as energetic as a child with a sugar rush. Their “dizzying rotational period” has them whizzing around every few milliseconds, “and emitting flashes of high-energy gamma and X-rays in the process.”
Under normal circumstances, “pulsars spin down and die quickly as they burn off a huge amount of energy. But every so often, a passing star can give a pulsar new life.” Those are the black widow systems.
Start from a globular cluster
Mass falling into the pulsar’s intense gravity well spins new energy back into the rotating core. At that point, the “recycled” pulsar “starts reradiating energy that further strips the star, and eventually destroys it.”
The MIT team concluded that “like with most black widow binaries, the triple system probably arose from a dense constellation of old stars known as a globular cluster.” This one “may have drifted into the Milky Way’s center, where the gravity of the central black hole was enough to pull the cluster apart while leaving the triple black widow intact.”
Until now, all the black widow binaries were “found through the gamma and X-ray radiation emitted by the central pulsar.” This time, the MIT team “used visible light, and specifically the flashing from the binary’s companion star, to detect ZTF J1406+1222.”
In these configurations, the companion star’s day side gets a whole lot hotter than the night side. “I thought, instead of looking directly for the pulsar, try looking for the star that it’s cooking,” Burdge relates.
His team “studied the brightness of stars to see whether any were changing dramatically by a factor of 10 or more, on a timescale of about an hour or less — signs that indicate the presence of a companion star orbiting tightly around a pulsar.”
That’s how he found his black widow. They found the the dozen known ones instantly, so know the method works. Then, they “spotted a star whose brightness changed by a factor of 13, every 62 minutes, indicating that it was likely part of a new black widow binary.“