Like black widow spiders, some pulsars bring death to their stellar partners. Astronomers have now discovered a particularly unusual representative of these destructive star remnants at a distance of around 3000 light years. Orbiting each other in close proximity, the pulsar and its companion take just 62 minutes to complete one orbit, making it the black widow with the shortest known orbital period. In addition, as the first pair of its kind, it was not detected by X-rays or gamma rays, but by the periodic illumination of the companion star in visible light.
Pulsars are rapidly rotating and highly magnetic neutron stars – relics of the supernova explosion of a massive star. Like a cosmic lighthouse, these ultra-dense stellar cores emit bundled rays that sweep through space at regular intervals. These beam pulses can be extremely energetic and flash several hundred times in one second in the case of extremely rotating pulses, the so-called millisecond pulsars. A special form of such stellar remnants is also known as a black widow, because they are pulsars that form a binary system with a small, normal companion star, but destroy their partners over time – like the spiders, which are known for their cannibalism. The cause of the pulsar’s destructive effect is its intense, high-energy radiation and its strong stellar wind, which gradually erode the companion star.
Flickering stars as search helpers
So far, only about two dozen black widow pulsars are known, so astronomers are always on the lookout for other representatives of such “cannibalistic” pairs – mostly they are looking for the high-energy X-rays and gamma rays that the pulsar emits. However, Kevin Burdge from the Massachusetts Institute of Technology (MIT) in Cambridge and his team looked for such pulsars in a different way: “I thought to myself: instead of looking for the pulsar directly, we could try to find the one it ‘cooked’ to find the star,” explains Burg. According to the idea behind it, this star should be heated up considerably on the side facing the pulsar and therefore be brighter. In theory, one should therefore be able to detect such pairs when looking for stars that change their brightness rapidly and drastically.
For their study, the astronomers evaluated observation data from the Zwicky Transient Facility in California, a telescope that scans large sections of the sky every night for changing or emerging phenomena. In the data on around 20 million stars, they looked for objects whose brightness fluctuated by a factor of ten or more at intervals of one hour or less. In fact, the astronomers found what they were looking for: They discovered an object that becomes around 13 times brighter every 62 minutes and then fades again. Dubbed ZTF J1406+1222, the system lies around 3,000 light-years away and its spectrum suggests that there are two stellar objects orbiting closely there, the team reports.
“Black Widow” with unusual features
According to astronomers, there is some evidence that this system is a black widow — a close pulsar-star pair. “What we do know for sure is that what we’re looking at here is a star whose day side is much hotter than the night side, and it’s orbiting something else over the course of 62 minutes,” says Burdge. “Everything seems to indicate that this is a black widow binary.” If so, this would be the first such pulsar to be detected by visible-light observations. It would also be the pulsar system with the shortest known orbital period. “However, there are some unusual things about this system, so it could also be something completely new,” the astronomer continues. No X-rays or gamma rays, which would normally be typical of a widow’s pulsar, have been detected from this system.
Complementary observational data from the Sloan Digital Sky Survey revealed that the close, rapidly rotating pair is orbited by a third, much more distant object. This star is about 600 astronomical units apart and takes about 12,000 years to orbit. In addition, spectral analysis suggested that the entire system is likely older than our solar system and must have originally formed in a globular cluster on the outskirts of the Milky Way. Since then, however, the pulsar and its companions may have moved close to the galactic center, where the system was ejected from its star cluster by gravitational turbulence, astronomers suspect.
“So this is a system that is unique to a black widow because we detected it in visible light, it has an additional distant companion, and it came out of the galactic center,” says Burdge. “There’s still a lot we don’t understand about this system.” The team of astronomers hopes that some questions will be clarified as they study ZTF J1406+1222 with other telescopes, but also as they potentially discover other representatives of such optically visible black widows discover.
Source: Kevin Burdge (Massachusetts Institute of Technology, Cambridge) et al., Nature, doi: 10.1038/s41586-022-04551-1