Astronomers Discover the Exotic ‘Black Widow’ – An Unusual System of ‘Cannibal’ Pulsar and Two Stars

Cosmic cannibal: astronomers have discovered an unusual representative of “black widows” – pulsars that destroy their companion stars. With an orbital period of just 62 minutes, the pulsar and its companion star form the closest and fastest pair of its kind. However, the absence of X-rays, a third star in Bunde and the origin of this system in a globular cluster that has migrated across the galaxy make it as enigmatic as it is extraordinary.

Like the spiders of the same name, cosmic black widows bring death to their stellar partners. These rapidly spinning pulsars form binary systems with a low-mass companion star, but over time destroy their stellar partner. The reason for this is the intense, high-energy radiation from the pulsar and its strong stellar wind, which gradually erode the companion star. So far, only about two dozen such “spider” pulsars are known.

Telltale flicker

But now astronomers have discovered a “black widow” unusual in several respects. It starts with their discovery: Kevin Burdge of the Massachusetts Institute of Technology (MIT) in Cambridge and his team did not detect the pulsar system via X-rays and gamma rays from the pulsar, but via the twinkling of its companion star. Since the side facing the pulsar is hotter than the side facing it, its brightness should vary rapidly and widely depending on orientation.

For their search for “spider pulsars,” Burdge and his team evaluated observational data from the Zwicky Transient Facility in California, a telescope that scans large areas of the sky for changing or emerging phenomena. In data on about 20 million stars, astronomers looked for objects that fluctuated in brightness by a factor of ten or more at intervals of an hour or less.

Cannibal Pulsar with a close companion

In fact, the researchers found what they were looking for: they discovered an object about 3,000 light-years away that becomes about 13 times brighter every 62 minutes and then fades again. According to the astronomers, this pattern and the system’s spectral characteristics suggest that it is a binary system in which one partner is unilaterally heated – as would be the case for a cosmic black widow.

“What we know for sure is that what we’re looking at here is a star whose dayside is much hotter than its nightside, and orbiting something else in 62 minutes,” Burdge said. “Everything seems to indicate that it is a black widow pulsar.” If so, the system, dubbed ZTF J1406+1222, 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.

Radiation missing and a third

But that’s not all: “There are some unusual things in this system, so it could also be something completely new,” says Burdge. So far, no X-rays or gamma rays emitted by this system have been detected, and no source of radiation could be identified at this position in the archived radio data either. It is unknown why this radiation, which is normally typical of a widow pulsar, is absent from the ZTF J1406+1222.

A third party is also unusual: Additional observational data from the Sloan Digital Sky Survey has revealed that the nearby, fast-rotating pair orbits a third, much more distant star. This metal-rich dwarf star is about 600 astronomical units distant and takes about 12,000 years to orbit – so it’s much more loosely bound than the inner pair.

Unique among pulsars

This triple hierarchical formation, combined with the short orbital period of the pulsar and its inner partner, sets this black widow apart from all spider pulsars known so far, the astronomers explain. Additionally, spectral analyzes suggest that ZTF J1406+1222 is likely older than our solar system. This raises the question of where and how this exotic system was born.

From their data, the team concludes that the pulsar and its companions originally resided in a globular cluster on the outskirts of the Milky Way. However, this cluster of stars must then have reached the center of the galaxy, where it was torn apart by gravitational turbulence. Astronomers have long speculated that the millisecond pulsars discovered near the galactic center could also come from such disrupted globular star clusters.

“So this system is unique for a black widow because we detected it in visible light, it has an additional distant companion, and because it exited the galactic center,” Burdge says. “There’s still a lot we don’t understand about this system.” (Nature, 2022; doi: 10.1038/s41586-022-04551-1)

Source: Massachusetts Institute of Technology

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