Rare “black widow” stars in deadly cosmic dance

Some species of black widow spiders are known for their deadly mating rituals – females infamously eat their diminutive partners after coitus. But this is an article about stars, right? Right. When a dense neutron star consumes a smaller neighbour, astronomers say they have witnessed a “black widow” binary system.

The powerhouse of black widow binaries are pulsars – rapidly spinning neutron stars which are the collapsed cores of now dead massive stars. Pulsars spin at a dizzying rate, rotating every few milliseconds, and shoot out high-energy radiation in the form of gamma and X-rays. Normally pulsars die down, but can become re-energised when they pick up smaller stars which stray too close to the pulsar’s immense gravitational pull.

The “recycled” pulsar gets new life, destroying the smaller star in the process. This is what eventually happens in black widow binary systems – hence the name and our diversion into the world of arachnids.

There are a couple of dozen known black widow binaries in the Milky Way. Massachusetts Institute of Technology (MIT) astronomers in the US have potentially found a new one, named ZTF J1406+1222, about 3,000 light years away. The discovery is the subject of a new Nature article.

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The new black widow binary has the shortest orbital period yet observed for this kind of star pair, at just 62 minutes. Three’s a crowd, but it seems this pair is actually a trio with a third star orbiting the two inner stars every 10,000 years.

Such a system raises questions about how it was formed. The team theorises that the triplet was originally a part of a large, dense cluster of stars. As the cluster moved closer to the supermassive black hole in the centre of the Milky Way, the black hole tore stars from the group, leaving behind the three we see today.

“It’s a complicated birth scenario,” says Kevin Burdge, a Pappalardo Fellow in MIT’s Department of Physics. “This system has probably been floating around in the Milky Way for longer than the Sun has been around.”

Normally, black widow binaries are found by observing the radiation emitted by the central pulsar. This time, the MIT team’s discovery was made by observing the flashing visible light coming from the pulsar’s smaller companion.

“I thought, instead of looking directly for the pulsar, try looking for the star that it’s cooking,” Burdge explains. Cooking is right. The companion star’s “day side” – the side which is perpetually facing the pulsar – can be several times hotter than the other side due to the constant radiation being shot at it by the pulsar.

The team looked through data taken from California’s Zwicky Transient Facility. If a star’s brightness changed by a factor of 10 or more on a timescale of about an hour or less, it was assumed to be a companion star orbiting tightly around a pulsar. The method was validated when the team correctly identified a dozen known black widow binaries.

They then spotted the star labelled ZTF J1406+1222, whose brightness changed by a factor of 13 every 62 minutes. Looking at data from the European Space Agency’s Gaia space telescope, the team noticed the third star trailing behind the central pair.

“This system is really unique as far as black widows go, because we found it with visible light, and because of its wide companion, and the fact it came from the galactic centre,” Burdge says. “There’s still a lot we don’t understand about it. But we have a new way of looking for these systems in the sky.”

Oddly, though, the team have not detected the usual gamma or X-ray emissions expected from a pulsar in a black widow binary. “The one thing we know for sure is that we see a star with a day side that’s much hotter than the night side, orbiting around something every 62 minutes,” Burdge says. “Everything seems to point to it being a black widow binary. But there are a few weird things about it, so it’s possible it’s something entirely new.”

Further research on the system will be done and the team also plans to use their new method to identify more black widows.

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