Mysterious 'Black Widow' Binary With Shortest Orbit Yet Discovered. Here's What We Know So Far

The stellar oddity is a new "black widow binary", which is a rapidly spinning neutron star, or pulsar, that is circling and slowly consuming a smaller companion star.

The pulsar and the companion star circle each other every 62 minutes. The black widow binary is unique because it appears to host a third, far-flung star that orbits the two inner stars every 10,000 years. | Representational Image

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New Delhi: Astronomers at the Massachusetts Institute of Technology (MIT) in the United States have discovered a new and mysterious system 3,000 light-years from Earth, thanks to the flashing of a nearby star. The stellar oddity is a new "black widow binary", which is a rapidly spinning neutron star, or pulsar, that is circling and slowly consuming a smaller companion star. This is what the arachnid "black widow" does to its mate. 

Pulsars are rapidly spinning neutron stars which are collapsed cores of massive stars, and have a dizzying rotational period. They spin around every few milliseconds, and emit flashes of high-energy gamma and X-rays in the process. 

As many as two dozen black widow binaries in the Milky Way galaxy are known to astronomers. The mysterious black widow binary, dubbed ZTF J1406+1222, has the shortest orbital period yet identified. The study was recently published in the journal Nature. 

What Makes The Black Widow Binary Unique?

The pulsar and the companion star circle each other every 62 minutes, according to the study. The black widow binary is unique because it appears to host a third, far-flung star that orbits the two inner stars every 10,000 years. 

How Do Triple Black Widows Form?

Astronomers are trying to figure out how systems such as this likely triple black widow form. The MIT team, based on its observations, has proposed an origin story, which states that the triple system, like most black widow binaries, could have arisen from a dense constellation of old stars known as a globular cluster. According to the researchers, the globular cluster may have drifted into the Milky Way's centre, where the gravity of the central black hole was enough to pull the cluster apart while leaving the triple black hole intact. 

In a statement issued by MIT, Kevin Burdge, the lead author of the study, said it is a "complicated birth scenario", and that the system has probably been "floating around in the Milky Way for longer than the sun has been around." 

How Did Astronomers Detect The Triple System?

A new approach was used to detect the triple system. The researchers used visible light, and specifically the flashing from the binary's companion star, to detect the new black widow binary, unlike most black widow binaries which are found through the gamma and X-ray radiation emitted by the central pulsar.

Burdge said that the system is really unique as far as black widows go, because the team found it with visible light. It is also unique because of the wide companion star, and the fact that the visible light came from the galactic centre. The researcher said there is still a lot that is not known about it, but astronomers have a new way of looking for these systems in the sky.

What Role Do Pulsars Play In Black Widow Binaries?

Pulsars power black widow binaries. Since pulsars burn off a huge amount of energy, they usually spin down and die quickly. However, a passing star can save a pulsar by giving it a new life. As a star approaches the pulsar, the former's material is pulled off due to the pulsar's gravity. The material provides new energy to spin the pulsar back up, following which the "recycled" pulsar starts re-radiating energy. This, in turn, further strips the star, and eventually destroys it.

Burdge explained that these systems are called black widows because of how the pulsar consumes the thing that recycled it, just as the spider eats its mate.

How Is The Discovery Important?

To date, every black widow binary has been detected through gamma and X-ray emissions from the pulsar. The discovery of ZTF J1406+1222 marks the first time a black widow binary has been observed through the optical flashing of the companion star.

According to the study, the companion star's day side, which is the side perpetually facing the pulsar, can be many times hotter than its night side. This is due to the constant high-energy radiation the star receives from the pulsar. Burdge said he thought that instead of looking directly for the pulsar, he should try looking for the star that it is cooking. If astronomers observed a star whose brightness was changing periodically by a huge amount, it would be a strong signal that it was in a binary with a pulsar, the researcher explained.

Surprising Findings Made By The Astronomers

In order to test the theory, the study authors looked through optical data taken by the Zwicky Transient Facility, an observatory based in California that takes wide-field images of the night sky. They 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. These are signs that indicate the presence of a companion star orbiting tightly around a pulsar. 

Using the new technique, the researchers were able to pick out the dozen known black widow binaries. This validated the new method's accuracy. Then, they observed a star whose brightness changes by a factor of 13, every 62 minutes. This indicated it was likely a part of a new black widow binary, which they named ZTF J1406+1222. 

According to the study, the researchers looked up the star in observations taken by Gaia, a space telescope operated by the European Space Agency. They looked back through decades old measurements of the star from the Sloan Digital Sky Survey, and found that the binary was being trailed by another distant star, which appeared to be orbiting the inner binary every 10,000 years. 

Surprisingly, the team has not directly detected gamma or X-ray emissions from the pulsar in the binary, which is the typical way to confirm black holes. Burdge said the one thing the astronomers know for sure is that they saw a star with a day side that is much hotter than the night side, orbiting something every 62 minutes. Since there are a few weird things about the cosmic object, it is possible that it is something entirely new.