The shortest-ever binary ‘black widow’ has been discovered by astronomers

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MIT researchers have discovered a new and strange system 3,000 light years away thanks to the flashing of a neighbouring star. Another “black widow binary” has been found in our galaxy. This pulsar is slowly devouring its smaller companion star, like the arachnid in the pair’s name does.

About a dozen black widow binaries have been discovered in the Milky Way. For the first time ever, the pulsar and companion star ZTF J1406+1222 have been found to orbit each other every 62 minutes. In this system, there appears to be a third star that orbits the two inner stars every 10,000 years.

The emergence of a system as complex as this triple black widow begs the question of how it came to be. The MIT team has come up with a genesis tale based on its observations: It is believed that the triple system, like most black widow binaries, evolved from a dense group of old stars known as a globular cluster. In this case, the gravitational pull of the Milky Way’s central black hole may have been enough to separate the cluster from the triple black widow.

A Pappalardo Postdoctoral Fellow in the MIT Department of Physics, Kevin Burdge, describes it as a “complex birth scenario. When it comes to the Milky Way’s history, “this system is possibly as old as the sun itself”

The team’s discovery is described in full in a study written by Burdge and published in Nature. Triple system was detected using an unconventional method. ZTF J1406+1222 was discovered by the team using visible light, and specifically the flashing of the binary’s companion star, rather than the gamma and X-ray radiation generated by the core pulsar.

For Burdge, “this system is extremely unique in terms of black widows” because of its broad companion and the fact that it originated from our galaxy’s nucleus. “A lot of it remains a mystery to us. However, we’ve discovered a new approach to search for these constellations in the sky.”

All of the researchers in the study are from different institutions, including the University of Warwick, Caltech, the University of Washington and McGill University.

During the day and at night

Neutron stars that have collapsed into black widow binaries are powered by pulsars; these are rapidly rotating neutron stars. Because of their fast rotational rate and the high-energy radiation they emit during this process, pulse radiated from them can be quite disorienting to observers.

It’s not unusual for a pulsar to rapidly spin down and die as it expels a lot of energy. A pulsar can be given fresh life by a passing star, although this only happens once in a while. When a nearby star approaches, the pulsar’s gravity pulls material away from the star, providing additional energy to spin the pulsar up again. The “recycled” pulsar then begins to radiate energy that weakens and eventually destroys the star.

In the same way as a spider eats its own kind, these systems are known as black widows because its pulsar devour the organism that recycled it.

The pulsar’s gamma and X-ray flares have spotted every black widow binary to date. Burdge discovered ZTF J1406+1222 by the companion star’s optical flashing, which was a first.

Due to the pulsar’s continual high-energy radiation, the companion star’s day side—the side that is always facing the pulsar—can be many times hotter than the night side.

After thinking about it, Burdge decided to search for the star it is frying instead of seeking for a pulsar in the night sky.

Astronomers would be alerted to the fact that the brightness of a star would fluctuate by a large amount if they saw it in the company of a pulsar, according to his theory.

The movement of the stars

A California-based telescope called the Zwicky Transient Facility (ZTF) was used to test Burdge’s idea by analysing optical data from the night sky. To see whether any stars were changing rapidly in brightness by a factor of 10 or more on a timescale of approximately an hour or less, the team looked at the brightness of stars to see if they had a partner star orbiting a pulsar.

Validation of the new method’s accuracy was achieved by identifying the twelve known black widow binaries. It was then that they discovered ZTF J1406+1222, a star whose brightness varied by a factor of 13 every 62 minutes, suggesting that it was part of a new black widow double.

The European Space Agency’s Gaia space telescope took measurements of the star’s position and velocity, and they used those findings to find out more about it. The team discovered that the binary was being pursued by another distant star, based on data taken decades ago by the Sloan Digital Sky Survey. This third star appeared to be circling the inner pair every 10,000 years, based on their estimates.

The pulsar in the pair has not been identified directly by the astronomers, which is usually how black widows are confirmed. As such, the team believes that ZTF J1406+1222 is a possible black widow binary.

A star with a day side significantly hotter than the night side, which orbits around something every 62 minutes, is all we know for sure, says Burdge. “A black widow binary seems to be the most logical conclusion to be drawn from the evidence. Is it anything completely new? There are a few oddities about it.”

This novel system will be studied in-depth, and the optical technology will also be used for the illumination of other nearby neutron stars and black widows in the sky.

Further information: Kevin Burdge, A 62-minute orbital period black widow binary in a wide hierarchical triple, Nature (2022). DOI: 10.1038/s41586-022-04551-1. www.nature.com/articles/s41586-022-04551-1

Journal information: Nature

Source: Massachusetts Institute of Technology