Rare ‘black widow’ binary star with shortest orbit ever identified

PTI, May 7, 2022, 8:45 AM IST

Image Source: MIT News

Scientists have discovered a rare ”triple black widow” system — a pair of stars that rapidly circle each other before one is consumed by the other — located some 3,000 light-years away.

The star system named ”ZTF J1406+1222” has the shortest known orbit of any black widow binary i.e. 62 minutes, according to researchers.

What makes this system unique is that it contains a third star that circles the central pair every 10,000 years, according to the finding published in the journal Nature on May 4.

A team led by scientists from Massachusetts Institute of Technology (MIT) in the US found the stellar oddity, which appears to be a new black widow binary — a rapidly spinning neutron star or pulsar that is circling and slowly consuming a smaller companion star.

The system derives its name from the ”black widow” spiders, in which the female eats the male after mating.

Astronomers know of about two dozen black widow binaries in the Milky Way.

The research, which also involves astronomers from the University of Sheffield in the UK, suggests that ”ZTF J1406+1222” has the shortest orbital period yet identified, with the pulsar and companion star circling each other every 62 minutes.

The study used HiPERCAM, a high-speed camera developed by researchers at the University of Sheffield that can take more than 1,000 optical images per second, to find the exotic triple black widow.

The discovery has raised questions about how such a system could have formed, with researchers speculating that as with most black widow binaries, the triple system likely arose from a dense constellation of old stars known as a globular cluster.

This particular 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 widow intact.

With the help of HiPERCAM, the research team used a new approach to detect the triple system. While most black widow binaries are initially found through the radio and gamma ray radiation emitted by the central pulsar, the team used visible light, and specifically the varying light from the binary’s companion star to discover ZTF J1406+1222.

”Thanks to the extraordinary sensitivity of the Sheffield-led HiPERCAM camera, we have discovered the most extreme member of the black widow class of binary star, along with a promising new method of detecting such systems,” Professor Vik Dhillon, co-author of the study from the University of Sheffield’s Department of Physics and Astronomy, said.

Black widow binaries are powered by pulsars — rapidly spinning neutron stars that are the collapsed cores of massive stars.

Pulsars have a dizzying rotational period, spinning around every few milliseconds, and emitting flashes of radio and high-energy gamma rays in the process.

Normally, pulsars spin down and die quickly as they burn off a huge amount of energy, but every so often, a passing star can give a pulsar new life.

As a star nears, the pulsar’s gravity pulls material off the star, which provides new energy to spin the pulsar back up. The “recycled” pulsar then starts reradiating energy that further strips the star, and eventually destroys it.

Astronomers discovered the companion star’s day side — the side perpetually facing the pulsar — can be many times hotter than its night side, due to the constant high-energy radiation it receives from the pulsar.

This allowed them to find the new black widow by looking for the companion star instead of looking directly for the pulsar.

The new method could make it easier to discover black widows in the future, as a star whose brightness is changing periodically by a huge amount is a strong signal that it’s a binary with a pulsar, said the research supported by the National Science Foundation.