Astronomers have spotted a “black widow binary”— a unique system that consists of a pulsar (rapidly spinning neutron star) that is circling and slowly consuming a smaller companion star, just like the female black widow spider does to its mater, hence the name. Astronauts have previously identified about two dozen black widow binaries in the milky way galaxy but the newest candidate has the shortest orbital period yet identified.
The discovery has been reported in a research article titled, “A 62-minute orbital period black widow binary in a wide hierarchical triple,” published in Nature Communications.
Named ZTF J1406+1222, the system has a pulsar and a companion star that circle each other every 62 minutes. Another thing that makes the system unique apart from the short orbital period is the fact that it seems to host a third far-flung star that orbits the other two every 10,000 years.
This possibly ‘triple black widow’ has raised questions about how it could have formed. The MIT researchers who made the discovery have proposed a theory: Just like with most black widow binaries, the triple system likely arose from a dense constellation of old stars known as a globular cluster.
The particular cluster from which this system formed may have drifted towards the supermassive black hole at the centre of the Milky way. The gravity of this central black hole must have been enough to pull the cluster apart while leaving the triple black widow intact.
“It’s a complicated birth scenario. This system has probably been floating around in the Milky Way for longer than the sun has been around. It 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,” said Kevin Burdge, a Pappalardo Postdoctoral Fellow in MIT’s Department of Physics, in a press statement.
Pulsars are rapidly spinning neutron stars that are the collapsed cores of massive stars. They have an incredibly fast rotational period, spinning around every few milliseconds and emitting flashes of high energy gamma and X-rays while doing so.
Typically, pulsars spin down and die quickly as they burn huge amounts of energy in a short amount of time. But every once in a while, a passing star can ‘refuel’ them. As a star nears a pulsar, the latter’s gravity pulls material off the star, providing new energy to spin the pulsar back up. This ‘reignited’ pulsar then starts reradiating energy that strips the star further until it is completely destroyed.
Every black widow binary discovered to date was detected due to the gamma and X-ray flashes from the pulsar. But for this system, Burdge cam upon it through the optical flashing of the companion star.
This was possible because the companion star’s dayside (the side always facing the pulsar) can be many times hotter than the night side due to the radiation it receives from the pulsar. Burdge reasons that if astronomers observed a star whose brightness was changing periodically by a huge amount, it would be a strong signal that it was a binary with a pulsar.