University of Warwick: Astronomers discover a rare ‘black widow’ binary, with the shortest orbit yet

The flashing of a nearby star has drawn a team of astronomers including the University of Warwick to a new and mysterious system 3,000 light years from Earth.

The stellar oddity 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, as its arachnid namesake does to its mate.

Astronomers know of about two dozen black widow binaries in the Milky Way. This newest candidate, named ZTF J1406+1222, has the shortest orbital period yet identified, with the pulsar and companion star circling each other every 62 minutes. The system is unique in that it appears to host a third, far-flung star that orbits around the two inner stars every 10,000 years.

The researchers used a new approach to detect the triple system. While most black widow binaries are found through the gamma and X-ray radiation emitted by the central pulsar, the team used visible light, and specifically the flashing from the binary’s companion star, to detect ZTF J1406+1222.

This new approach took advantage of the highly sensitive HiPERCAM instrument. HiPERCAM was funded through a European Research Council (ERC) grant and is the latest development from long-term support for researchers at the Universities of Warwick and Sheffield working on high-speed astronomical cameras funded by the Science and Technology Facilities Council (STFC).

Professor Tom Marsh from the University of Warwick Department of Physics said: “With the unique sensitivity of HiPERCAM on the Gran Telescopio Canarias in La Palma we could directly detect the system at its faintest and in the ultraviolet, allowing us to rule out the presence of a white dwarf. This left a neutron star as the only explanation for the dramatic heating of the star that is visible.”

This likely triple black widow is raising questions about how such a system could have formed. Based on its observations, the MIT team proposes an origin story: 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 center, where the gravity of the central black hole was enough to pull the cluster apart while leaving the triple black widow intact.

“It’s a complicated birth scenario,” says Kevin Burdge, a Pappalardo Postdoctoral Fellow in MIT’s Department of Physics. Burdge is the lead author of a study appearing in Nature that details the team’s discovery. “This system has probably been floating around in the Milky Way for longer than the sun has been around.”

The study’s co-authors are collaborators from multiple institutions, including the University of Warwick, Caltech, the University of Washington, McGill University, and the University of Maryland.

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 high-energy gamma and X-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.

“These systems are called black widows because of how the pulsar sort of consumes the thing that recycled it, just as the spider eats its mate,” Burdge says.

Every black widow binary to date has been detected through gamma and X-ray flashes from the pulsar. In a first, Burdge came upon ZTF J1406+1222 through the optical flashing of the companion star.

It turns out that 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.