Record-breaking 'black-widow' pulsar found just 3,000 light-years from Earth
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Record-breaking ‘black-widow’ pulsar found just 3,000 light-years from Earth

An artist’s impression of a black-widow pulsar tearing material from a companion star. (Image credit: NASA’s Goddard Space Flight Center)

A fast-spinning pulsar hungrily feeding on a nearby companion star has been discovered 3,000 light-years away in a black-widow binary, a rare type of cannibalistic system.

There are currently approximately two dozen black-widow binaries known in the Milky Way galaxy. A pulsar is a spinning neutron-star remnant of a big star that burst in a supernova, stealing matter from a companion star. As the infalling matter accumulates on the pulsar’s surface, a torrent of X-rays and gamma rays is unleashed, further eroding and destroying the companion.

The newly discovered black-widow binary, designated ZTF J1406+1222 (the numbers refer to its coordinates on the sky), is the tightest black-widow binary known, with the pulsar and the companion locked in an orbital dance with a period of just 62 minutes. There’s also a third star in the system on a very wide orbit of 10,000 years.

ZTF J1406+1222 was discovered in data collected by the Zwicky Transient Facility at Palomar Observatory in California by scientists led by Kevin Burdge of the Massachusetts Institute of Technology. As the accreting matter falls onto the pulsar, the extra angular momentum that it provides acts to spin the pulsar up by such a degree that it begins rotating at hundreds of times per second. Such rapidly spinning pulsars are referred to as millisecond pulsars.

As the pulsar spins up, it unleashes increasing amounts of radiation that heat the companion star’s pulsar-facing side. Burdge’s team reasoned that this would result in a significant brightness difference between the companion’s two hemispheres, and that as the companion star and pulsar orbits the earth one another, we would see the star periodically brighten and dim as we saw first its brighter pulsar-facing side and then its cooler far side.

“I thought, instead of looking directly for the pulsar, try looking for the star that it’s cooking,” said Burdge in a press statement.

Burdge’s team first used this method to detect some of the previously known black-widow binaries to verify that it would work. After analyzing through data collected by the Zwicky Transient Facility, they discovered a new object whose brightness changed by a factor of 13 every hour and did not appear to be related to any previously recognized form of variable star. This was ZTF J1406+1222, and further analysis of archive data from the European Space Agency’s Gaia astrometric mission revealed the presence of a third star in a broad orbit.

ZTF J1406+1222, on the other hand, does not appear to be a typical black-widow binary, as no X-rays or gamma-rays have been found coming from it. The detection of this radiation is necessary to confirm the existence of a pulsar accreting matter. Consequently, the system is only considered a candidate for now.

“Everything seems to point to it being a black-widow binary,” said Burdge. “But there are a few weird things about it, so it’s possible it’s something new entirely.”

Most black-widow binaries, according to astronomers, form inside globular clusters. If such clusters get too close to the center of our Milky Way galaxy, gravitational tides from the supermassive black hole there can break them apart and disperse their stars across the galaxy.

“It’s a complicated birth scenario,” said Burdge. “This system has probably been floating around the Milky Way for longer than the sun has been around.”

The discovery is detailed in a paper.

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