A fast-spinning pulsar hungrily feeding on a close companion star has been found 3,000 light-years from Earth in a rare type of cannibalistic system known as a black-widow binary.
Around two dozen black-widow binaries are currently known in the Milky Way galaxy. They typically feature a pulsar, which is the spinning neutron-star remnant of a massive star that exploded in a supernova, stealing matter from a companion star. As the infalling matter accumulates onto the surface of the pulsar, a torrent of X-rays and gamma rays are unleashed, which further erodes and destroys 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.
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Scientists from the Massachusetts Institute of Technology, led by Kevin Burdge, found ZTF J1406+1222 in data collected by the Zwicky Transient Facility at Palomar Observatory in California. 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 create a substantial difference in brightness between the companion’s two hemispheres, and that as the companion star and pulsar orbit one another, we would see the star periodically brighten and dim as we see 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.
To verify that this would work, Burdge’s team first used this method to identify some of the previously known black-widow binaries. Then, after sifting through data collected by the Zwicky Transient Facility, they identified a new object varying in brightness by a factor of 13 every hour, and which did not seem related to any previously known type of variable star. This was ZTF J1406+1222, and further study of archive data from the European Space Agency’s Gaia astrometric mission revealed the presence of the third star on a wide orbit.
However, ZTF J1406+1222 doesn’t seem to be a typical black-widow binary, since no X-rays or gamma-rays have been detected 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.”
Astronomers believe that most black-widow binaries form inside globular clusters. If such clusters wander too close to the center of our Milky Way galaxy, gravitational tides from the supermassive black hole there can tear the clusters apart and disperse their stars throughout 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 reported in a paper in Nature that was published online today (May 4).
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