Astronomers just spotted a record-breaking "black widow" pulsar consuming its victim

"We don't understand how the stars could get into such a tight orbit."
Grant Currin
ZTF at night.Caltech

Every two nights, a telescope near San Diego, California, meticulously scans the entire night sky. That telescope, the Zwicky Transient Facility (ZTF), scans millions of stars in search of celestial objects exhibiting unusual behavior.

In a paper published Wednesday in the peer-reviewed journal Nature, a team of authors describes what appears to be a pulsar and a small star orbiting each other faster than researchers have ever seen. But it won't continue forever. This pulsar — a "black widow" to astronomers — is consuming matter and energy from its doomed companion. 

Stellar mysteries are encoded in the flicker of starlight

Stars are not "boring, static objects," says physicist Tom Prince, a researcher at ZTF and co-author on the new paper. "[A] large fraction of stars exhibit dips, pulsations, or periodic brightenings that are a key to understanding their nature." ZTF makes it possible for researchers to get a more detailed look at some of these patterns than ever before.

The telescope spotted the unusual system described in the new paper when post-doctoral researcher Kevin Burdge, now at MIT, designed an algorithm that finds objects that get much brighter or much dimmer over the course of just 80 minutes. The algorithm brought J1406+1222, the subject of the new paper, to the researchers' attention. Its brightness varies by a factor 13 every 62 minutes. The telescope can't see it well enough to directly observe what's going on, but it can see enough for researchers to know that something strange is going on.

"This 62-minute orbit is remarkable because we don't understand how the stars could get into such a tight orbit," Burdge says. "The process of the pulsar ablating its companion should actually drive them apart. This is pushing the boundaries of what we thought possible."

The "black widow" pulsar devours its prey

If the researchers are right, what they're seeing is the small companion star, a relatively cool brown dwarf, spinning on its axis. The black widow pulsar's influence on the brown dwarf has caused one side of the smaller star to grow colder than the other side. When the hot side faces Earth, the whole system appears bright to observers in San Diego. When it spins away, the system gets dim.

The two objects aren't the only stars in the system. Researchers have spotted a third star that orbits the other two — from a great distance. It takes the two primary stars in the system just over an hour to orbit each other. The third star's journey takes 10,000 years.

This is a convincing explanation for what ZTF observed, but it may not be the whole story. "Our data indicate we are looking at a black widow binary, but it could be something entirely new," Burdge says. Data from NASA's Chandra X-ray Observatory, a space telescope that orbits in a large ellipse around Earth, will confirm the finding.

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