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We May Finally Know Where Vicious 'Black Widow' Pulsars Come From

Pulsar in Binary System
An illustration shows a pulsar in a binary system.
(Image: © ESA)

Vicious, fast-blinking "black widow" and "redback" pulsars dot the night sky. These violent stars blast their smaller stellar partners to bits as they whip them around in tight binary orbits, cannibalizing the smaller partners in the process. And, in a new paper, scientists have revealed the origin story behind these hungry stars.

It's no coincidence that astronomers named these systems — places in space where a tiny, heavy, fast-spinning neutron star is energizing itself by ripping apart a small binary partner — after deadly spiders. Both redback and black widow females eat the male alive after sex. (In stars, as in spiders, black widows hook up with smaller partners.) Redback and black widows are subcategories of "millisecond pulsars," neutron stars that spin so fast that they flash Earth every few fractions of a millisecond. But, until now, no one could explain how these nasty stars formed.

Neutron stars are the ultradense remnants of collapsed stars. No wider than a small city, they nevertheless outweigh our sun. Scientists have had to invent all-new physics to explain how matter behaves inside of them. (But unlike black holes, they aren't quite dense enough to form singularities.) Scientists call them pulsars, because they often appear to telescopes as regularly pulsing light sources. Most spin far faster than normal stars, and their regular rotations can act like clocks ticking away in space.

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But a neutron star on its own won't typically spin fast enough to be a millisecond pulsar, the researchers wrote in the new study. Some external source of energy must kick the pulsar up to its rotational speed. That's why most millisecond pulsars turn up in binary systems. Astronomers believe that typically, a white dwarf collapses into a neutron star, then at some point down the line starts sucking a stream of matter off its binary twin. The energy from that stream of matter sets the neutron star spinning much faster than it did at birth.

Redbacks and black widows don't generally fit this model, though. Often the heavier partner in their little binary systems, locked in tight orbits, their intense X-ray beams blast matter off the surfaces of their companion star, knocking that miniature star into space and then sucking it back in with gravity. The masses and energies moving around these systems are very unusual compared with typical millisecond pulsar systems. As a result, the researchers wrote, the normal model for how companion stars accelerate millisecond pulsars doesn't seem to apply.

In the new paper, published Aug. 14 in The Astrophysical Journal, a team of researchers refined that model. Their paper takes into account the powerful magnetic energy of neutron stars and shows how a neutron star's magnetism could confine all the matter blasted off the companion star at the neutron star's north and south poles. That changes the underlying mechanics of the situation, they wrote, and shows that even the smaller partner in redback systems and many black widow systems could accelerate the pulsars to millisecond speeds.

This magnetism theory can’t explain all the black widows we know about, however. But this work should eliminate the need for certain more dramatic theories — like the one published in The Astrophysical Journal in 2015, suggesting that perhaps these sorts of neutron stars are simply born as millisecond pulsars and don't need any help accelerating.

Originally published on Live Science.