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Space spider caught spinning a web of gamma rays

In a binary "spider" star system, a pulsar devours the gaseous outer layers of its trapped victim.

By Elizabeth Rayne
Liz Pulsar GETTY

Spiders in space? The actual creepy crawlies have made the journey beyond our atmosphere, but this is what happens when a star morphs into a spider.

Astronomers discovered the first example of a binary star system known as a “spider” in a phase that had never been observed before. This system is 2,600 light years away and thought to include a millisecond pulsar (ridiculously fast-spinning neutron star). Orbiting each other are the supposed pulsar and a dying star slowly turning into a white dwarf. Like a spider spinning silk around prey and sucking its life out, the pulsar in this death spiral devours the gaseous outer layers of its trapped victim.

This system is also the source of gamma rays whose origins were a mystery until now. Astronomer Sam Swihart of the U.S. Naval Research Laboratory, and his research team used the SOAR Telescope in Chile to catch this phenomenon just one phase short of its end. The star currently morphing into a white dwarf will actually become one for real after it has contracted enough, now that the pulsar has eaten the outside. Swihart led a study recently posted on the preprint server arXiv.

“We have found this binary system in kind of a ‘second to last’ stage of its evolution,” he told SYFY WIRE. “We have observed many systems in the end stage but not in this penultimate stage, so finding this system is a missing link in our understanding of binary millisecond pulsar evolution.”

It was possible to tell that the proto-white dwarf was orbiting something huge without actually seeing what it was orbiting. When an object moves towards the observer, wavelengths of light get shorter, moving into the ultraviolet realm (blueshift), while wavelengths grow longer and shift to the infrared when that object moves further away (redshift). This was visible in the absorption lines of the white dwarf. Absorption lines show up in a spectrum when there is an absorbing material, like a cloud of gas, between the source of light and the observer.

Further measurements of radial velocity — velocity from the observer’s line of sight — revealed the white dwarf was orbiting an unseen object about every 15 hours. How high the radial velocity curve went eventually gave away how massive this object was. It was also spewing out gamma rays nonstop, which made it most likely to be a neutron star and even more likely to be a millisecond pulsar, since all millisecond pulsars emit gamma rays. Observations by NASA’s Swift and ESA’s XMM-Newton space telescopes found it was also emitting X-rays.

“We only know of about 5 extremely-low-mass (ELM) white dwarfs with these short orbital periods (about a day or shorter) around a confirmed millisecond pulsar,” Swihart said. “The main difference between those systems and this one is that the radius of the companion in this system is around 5 times larger.”

Swihart and his team were able to find out the white dwarf’s orbital period, surface gravity, temperature, and radius so they could figure out how long it took to evolve and ultimately, about how old it is. This is also how they realized that the white dwarf is inflamed and still contracting. It will take two billion years for it to to finish evolving so it ends up like the other ELM white dwarfs in “spider” systems. If the gamma rays really are from a millisecond pulsar, that could mean there are other undiscovered spiders lurking in the darkness.

“This system is relatively nearby, so confirmation of a millisecond pulsar would provide strong evidence that the millisecond pulsar census is far from being complete, even relatively close to the Sun,” said Swihart.