Rare 'black widow' star system could help unlock the secrets of space-time
Universe

Rare ‘black widow’ star system could help unlock the secrets of space-time

This cannibal star system could be a gravitational wave detecting machine.

A dead star blasts a powerful blast of radiation toward our planet every 4 milliseconds. Don’t be worried; Earth will be OK. The tiny companion of the dead star is in danger.

Researchers explain this ill-fated binary star system — a rare form of celestial object known as a black widow pulsar — in an article published on the pre-print database arXiv. The larger member of the duo appears intent on devouring and eliminating its smaller friend, much like the cannibal spider from which this sort of system gets its name. (Female spiders are frequently larger than males.)

However, there will be no quick decapitation for this black widow; the larger star appears to be killing its partner much more slowly. Over hundreds or thousands of years, the larger star has sucked matter from the smaller star’s vicinity while simultaneously blasting the smaller star with strobing blasts of energy, pushing even more matter into space.

According to main study author Emma van der Wateren, a doctoral student at the Netherlands Institute for Radio Astronomy (ASTRON), the larger star could eventually swallow the smaller one completely. However, scientists want to put this strange system to use before then. By monitoring the larger star’s remarkably steady pulses for sudden irregularities, the study’s authors hope this pulsar could help them detect rare ripples in the fabric of space-time known as gravitational waves.

“To detect gravitational waves, you need many, many very stable pulsars,” van der Wateren said. “And unlike earlier black widow pulsars that have been discovered, this system is very stable.”

Cannibal corpses

Scientists discovered J06102100, a star system around 10,000 light-years from Earth, in 2003 after seeing its periodic pulsing with a radio telescope. Researchers pegged the system as a pulsar — a type of small, dense, collapsed star that rotates extremely quickly.

These dead stars are highly magnetized and blast beams of electromagnetic radiation as they spin. When one of those beams points toward Earth, the effect is like a lighthouse, with the light blinking on and off as the beam strobes past us. If the light blinks once per 10 milliseconds or less (like J06102100, which blinks once every 3.8 milliseconds), the star is classified as a millisecond pulsar.

Many millisecond pulsars orbit sun-like companion stars, which the pulsars slowly consume. As the pulsars consume the companion star’s spinning disks of materials, they emit X-ray radiation that can be seen throughout the galaxy.

An illustration of a pulsar consuming matter from its companion star. The companion star in black widow pulsars has been reduced to one-tenth the mass of the sun, or less. (Image credit: NASA Goddard )

At times, a pulsar may steal more than its fair share of matter from its companion. If a pulsar’s companion star has a mass less than one-tenth that of the sun, the star system is known as a black widow pulsar.

J06102100 is the third black widow pulsar discovered, and it appears to be one of the most hungry. According to the study, the pulsar’s companion star weighs only 0.02 solar masses and orbits the pulsar every seven hours or so.

Van der Wateren and her colleagues investigated 16 years of radio telescope data from this cannibal star system for their new paper. While the system is obviously a black widow pulsar, the team was disappointed to discover that it lacked a few distinctive characteristics.

For example, the star system never showed what’s known as a radio eclipse — a nearly universal phenomenon in other black widow pulsars.

“Typically, for a portion of the binary orbit, the radio emissions from the pulsar completely disappear,” van der Wateren said. “This occurs when the companion star moves close to the front of the pulsar, and all this irradiated material coming off of the companion eclipses the pulse emission from the pulsar.”

The star system also never revealed any timing irregularities — sudden, tiny variations in the timing of a pulsar’s pulse compared to astronomers’ predictions — throughout the course of 16 years.

Waves that move the universe

An artist’s rendering of a pulsar surrounded by a glowing disk of matter. In ‘black widow’ pulsars, that matter comes from a smaller companion star that’s slowly being irradiated out of existence. (Image credit: NASA/JPL-Caltech)

According to van der Wateren, the absence of these two common events is difficult to explain. It’s possible that the pulsar’s line of sight is skewed, making radio eclipses invisible to Earth-based telescopes, or that the pulsar’s companion star isn’t being irradiated as strongly as other known pulsars that exhibit these features. In any event, the black widow system is extremely stable and predictable, making it an excellent choice for detecting gravitational waves, according to the researchers.

When the universe’s most massive objects interact, such as when black holes or neutron stars collide, these waves (originally predicted by Albert Einstein) . The waves travel at light speed through time and space, warping the fabric of the cosmos as they pass.

Astronomers seek to discover gravitational waves by concurrently monitoring dozens of millisecond pulsars using equipment known as pulsar timing arrays. If every pulsar in the array suddenly had a timing irregularity around the same time, it may indicate that something massive, such as a gravitational wave, disrupted their pulses on their way to Earth.

“We have not detected gravitational waves in this way yet,” van der Wateren said. “But I think we are coming close.

That is why the discovery of highly predictable black widow pulsars like this one is so significant,  van der Wateren continued.

Black widow pulsars are rarely good candidates for gravitational wave detection because they are temperamental due to radio eclipses and timing irregularities. However, J06102100 could be an exception — and the fact that it exists suggests that there may be other acceptable exceptions out there too. This black widow’s cannibal bite, like that of its arachnid namesake, may serve a larger purpose in the end.

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