Astronomers witness white dwarf star rapidly ‘switch on and off’

Astronomers have discovered a new astronomical phenomenon in which a white dwarf star has been switching “off and on,” with its brightness abruptly dimming and intensifying in just 30 minutes.

The discovery was based on observations from NASA’s Transiting Exoplanet Survey Satellite (TESS) searching for alien planets outside our solar system. The team of astronomers at Durham University and their partners across the world describe it as “extraordinary.”

At first, they believed that the fluctuations in luminosity were the result of something interfering with the way thatthe white dwarf was feeding from its companion star (i.e. another object with a strong gravitational pull), but they didn’t expect the cause to turn out as unique as it was.

Since the flow of gas and dust from the donor star to the main white dwarf is governed by gravity, it should remain relativity constant. This perplexed researchers as there was no obvious reason why the star’s luminosity would change in such a short period of time.

Render of a white dwarf star feeding on a nearby companion star. The spiral disc around the white dwarf, also known as an accretion disc, is formed by the star’s strong gravitational influence and spin. Image credit: NASA/CXC/M.Weiss

Using the precise observational details offered by NASA‘s TESS, they found something that hadn’t been seen before – recurrent and rapid reconfigurations of the white dwarf’s surface magnetic field.

According to the study published Monday in the journal Nature Astronomy, the reason is that the white dwarf, TW Pictoris, has an unusually fast magnetic field that is spinning so rapidly that it’s creating a wall around the main star, known as a centrifugal barrier.

This “has never been seen in other accreting [feeding] white dwarfs,” said Simone Scaringi, an astronomer at Durham University’s Centre for Extragalactic Astronomy and the lead author of the published study.

In decoding the mystery, the team found that during its “off” phase, the white dwarf’s accretion disc was spinning so fast it created a centrifugal force strong enough to prevent any matter from ever reaching the central star. This explains why the white dwarf didn’t have anything to matter feeding it and began dimming.

After the “off” phase and during the “on” phase, the speed of the star’s magnetic field went back to normal. This allowed matter to, once again, escape the spin and fall in toward the white dwarf.

The phenomenon is similar to swinging a bucket around with water in it. If you spin fast enough, water is pushed up against the bucket and will not fall out. Once you slow down though, the water will begin to splash out of the bucket.

Researchers are still unsure what exactly is causing the white dwarf’s magnetic field to change so suddenly.

“This really is a previously unrecognized phenomenon,” Scaringi concluded, adding that “because we can draw comparisons with similar behavior in the much smaller neutron stars, it could be an important step in helping us to better understand the process of how other accreting objects feed on the material that surrounds them, and the important role of magnetic fields in this process.”

The main star in the TW Pictoris binary system could represent a whole new class of dynamically changing white dwarf stars, or it could just be an oddball in the cosmos. Either way, there’s still a lot we don’t about how magnetic fields operate around white dwarfs, which is important for understanding the birth and life of these cosmic curiosities.

Featured image: White dwarf ZTF J1901+1458 depicted with our Moon. (Credit: Giuseppe Parisi)

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