First time observation: white dwarf 'switching on and off'

Astronomers have used a planet-hunting satellite to see a white dwarf abruptly switching on and off for the first time. The researchers used NASA's Transiting Exoplanet Survey Satellite (TESS) to observe the unique phenomenon.

White dwarfs are what most stars become after they have burned off the hydrogen that fuels them. They are approximately the size of the Earth, but have a mass closer to that of the Sun. The white dwarf observed by the team is known to be accreting, or feeding, from an orbiting companion star.

With the new observations astronomers saw it lose brightness in 30 minutes, a process only previously seen to occur in accreting white dwarfs over a period of several days to months. The brightness of an accreting white dwarf is affected by the amount of surrounding material it feeds on so the researchers say something is interfering with its food supply. They hope the discovery will help them learn more about the physics behind accretion—where objects like black holes, white dwarfs and neutron stars feed on surrounding material from neighboring stars.

The team observed the phenomenon in the white dwarf binary system, TW Pictoris, which is found about 1,400 light years from Earth. TW Pictoris consists of a white dwarf that feeds from a surrounding accretion disk fuelled by hydrogen and helium from its smaller companion star. As the white dwarf eats—or accretes—it becomes brighter.

Using the precise observational detail offered by TESS—normally used to look for planets outside our solar system—the  team saw abrupt falls and rises in brightness never before seen in an accreting white dwarf on such short timescales. Because the flow of material on to the white dwarf's accretion disk from its companion star is relatively constant it should not drastically affect its luminosity on such short timescales. Instead, the researchers think what they are witnessing could be reconfigurations of the white dwarf's surface magnetic field.

During the "on" mode, when the brightness is high, the white dwarf feeds off the accretion disk as it normally would. Suddenly and abruptly the system turns "off" and its brightness plummets.

The researchers say that when this happens the magnetic field is spinning so rapidly that a centrifugal barrier stops the fuel from the accretion disk constantly falling on to the white dwarf.

During this phase the amount of fuel the white dwarf is able to feed on is being regulated through a process called magnetic gating.

In this case, the spinning magnetic field of the white dwarf regulates the fuel passing through a "gate" on to the accretion disk, leading to semi-regular small increases in brightness seen by the astronomers.

After some time, the system sporadically turns "on" again, and the brightness increases back to its original level.

Simone Scaringi, An accreting white dwarf displaying fast transitional mode switching, Nature Astronomy (2021). DOI: 10.1038/s41550-021-01494-x. www.nature.com/articles/s41550-021-01494-x

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