TW Pictoris is a cataclysmic variable system in the constellation Pictor that is roughly 1,400 light-years away.
It’s made up of a white dwarf that gets its energy from an accretion ring around it, which is fueled by hydrogen and helium from its smaller partner star. The white dwarf becomes brighter as it grows.
Using TESS data, Durham University astronomer Simone Scaringi and colleagues discovered that the white dwarf in TW Pictoris dimmed in brightness in 30 minutes, a phenomenon that had previously only been seen in accreting white dwarfs over many days to months.
“Brightness fluctuations in accreting white dwarfs are often gradual, happening on timeframes of days to months,” Dr. Scaringi said.
“Seeing TW Pictoris’ brightness plunge in 30 minutes is remarkable in and of itself since it has never been observed in other accreting white dwarfs and is completely unexpected given our knowledge of how these systems are meant to feed via the accretion disc. It seems like it’s turning on and off.”
On such short timeframes, the flow of material onto the white dwarf’s accretion disc from its partner star should not have a significant impact on its brightness.
Instead, the scientists think they are seeing reorganizations of the white dwarf’s surface magnetic field.
When the brightness is high in the ‘on’ state, the white dwarf feeds on the accretion disc as it usually would. The system immediately shuts ‘off,’ and the brightness drops dramatically.
When this occurs, the magnetic field spins so fast that a centrifugal barrier prevents the fuel from the accretion disc from landing on the white dwarf all the time.
The quantity of fuel the white dwarf may consume during this phase is controlled by a mechanism known as magnetic gating.
In this situation, the white dwarf’s rotating magnetic field governs the fuel streaming through a ‘gate’ into the accretion disc, resulting in semi-regular modest brightness increases.
The system periodically comes ‘on’ again after some time, and the brightness returns to its normal level.
“Because we can draw comparisons with similar behaviour in much smaller neutron stars, this could be an important step in helping us 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,” said Dr. Scaringi