Astronomers have observed an accreting white dwarf abruptly lose and gain its brightness for the first time. White dwarfs are stars that have run out of their basic fuel: hydrogen. Usually, researchers have observed white dwarfs losing their brightness over days or months. Astronomers were studying the white dwarf binary system, TW Pictoris, and the white dwarf in it lost its brightness in 30 minutes, which surprised astronomers. Astronomers said it was accreting, or feeding on the energy from an orbiting companion star. To observe the unique phenomenon, astronomers used NASA's Transiting Exoplanet Survey Satellite (TESS) that helps in planet-hunting processes.
The brightness of a white dwarf is not dependent on its own hydrogen source, as it has already exhausted it. Its brightness depends on the amount of surrounding material that it feeds on. The abrupt switching on and off of the white dwarf may be due to some interference with food supply, said researchers.
The research was led by England's Durham University. The findings were published on October 18, 2021, in the Nature Astronomy journal.
TW Pictoris comprises a white dwarf and a smaller companion star. The white dwarf feeds from the accretion disc that is fuelled by hydrogen and helium from the companion star. The more energy it feeds on, the brighter it becomes.
Researchers noticed the abrupt rise and fall in the brightness of this white dwarf, the likes of which they had never seen before. They believe this could be caused by reconfiguration of the white dwarf's surface magnetic field.
At its brightest phase, the white dwarf feeds off the accretion disc. But suddenly the magnetic field starts spinning so rapidly that a centrifugal barrier is created. This stops the fuel from the accretion disc to reach the white dwarf.
The "magnetic gating" regulates the fuel passing from the disc to the white dwarf. Due to the small amount of fuel passing through the "gate" the white dwarf experiences semi-regular small increases in brightness.
Then, the system switches "on" again, allowing the white dwarf to feed normally from the disc. The brightness reaches its peak again.
Lead author Dr. Simone Scaringi said, "This really is a previously unrecognised phenomenon and because we can draw comparisons with similar behaviour 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."
This extraordinary observation could lead to further studies of white dwarfs and neutron stars.
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