For the first time, astronomers have seen a white dwarf ‘turning on and off’. The star goes from light to relatively dark in just 30 minutes. Astronomers from Durham University and Radboud University, among others, see this in observations of NASA’s TESS satellite, which is mainly intended to investigate exoplanets. Their results, which provide more insight into the physics surrounding binary stars, they published monday in Nature Astronomy.
A star becomes a white dwarf when it has ‘burned out’ and weighed less than eight times as much as the Sun. The dwarf remains hot and therefore also radiates.
The studied white dwarf is part of the binary star TW Pictoris in the constellation Schilder (Pictor), at a distance of 1,400 light-years from Earth. A normal star revolves around it. The two are so close together that the white dwarf’s gravitational pull pulls hydrogen and helium from the paired star. The dwarf slowly ‘eats’ the donor star through a process called accretion.
The magnetic field of the white dwarf influences that gas flow. “Normally, the gas follows the magnetic field lines and reaches the poles of the dwarf. Just like, for example, particles from the sun do on Earth with the northern lights. The moment the gas hits the white dwarf, you see light,” explains Paul Groot, professor of astronomy at Radboud University and co-author of the study.
A ricocheting stone
“But the magnetic field can also act as a barrier. Think of a paddle wheel spinning hard on its axis. If you throw a stone at that, it bounces off instead of being absorbed.” Whether that happens depends on the strength and rotational speed of the magnetic field, but also on how much gas gets onto this field.
More likely, the gas flow will increase. “A slow gas flow can absorb the white dwarf. But if it goes faster, it will bounce off. When that happens, the system gives less light.” It is not yet certain why the gas flow suddenly changes. Perhaps the donor star sputters, „similar to the volcano in La Palma now. Sometimes the lava flows very easily, sometimes it sputters a bit.” But it is also possible that the gas flow from the donor star to the accretion disk around the white dwarf is constant, but that the flow rate from that disk to the dwarf varies.
The ‘turning on and off’ of star remnants in a short period of time had already been observed in the so-called transitional pulsars, a subgroup of neutron stars. There the effect is more spectacular. When the pulsars are ‘on’, they emit high-energy X-rays. If the pulsar is ‘off’, because the gas stream is bouncing, it will emit radio waves with very low energy. The step is not that big with white dwarfs. There the light is in both positions in the optical spectrum, which runs from UV to infrared radiation. In the ‘off’ position, the white dwarf still emits light, but much less.
The question was always whether this blinking was a typical phenomenon of neutron stars, or something more general. So the second seems to be the case. Accretion changes not only occur relatively quickly, as with TW Pictoris and pulsars, but in the case of supermassive black holes at the centers of galaxies, the same happens over millions of years. “This makes those systems much more difficult to study. But much of what we observe does not depend on which object is extracting the mass. It has more to do with the underlying physics of the accretion disks.” Studying TW Pictoris and pulsars allows astronomers to better understand that physics, thanks to systems that make them easier and faster to study.
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