At white dwarfs they are the objects that most stars become after depleting the hydrogen that fuels them. Its size is approximately that of the Earth, but its mass is more like that of the Sun.
Now, an international team of researchers led by Durham University (UK) has used TES (Transiting Exoplanet Survey Satellite), a NASA satellite that studies exoplanets, to see a unique and different phenomenon: how a white dwarf suddenly turns on and off. The results are published in the journal. Astronomy of Nature.
With NASA’s TESS satellite, a sharp decrease in brightness in the two white dwarf system TW Pictoris was observed within 30 minutes, as one “feeds” on the other.
The authors observed the event in a white dwarf from the binary system. TW Pictoris, located about 1400 light years from Earth. Specifically, in the larger one, while it swelled or swelled at the cost of hydrogen and helium that snatched its smaller companion.
Astronomers verified how the system lost its luster in a matter of half an hour, a phenomenon seen earlier in the accumulation of white dwarfs, but for much longer periods of several days or months.
TW Pictoris is a relatively bright binary system in which a white dwarf accumulates material from a companion star. Observations made with the TESS satellite revealed how the system suddenly and abruptly disappears in less than 30 minutes. In blue, the light mode or “on”, and in red, the off mode. / Simone Scaringi
“Seeing the glow of TW Pictoris plummet in 30 minutes is extraordinary in itself, as it has never been seen in other white dwarfs that accumulate and is totally unexpected: it seems to turn on and off,” explains lead author, Simone scaring.
“It’s really a phenomenon not known until now”, he emphasizes, “and how can we draw comparisons with a behavior similar in neutron stars, much smaller, could be an important step to help us better understand the process of how other accretion objects feed on the material that surrounds them and the important role of magnetic fields in this process”.
Magnetic Field Resets
The brightness of a forming white dwarf is affected by the amount of surrounding material it engulfs, so researchers say something is interfering with its supply on these very short timescales. They believe that what they are witnessing may be reconfigurations of the white dwarf’s surface magnetic field.
During “on” mode, when the brightness is high, the white dwarf feeds on the accretion disk as it normally would. Suddenly and abruptly, the system shuts down and its brightness quickly fades. When this occurs, the magnetic field rotates so quickly that a centrifugal barrier prevents fuel from the accretion disk from constantly falling onto the white dwarf.
The researchers hope this discovery will help them learn more about the physics behind accretion, not just in white dwarfs, but also in other objects such as neutron stars and black holes.
During this phase, the amount of fuel that the white dwarf can feed is regulated by a process called magnetic gate (a kind of magnetic door). The white dwarf’s rotating magnetic field regulates the fuel that passes through a ‘gate’ to the accretion disk, leading to small, semi-regular increases in brightness observed by astronomers.
After a while, the system is sporadically ‘turned on’ again and the brightness returns to its original level. Since white dwarfs are more common in the universe than neutron stars, astronomers hope to find more examples of this behavior.
They believe this new discovery will help them learn more about the physics behind adding these two types of objects, but also others like black holes, which feed on material from neighboring stars.
Reference:
S. Scaringi et al. “An add-on white dwarf exhibiting a quick transition mode change.” Astronomy of Nature, 2021.
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