Astronomy

True Identity of Exotic White Dwarf Revealed — and It's One of a Kind

AR Scorpii is home to the first ever white dwarf pulsar discovered, an energetic stellar remnant that brutalizes its red dwarf partner.

Like finding a shiny diamond in the dirt, astronomers have realized the true identity of a white dwarf star that has been accused of red dwarf brutality - it's the first "white dwarf pulsar" ever discovered.

The true binary nature of AR Scorpii (AR Sco) was revealed when the Hubble Space Telescope took a close look at what was believed to be a lone variable star, around 380 light-years away. Instead, astronomers noticed the object was actually two stars, meaning it was a binary system composed of a cool red dwarf and tiny white dwarf orbiting one other at a very close 870,000 miles - roughly three times the Earth-moon distance. As revealed in research last year, this compact binary system may be small, but what it lacks in stature it certainly makes up for in energy.

The perplexing thing about AR Sco was its excessively bright pulses of radiation across the electromagnetic spectrum, and it wasn't until Hubble realized there was a white dwarf in tow that the possible mechanism driving its bright pulses presented itself. It appeared that the white dwarf was somehow energizing the binary system, causing the rapid acceleration of electrons, generating an incredibly powerful beam of radiation as the white dwarf spun once every two minutes. This beam would regularly - and relentlessly - lash the red dwarf.

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Known to have a magnetic field 100 million times more powerful than Earth's, the white dwarf in AR Sco is basically a souped-up particle accelerator and every time the white dwarf spins, it produces a lighthouse-like beam of radiation that is directed toward Earth that we see as regular pulses.

"AR Sco is like a gigantic dynamo: a magnet, size of the Earth, with a field that is ~10,000 stronger than any field we can produce in a laboratory, and it is rotating every two minutes," said Boris Gänsicke of the University of Warwick. "This generates an enormous electric current in the companion star, which then produces the variations in the light we detect."

The nature of this white dwarf certainly sounds an awful lot like a pulsar and, indeed, in research published in Nature Astronomy, the researchers have called it such. So there we have it, the first ever "white dwarf pulsar" discovered in the universe.

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Classical pulsars consist of more compact neutron stars, which are produced after a massive star goes supernova. The entire stellar magnetic field is retained by the rapidly-spinning neutron star, which then generates powerful beams of radiation blasting from the poles. As the pulsar spins, we see very precise flashes as the lighthouse-like beams sweep in our direction.

White dwarfs, however, are not produced by supernovae, though they are created after a star dies. Less massive stars like our sun do not explode, instead reaching the end of their lives as red giant stars, when the outer layers of superheated gas blasts from the dying star, creating a planetary nebula. When this death cycle is complete, a small stellar remnant - a white dwarf that is approximately the size of Earth, only 200,000 times more massive- remains in its wake.

Known to also possess powerful magnetic fields, white dwarfs haven't been known for their pulsar-like qualities - until now.

"The new data show that AR Sco's light is highly polarized, showing that the magnetic field controls the emission of the entire system, and a dead ringer for similar behavior seen from the more traditional neutron star pulsars," said Tom Marsh, also from the University of Warwick, in a statement.

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