First eclipsing magnetic propeller discovered in a binary star system

We actually can see blobs of gas as they’re ejected by the propeller.


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In 2020, J0240 star system was identified as an unusual cataclysmic variable. It is a binary system consisting of a white dwarf star and a mass-donating red star.

Usually, the white dwarf star collects the gas and grows in mass. But, in the case of J0240, the dwarf rejects the gas and throws it out of the binary system.

Recently, scientists from the University of Notre Dame have identified the first eclipsing magnetic propeller in J0240. Scientists recorded the occurrence of flares and eclipses from the star that illustrated the rapid spinning of the white dwarf star and the pull of the magnetic field — which expels incoming gases that would otherwise be added to the star but instead creates a spiral of gas expanding away from the two leads.

The observations made using the Large Binocular Telescope in Safford, Arizona, in September, October, and November of 2020.

Peter Garnavich, professor of astrophysics and cosmology physics and chair of the Department of Physics at Notre Dame, said, “What’s unique about the system is that we actually can see blobs of gas as the propeller ejects them. The flares we see are mini-explosions that blow off the gas at 6 million miles per hour or 1 percent of the speed of light.”

“The flaring disappears when the red companion gets in the way during an eclipse. From the timing of the eclipses, we pinpointed the location of the flares. The flaring is coming from very close to the compact companion, likely from the whack the gas receives as it approaches the rapidly spinning magnetic field.”

Further observations could help scientists learn more about the J0240. One of the big unknowns is the white dwarf spin period, which the team could not detect.

Garnavich said, “The energy of the propeller is coming from the spinning white dwarf, so we expect the spin rate to be slowing over time. When it runs down, the propeller will stop, and the system will look like an ordinary cataclysmic variable.”

“The biggest question is exactly how do you get into this state. It’s a very short-lived phase where you have a magnetic white dwarf spinning about as fast as it can spin without actually flying apart. Spinning so fast with a strong magnetic field — seems like it can’t be just coincidence.”

Journal Reference:
  1. Peter Garnavich et al. Confirmation of a Second Propeller: A High-Inclination Twin of AE Aquarii. arXiv: 2102. 08388v1