White dwarf blasts companion with radiation

A new type of binary star system has been observed in which a white dwarf blasts its red dwarf companion with radiation, yet the source of the radiation remains unclear.

Artist's depiction of the white dwarf star (top right) blasting radiation onto its companion red dwarf
Credit: M. Garlick/University of Warwick, ESA/Hubble

Astronomers have discovered a new type of binary star consisting of a spinning white dwarf that blasts radiation onto its companion red dwarf causing bright, energetic pulses.

Star system AR Scorpii lies in the constellation of Scorpius. Its white dwarf star is about the same size as Earth but contains about 200,000 times more mass. The red dwarf companion is about one third the mass of the Sun. The two orbit each other every 3.6 hours.

The binary system was first discovered in the 1970s and was wrongly thought to be a lone variable star. But new observations by a team of amateur astronomers from the UK, Germany and Belgium revealed it to be a binary system exhibiting unusual activity.

The white dwarf is spinning so fast it accelerates electrons to almost the speed of light, which causes bursts of radiation in strong beams to fire across the face of the red dwarf star. This causes the whole system to brighten in regular bursts every 1.97 minutes.

The pulses include radiation at radio frequencies, which has never before been detected in a white dwarf system. Similar pulsing behaviour has been observed in neutron stars, however.

“AR Scorpii was discovered over 40 years ago, but its true nature was not suspected until we started observing it in June 2015. We realised we were seeing something extraordinary the more we progressed with our observations,” says lead researcher Tom Marsh of the University of Warwick’s Astrophysics Group.

However, while the radiation frequencies can be explained by the electrons being accelerated in magnetic fields, astronomers can’t yet explain the source of the electrons.

Co-author Boris Gänsicke of the University of Warwick says: "We've known about pulsing neutron stars for nearly fifty years, and some theories predicted white dwarfs could show similar behaviour. It's very exciting that we have discovered such a system, and it has been a fantastic example of amateur astronomers and academics working together."


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