The first white dwarf pulsar discovered

Artist’s impression of the AR Scorpii system (Image M. Garlick/University of Warwick/ESO)
Artist’s impression of the AR Scorpii system (Image M. Garlick/University of Warwick/ESO)

An article published in the journal “Nature Astronomy” describes the discovery of the first white dwarf pulsar, an object so far only hypothesized but never found. A team of researchers from the British University of Warwick identified it in the AR Scorpii (AR Sco) System, composed of a red dwarf and the white dwarf pulsar that a rotation period of just under two minutes.

The Investigations on the AR Scorpii system started in May 2015, when a group of amateur astronomers from different countries realized that it had strange behavior. The issue got many observatories interested bringing the University of Warwick’s involvement but also ESO’s and among the telescopes used in the observations there was the VLT as well. Eventually, they realized that what was initially considered an odd variable star was actually a pair of stars.

The AR Scorpii system is relatively close, 380 light years away from Earth, however, the stars that compose it are small and their behavior is abnormal, elements that made it more difficult to understand what was going on in a system that apparently was not interesting. The red dwarf is a star of the most common type existing in the universe with a mass that is about one third of the Sun’s. Its companion is a white dwarf, what remains at the end of a medium-small mass star’s lifecycle, one like the Sun. Between the two stars there’s a distance of approximately 1.4 million kilometers.

Initially, the AR Scorpii system was classified as a Delta Scuti variable, a type of variable star with a period of 3.56 hours. However, the new observations allowed to establish that that’s actually the orbital period of the two stars orbting each other. An article published in the journal “Nature” in July 2016 described these results as well as the white dwarf’s strange emissions.

What remains after the death of a star collapses to a point that depends on the mass remained. The white dwarf in the AR Scorpii system has a size similar to the Earth but its mass is 200,000 times greater. Among the consequences of that collapse there are a very powerful magnetic field and a quick spin. In this case there’s a unique phenomenon because the white dwarf accelerates electrons to near the speed of light. They’re projected in space and when they hit the red dwarf they cause a kind of giant lightning that occurs every 1.97 minutes.

The investigations by the University of Warwick researchers continued and now the findings have been published in “Nature Astronomy”. According to Professor Tom Marsh, one of the authors of this article and the one published last year in “Nature”, the new data show that the AR Scorpii light is strongly polarized, showing that the magnetic field controls the emission of the whole system. From these characteristics, the researchers concluded that the white dwarf is a pulsar, the first of its kind ever discovered.

Pulsars are well-known objects but until now they were always neutron stars, another possible end of a star’s life. Neutron stars have masses larger than that the Earth’s and for this reason their gravity makes them collapse into much smaller volumes. The white dwarf in the AR Scorpii system has a size similar to the Earth’s, which has a diameter of about 12,700 km, a neutron star can have a diameter of 10 kilometers, less than one thousandth.

The mass concentration of a neutron star determines the characteristics that can make it a pulsar. A white dwarf has a magnetic field much less intense and rotates more slowly but in the case of AR Scorpii system there’s magnetic interaction between two stars that makes the difference. The authors believe that further observations, in particular at X-rays and radio frequencies, will be important to determine the exact nature of the mechanisms that generate the emissions in that system.

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