White dwarf pulsar discovery confirms new class of star

The discovery of a white dwarf blasting out radiation every 5 minutes is the second white dwarf found to "pulse" at a periodic rate, confirming it as a new class of star.
John Loeffler
A white dwarf spinning 300 times faster than the Earth pulses out radiation every 5 minutes
A white dwarf spinning 300 times faster than the Earth pulses out radiation every 5 minutes

Dr Mark Garlick/University of Warwick

The discovery of a new “pulsing” binary star system has shed new light on the evolution of stars while confirming a new exotic class of stellar object: the white dwarf pulsar.

The star in question, J191213.72-441045.1 (J1912-4410), is a white dwarf about 773 light-years from Earth and is part of binary star system with a companion red dwarf. It is thought to complete a rotation of its axis once every roughly five minutes, which coincides with a regular, periodic pulse of X-ray radiation that is observable from the system.

This isn’t the first white dwarf system we’ve seen that behaves in such a way. The first was AR Scorpii (AR Sco), discovered in 2016, and the discovery of a second white dwarf system exhibiting this same quirk is significant.

A white dwarf is a stellar relic of sorts, the left-over core of a low-mass star that has exhausted its fuel and shed its outer layers. Typically the size of the Earth, but possessing a mass comparable to the sun, they can spin incredibly fast

In the case of J1912-4410, the white dwarf spins on its axis about 300 times faster than the Earth, despite being roughly the same size. This is due to the extreme density of a white dwarf, which must conserve the angular momentum of the original star, despite being a fraction of its size. 

This density and rate of rotation might also contribute to the creation of a cosmological “dynamo,” or electrical generator, that can also lead to incredibly strong magnetic fields emanating from the object. The Earth itself has a dynamo in its core, as does the sun. 

White dwarf pulsar discovery confirms new class of star
A binary star system with a white dwarf and a red dwarf

“The origin of magnetic fields is a big open question in many fields of astronomy, and this is particularly true for white dwarf stars,” Dr. Ingrid Pelisoli, of the University of Warwick’s Department of Physics and the lead author of the new study in Nature Astronomy, said in a statement. “The magnetic fields in white dwarfs can be more than a million times stronger than the magnetic field of the Sun, and the dynamo model helps to explain why. The discovery of J1912−4410 provided a critical step forward in this field.”

The magnetic fields of the white dwarfs in AR Sco and J1912-4410 might be the key to understanding the cause of their systems’ characteristic pulse. 

“We used data from a few different surveys to find candidates, focusing on systems that had similar characteristics to AR Sco … After observing a couple dozen candidates, we found one that showed very similar light variations to AR Sco,” Pelisoli said. “Our follow-up campaign with other telescopes revealed that every five minutes or so, this system sent a radio and X-ray signal in our direction.”

The thinking is that the magnetic field of the spinning white dwarf is striking the companion star with radiation and high-energy, electrically charged particles at regular intervals as the white dwarf rotates. These periodic magnetic lashings of the companion star then create a burst of radio and X-ray radiation, which we can observe when looking at the system.

"We are excited to have independently found the object in the X-ray all-sky survey performed with SRG/eROSITA,” Dr. Axel Schwope, the head of X-ray astronomy at the Leibniz Institute for Astrophysics Potsdam who is leading a complementary study soon to be published as a letter in Astronomy and Astrophysics, said. “The follow-up investigation with the ESA satellite XMM-Newton revealed the pulsations in the high-energy X-ray regime, thus confirming the unusual nature of the new object and firmly establishing the white dwarf pulsars as a new class.”

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