The white dwarf he thought was a pulsar

Like beacons guiding sailors caught in a storm, pulsars sweep across space with their powerful beams of electromagnetic waves with incredible regularity. These stars are rapidly spinning neutron stars, and are some of the densest objects in the universe. But pulsars wouldn’t be the only ones emitting such periodic electromagnetic signals… This behavior has already been observed in a so-called “white dwarf” star. Unique oddity? no ! By discovering a second white dwarf that behaves like a pulsar, Ingrid Pelisoleli of the University of Warwick in the UK and her collaborators have formally demonstrated the existence of a new class of stellar object.

When a star less than eight times the mass of the Sun uses up all of its nuclear fuel, its core can no longer support its weight and collapses in on itself. On the other hand, the outer layers of the star swell and gradually spread out into space, leaving behind only the remnant of the protostar – the white dwarf.

White dwarfs are the most common stellar fossils in the universe. However, it took until 2016 for AR Scorpii, the first white dwarf star, to be discovered. Before AR Scorpii, the only known pulsars were neutron stars, the remnants of larger stars. Because they spin so fast, neutron stars have an intense magnetic field, causing beams of radiation to be ejected from the magnetic poles. The star’s rotation causes the beam to sweep through space, creating the illusion of a periodic signal.

But white dwarfs have much slower rotation periods, on the order of days. The speed is not enough to generate a magnetic field by the effect of the dynamo. How can white dwarfs emit a periodic signal, like that of pulsars?

To find out, Ingrid Bellesoli and her colleagues set out to find other objects, using ULTRACAM, an ESO (European Southern Observatory) instrument capable of detecting rapid changes in brightness at different wavelengths. After observing dozens of white dwarf candidates, astrophysicists finally detected one 773 light-years from Earth, named J1912−4410, whose luminosity contrasts matched those of AR Scorpii. A follow-up campaign with other telescopes recently revealed that every five minutes, this white dwarf sends a signal in radio waves and X-rays in our direction. Thus J1912−4410 is a white dwarf star.

To reach rotational speeds high enough to act like a pulsar, these white dwarfs interact with another star, in this case a red dwarf, ripping material from it. By falling on the white dwarf, this matter brings in enough energy to accelerate its spin. During the billions of years during which this acceleration occurs, the white dwarf’s core, a plasma of carbon and oxygen, cools to the point of crystallization. This solid core is then covered with a mantle consisting of charged particles. As the white dwarf rotates faster, the particles in the mantle end up being accelerated enough to produce a strong magnetic field via the dynamo effect.

However, the rays of white dwarfs are of a different nature than those in neutron stars, and the mechanism behind them remains undetermined. “There are many theories trying to explain the source of the pulses,” says Ingrid Bellesoli. They may be the result of the interaction of the magnetic field with surface particles of either the white dwarf or the red dwarf. “To make a decision, more data is needed,” the researcher continues. However, in either case, it is the white dwarf’s magnetic field that provides energy to the particles, energy which they then re-emit as radiation, giving rise to the characteristic beams of pulsars.