The discovery of the white dwarf pulsar sheds light on stellar evolution

June 15, 2023

(Nanowerk News) White dwarfs are the remains of stars that are very dense with the mass of our Sun but smaller than our planet Earth. They form when a low-mass star has used up all its fuel, lost its outer layers, and its interior contracts forcefully. They are also called “stellar fossils” and offer insight into various aspects of star evolution.

Pulsars, on the other hand, have been known since the 1960s and more than 3000 have been discovered. They are fast rotating, highly magnetic neutron stars where charged particles are ripped from the surface by a very strong electric field and then accelerated to nearly the speed of light. Consequently, they emit radiation, that is light, on the radio into the X-ray or even the gamma range. Due to the fast rotation of the star, short pulses of radiation arrive on Earth, hence the reason for its name – pulsar. Artist’s impression of a white dwarf pulsar. (Image: M. Garlick / University of Warwick / ESO)

To the great surprise of the scientific community, the pulsar phenomenon was observed for the first time on a white dwarf in 2016. The surprise lay in the fact that in this star, AR Scorpii, neither the extremely fast rotation nor the strong electric field of a real pulsar is present. However, white dwarfs are found in very close binary systems and are supplied with particles by their nearest neighbor, a sun-like red dwarf, by injection into their magnetic field. This triggers the pulsar phenomenon from outside and illuminates a red companion star as if with a stroboscope, causing the entire system to become dramatically brighter and dimmer at regular intervals. The two stars, the white dwarf and the red dwarf, are so close together that they fit inside our Sun.

The decisive factor is the presence of a strong magnetic field, the cause of which, however, is unknown to astrophysicists. A key theory explaining strong magnetic fields is the “dynamo model”, which says that white dwarfs have a dynamo, a generator of electricity, at their core, just like Earth’s, only much more powerful. But to test this theory, the researchers had to look for other white dwarf pulsars to see if their predictions held true.

In two new studies published in parallel in Natural Astronomy (“White dwarf pulsates with a period of 5.3 minutes in binary detected from radio to X-rays.”) And Astronomy & Astrophysics (“X-ray properties of the white dwarf pulsar eRASSU J191213.9−441044”), an international team with AIP participation described the newly discovered white dwarf pulsar J1912-4410 (eRASSU J191213.9-441044). It is 773 light years from Earth and rotates once on its own axis in five minutes, 300 times faster than our planet. The white dwarf pulsar is similar in size to Earth, but at least as massive as the Sun. This means that one teaspoon of a white dwarf would weigh about 15 tons. White dwarfs start life at very high temperatures before cooling for billions of years. The low temperature of J1912-4410 indicates that it is very old.

The study confirmed that there were much more white dwarf pulsars, as earlier models had predicted. There are other predictions from the dynamo model which are confirmed by the discovery of J1912-4410. Because of their very old age, white dwarfs in the pulsar system should be cool. Their companions must be close enough for the white dwarf’s gravitational pull in the past to be strong enough to extract mass from its partner, causing them to spin rapidly. All of these assumptions apply to the newly discovered pulsar: the white dwarf is cooler than 13,000 Kelvin, has a high rotational frequency of about five minutes, and the white dwarf’s gravitational pull has a strong effect on its companion.

One team used data from Gaia and WISE to find candidates, focusing on those with similar traits to AR Scorpii. After surveying several dozen candidates, they found one with very similar light variations. Follow-up observations with other telescopes revealed that this system sends radio signals and X-rays to Earth every five minutes. Another team used data from the eROSITA X-ray telescope on the Spectrum-X-Gamma satellite to find a nearby white/red dwarf pair. The two teams join forces to further investigate their new discovery.

“We are very pleased to have found the object in the X-ray survey conducted with SRG/eROSITA,” notes Dr Axel Schwope, head of the X-ray Astronomy group at AIP and first author of the study published in Astronomy & Astrophysics. “Further surveys with ESA’s XMM-Newton satellite show pulsations in the high-energy X-ray region, the last missing piece of evidence to identify the object as a white dwarf pulsar. This confirms the unusual nature of the new object and establishes the white dwarf pulsar as a new class, even though it currently only has two members.”

Dr Ingrid Pelisoli from the Department of Physics at the University of Warwick and first author of the Nature Astronomystudy, added: “The origin of the magnetic field is a big question in many fields of astronomy, and this is especially true for white dwarf stars. The magnetic field in a white dwarf can be more than a million times stronger than that of the Sun, and dynamo models help explain why. The discovery of J1912−4410 provides an important step forward in this field.”

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