Astronomers have discovered a rare white dwarf pulsar, deepening our understanding of stellar evolution and the magnetic fields of white dwarfs. The discovery supports the dynamo model and suggests there are more such pulsars in the universe. In two independent studies, the University of Warwick and the Leibniz Institute for Astrophysics Potsdam (AIP) have discovered a rare star system that provides new insights into dynamical model predictions of stellar evolution.
This new white dwarf pulsar is an extremely close binary system composed of a white dwarf star and a red dwarf star. Together they can fit into the interior of the sun. It is the second known pulsar of its kind.
White dwarfs are extremely dense stellar remnants that are as massive as the sun but only as small as the Earth. They are formed when low-mass stars burn up all their fuel, lose their outer layers, and contract violently inside. Also known as "stellar fossils," they can provide insight into various aspects of stellar evolution.
Pulsars, on the other hand, have been known since the 1960s, and more than 3,000 have been discovered. They are rapidly rotating, strongly magnetic neutron stars, in which charged particles are torn from the surface by ultra-strong electric fields and then accelerated to nearly the speed of light. Therefore, they emit radiation, or light, ranging from radio to X-rays and even gamma rays. Due to the star's rapid rotation, short pulses of radiation reach Earth, which is why they are called pulsars.
To the surprise of the scientific community, in 2016, the pulsar phenomenon was observed for the first time on a white dwarf star. Surprisingly, there is neither extremely fast rotation nor the strong electric fields of a true pulsar in the star, called ARScorpii.
The white dwarf was discovered in a very close binary star system, and its neighbor - a sun-like red dwarf - powers it by injecting particles into its magnetic field. This ignites the pulsar phenomenon from the outside and shines a stroboscope on the red companion, causing the entire system to sharply brighten or dim at intervals. The two stars - a white dwarf and a red dwarf - are so close together that they could fit into our sun.
Exploring magnetic fields and 'dynamo models'"
The decisive factor is the presence of a strong magnetic field, but astrophysicists don't know why. A key theory to explain the strong magnetic field is the "dynamo model", which suggests that there is a dynamo in the core of the white dwarf star, just like the Earth, but much stronger than the Earth. But to test this theory, the researchers must look for other white dwarf pulsars to see if their prediction is correct.
In two new studies published simultaneously in Nature Astronomy and Astronomy & Astrophysics, an international research team involving the National Institute of Astronomy (AIP) described the newly discovered white dwarf pulsar J1912-4410 (eRASSUJ191213.9-441044). It is 773 light-years away from Earth and rotates once every 5 minutes, 300 times faster than our Earth. White dwarf pulsars are similar in size to Earth but at least as massive as the Sun. This means that one teaspoon of a white dwarf weighs about 15 tons. A white dwarf's life begins at extremely high temperatures and then cools over billions of years. J1912-4410's low temperature indicates that it is very old.
This study confirms that there are more white dwarf pulsars, as earlier models predicted. The discovery of J1912-4410 also confirms other predictions of dynamical models. White dwarfs in pulsar systems should be very cold due to their age. Their companions should be close enough that the white dwarf's past gravity is enough to extract mass from the companion, causing them to spin rapidly. For the newly discovered pulsar, all these assumptions hold true: the white dwarf has a temperature below 13,000 Kelvin, a high rotation rate of about 5 minutes, and the white dwarf's gravity has a strong influence on the companion star.
Collaborative research and future impact
A research team used data from Gaia and WISE to search for candidate objects, focusing on those with similar properties to AR Scorpii. After observing dozens of candidate objects, they discovered an object with very similar light variations. Follow-up observations with other telescopes found that the system was sending radio and X-ray signals to Earth about every five minutes. Another research team discovered a nearby white dwarf/red dwarf pair using data from the eROSITAX ray telescope on the Spectrum-X-Gamma satellite. The two groups joined forces to further investigate their new findings.
"We are delighted to have discovered this object in the X-ray survey conducted by SRG/eROSITA," said Dr. Axel Schwope, leader of the AIP X-ray Astronomy Group and first author of the study published in Astronomy & Astrophysics. "Following observations using ESA's XMM-Newton satellite revealed pulses in the high-energy X-ray region, the last piece of evidence missing to identify the object as a white dwarf pulsar. This confirms the unusual nature of this new object and identifies white dwarf pulsars as a new class, albeit one that currently only has two members."
Dr. Ingrid Pelisoli from the Department of Physics at the University of Warwick, first author of the study in Nature Astronomy, added: "The origin of magnetic fields is a major open question in many areas of astronomy, but this is particularly true for white dwarfs. The magnetic field of white dwarfs may be more than a million times stronger than that of the Sun, and dynamical models help explain why. The discovery of J1912-4410 is a key step forward for research in this field."
Compiled source: ScitechDaily