A Rare White Dwarf Pulsar Found 773 Light-Years Away in the Milky Way

A recently discovered star, known as J1912-4410, belongs to one of the most uncommon categories in our galaxy. This white dwarf pulsar is an exceptionally rare type of star, with only one other known to exist in the entire Milky Way. Its discovery not only confirms the existence of these stars in a unique class of their own but also provides a valuable tool for interpreting enigmatic signals detected throughout our galaxy that defy conventional explanations.

The findings of this discovery suggest that the magnetic field of a white dwarf is generated by an internal dynamo, akin to how Earth’s liquid core generates its magnetic field, but on a significantly more powerful scale.

“The origin of magnetic fields is a significant unresolved question in various astronomical fields, particularly when it comes to white dwarf stars,” explains astrophysicist Ingrid Pelisoli from the University of Warwick in the UK.

“Magnetic fields in white dwarfs can be over a million times stronger than the Sun’s magnetic field, and the dynamo model helps elucidate the reasons behind this. The discovery of J1912-4410 represents a crucial advancement in this field.”

Traditionally, pulsars are a type of neutron star, the remnants of massive stars that have exhausted their hydrogen fuel in their cores and subsequently shed their outer material. The core, no longer supported by fusion’s outward pressure, collapses under gravity to form an ultradense object. In the case of a pulsar, the neutron star spins rapidly, sometimes reaching millisecond scales, and beams of electromagnetic radiation, produced by the intense spin and powerful magnetic field, emanate from its magnetic poles. As the star rotates, these beams sweep across our line of sight, resembling the pulsing light of a celestial lighthouse.

White dwarfs, which are remnants of dead stars with masses below approximately 8 times that of our Sun, represent a similar type of stellar remnant. They possess larger radii and lower density compared to neutron stars. Until just a few years ago, it was believed that white dwarfs do not transform into pulsars.

However, in 2016, astronomers made a significant discovery with the identification of the first white dwarf pulsar named AR Scorpii. AR Scorpii differs slightly from traditional pulsars as it exists in a binary system alongside a red dwarf star. As the white dwarf rotates, its beams sweep past the red dwarf, causing periodic brightening across multiple wavelengths every 1.97 minutes. The observed pulsations are not directly emitted by the white dwarf’s beams but are a result of their influence on the red dwarf companion.

The system of AR Scorpii challenges our existing understanding of white dwarfs, as its spin rate is typically achieved through mass transfer from the red dwarf, which accelerates the white dwarf’s rotation. However, the white dwarf’s decreasing spin rate suggests the presence of a strong magnetic field, requiring a substantial amount of mass to be transferred to attain such a rapid spin.

One possible explanation lies in the changes white dwarfs experience as they cool and crystallize. It is plausible that the white dwarf in the AR Scorpii system initially lacked a magnetic field, allowing its spin rate to increase gradually as it gradually accreted mass from the red dwarf.

However, the process of cooling in white dwarf stars, combined with changes in interior density and convection as heat escapes, could potentially initiate a dynamo mechanism. This dynamo involves a rotating, conducting, and convecting fluid that converts kinetic energy into magnetic energy, generating a magnetic field that extends outward from the white dwarf.

Despite our understanding of white dwarf stars being limited, we do know that they possess incredibly high densities, with the mass of the Sun compressed into an object the size of Earth. The collapse of the white dwarf is prevented by the electron degeneracy pressure, which prohibits electrons from occupying the same quantum state beyond a specific critical threshold. However, the precise internal structure and behavior of white dwarfs remain largely speculative. The discovery of AR Scorpii may suggest that the interior of a white dwarf has the potential to generate a dynamo.

To confirm this hypothesis, astrophysicist Ingrid Pelisoli and her colleagues conducted an extensive search for additional white dwarf stars exhibiting similar characteristics to AR Scorpii. They examined survey data to identify candidate stars and subsequently conducted follow-up observations to determine if these candidates demonstrated comparable behavior.

“After observing several potential candidates, we discovered one star that displayed light variations resembling those of AR Scorpii. Further investigations using other telescopes revealed that this particular system emitted radio and X-ray signals toward our direction approximately every five minutes,” explains Pelisoli.

“This confirmation indicates the presence of additional white dwarf pulsars, aligning with predictions from previous models.”

The recently discovered J1912-4410 aligns with several key characteristics of the dynamo model. As expected for white dwarf pulsars, J1912-4410 exhibits a relatively cool temperature, indicating ongoing crystallization in its interior. Additionally, it is positioned in close proximity to its binary companion, suggesting the possibility of past mass transfer events that could have accelerated the white dwarf’s spin. J1912-4410 perfectly matches these criteria.

In a separate study led by astrophysicist Alex Schwope from the Leibniz Institute for Astrophysics Potsdam in Germany, the identification of J1912-4410 in data from the X-ray space observatory eROSITA further supports its classification as a white dwarf pulsar akin to AR Scorpii. This finding strongly implies that more of these objects exist in the cosmos.

The discovery of white dwarf pulsars like J1912-4410 holds the potential to shed light on ongoing astronomical mysteries. For instance, there are regular radio wave emissions occurring near the galactic center with an 18.18-minute periodicity. It is conceivable that one such source could be a white dwarf pulsar, potentially lacking a binary companion as it may not exhibit all the characteristics observed in AR Scorpii and J1912-4410.

This breakthrough provides astronomers with a fresh tool to aid in the comprehension of the peculiar phenomena detected within our Milky Way galaxy.

“We are thrilled to have independently detected this object in the all-sky X-ray survey conducted with SRG/eROSITA,” remarks Schwope. “Subsequent investigations using the ESA satellite XMM-Newton confirmed the presence of pulsations in the high-energy X-ray range, thereby verifying the extraordinary nature of this newfound object and establishing white dwarf pulsars as a novel class.”

The two papers have been published in Nature Astronomy and Astronomy & Astrophysics.

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