A newly discovered star just 773 light-years away belongs to one of the rarest classes in the Milky Way.
J1912-4410 is a white pulsar, a type of star so rarely seen that only one other star is known in the entire galaxy. His discovery confirms that these stars exist in a class of their own, and gives us a new tool for interpreting not only the evolution of stars but also strange signals detected throughout the Milky Way that defy conventional explanation.
The discovery seems to confirm that the white dwarf’s magnetic field is generated by an internal dynamo, similar to the method Earth’s liquid core generates its magnetic field But in a more powerful way.
“The origin of magnetic fields is a big open question in many areas of astronomy, and this is especially true of white dwarf stars,” explains astrophysicist Ingrid Pelisoleli from the University of Warwick in the UK.
“Magnetic fields in white dwarfs can be up to a million times stronger than the magnetic field of the Sun, and the dynamo model helps explain why. The discovery of J1912-4410 provided a crucial step forward in this field.”
Traditionally, pulsars are a type of dead star called neutron stars. They are the remains of stars between about 8 and 30 times massive that have run out of hydrogen fuel to fuse in their cores. The star ejects its outer matter, and the core, no longer supported by the outward pressure of fusion, collapses under the action of gravity into a superdense body.
In the case of a pulsar, the neutron star spins rapidly, down to millisecond scales, while beams of electromagnetic radiation, generated by the rapid rotation and strong magnetic field, erupt from the magnetic poles. As the star rotates, these rays cross our field of view like a cosmic beacon, making the star appear to pulsate.
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White dwarfs are a similar type of star remnant. They are the collapsed cores of dead stars under about 8 solar masses. They are less massive than neutron stars and have larger radii. As far as we know until a few years ago, they didn’t seem to turn into pulsars.
Then, in 2016, astronomers found what they called the first white dwarf pulsar, a star named AR Scorpii. AR Scorpii is a bit different than a traditional pulsar. It is a white dwarf in a binary system with a red dwarf star. As it spins, its beams sweep across the red dwarf, causing it to shine across multiple wavelengths on regular 1.97-minute timeframes; The pulses we see are not directly from the white dwarf’s rays, but from the effect of those beams on the red dwarf’s companion.
However, the AR Scorpii system has challenged our understanding of white dwarfs, with a spin rate normally achieved only by transferring mass from the red dwarf, causing the white dwarf to spin faster. However, the white dwarf’s spin rate indicates a strong magnetic field, which would require a large amount of mass to be transferred to reach the white dwarf’s amazing spin rate.
One possible explanation These are the changes that white dwarfs go through as they cool and crystallize. The white dwarf AR Scorpii likely started without a magnetic field, which allowed its spin rate to increase as it slowly stole mass from the red dwarf.
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However, as the white dwarf cools, the internal density changes along with convection as escaping heat can trigger a dynamo. This rotating, conductive, and convective fluid converts kinetic energy into magnetic energy that rotates off the body as a magnetic field.
We don’t really know what happens inside white dwarf stars. We know that it’s incredibly dense, about the mass of the Sun packed into an Earth-sized body, and only the refusal of the electrons to occupy the same state below a certain critical threshold prevents them from collapsing further, but what this looks like and how it behaves is purely hypothetical. AR Scorpii could mean that the interior of a white dwarf is capable of generating a dynamo.
But a sample size of one star makes it impossible to confirm, so Bellisoli and her colleagues looked for more. They combed the survey data, looking for stars with similar properties to AR Scorpii. Then they followed up with their candidates to see if they were a match.
“After observing dozens of candidates, we found one that showed very similar optical differences to AR Scorpii. Our follow-up campaign with other telescopes revealed that every five minutes or so, this system sends a radio and X-ray signal in our direction,” he said. Says.
“This confirms the presence of more white pulsars, as previously predicted by models.”
The newly discovered J1912-4410 also fits many of the other characteristics of the dynamo model. The white pulsars should be relatively cool, indicative of crystallization occurring within, and close enough to their binary companion that mass transfer could have occurred in the past to increase the white dwarf’s spin. J1912-4410 matches these characteristics perfectly.
A second study led by astrophysicist Alex Schwope of the Leibniz Institute for Astrophysics Potsdam in Germany independently found J1912-4410 in data from the X-ray space observatory eROSITA. They also concluded that the object is a white dwarf pulsar like AR Scorpii, which strongly suggests that there are more such objects out there.
And it could help astronomers solve persistent mysteries. For example, something near the galactic center flashes radio waves on a regular, 18.18-minute interval. This could be a white dwarf pulsar, perhaps without a binary companion, since it doesn’t tick all the boxes seen in AR Scorpii and J1912-4410.
But this discovery gives us a new tool for understanding the strange things we’re discovering in the Milky Way.
“We are excited that we found the body independently in an X-ray scan performed with SRG/eROSITA,” Schwope says. “A follow-up investigation with the ESA XMM-Newton satellite detected pulsations in the high-energy X-ray system, confirming the unusual nature of the new object and firmly establishing white pulsars as a new class.”
Both papers have been published in natural astronomy And Astronomy and astrophysics.
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