The newly discovered star, 773 light-years away, is in one of the rarest categories in the Milky Way.
J1912-4410 is a white dwarf pulsar, a very rare type of star and only one known in the entire galaxy. Its discovery confirms that these stars exist in a class of their own and provides new tools for interpreting not only stellar evolution, but strange signals detected throughout the Milky Way that defy conventional explanations.
This discovery appears to confirm that the white dwarf’s magnetic field is generated by an inner dynamo. This is similar to how Earth’s liquid core creates a magnetic field, but much more powerful.
“The origin of the magnetic field is a big open question in many fields of astronomy, and this is particularly true of white dwarf stars,” explains astrophysicist Ingrid Pelisoli of Warwick University, UK.
“The magnetic field of a white dwarf can be over a million times stronger than that of the Sun, and the dynamo model helps explain why. The discovery of J1912-4410 provided an important step in this field.”
Traditionally, pulsars are a type of dead star called neutron stars. They are the remnants of massive stars between about 8 and 30 times as large that have run out of hydrogen fuel to fuse in their cores. Stars expel foreign material, and the core, no longer supported by external fusion pressure, collapses under gravity into super-dense objects.
In the case of a pulsar, a neutron star spins as fast as milliseconds, and electromagnetic radiation produced by the fast spin and strong magnetic field is ejected from the magnetic pole. As the star rotates, these rays pass through our field of view like cosmic lighthouses, making the star appear to pulsate.
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White dwarfs are similar types of stellar remnants. They are the collapsed cores of dead stars below about 8 solar masses. They are less dense than neutron stars and have a larger radius. As far as we know, until just a few years ago, they don’t seem to turn into pulsars either.
Then, in 2016, astronomers discovered a star called AR Scorpii, which they call the first white dwarf pulsar. AR Scorpii is a bit different from a traditional pulsar. It is a white dwarf in a binary system with red dwarf stars. As it rotates, its rays pass the red dwarf and brighten across several wavelengths in a regular time frame of 1.97 minutes. The pulsations we see do not come directly from the white dwarf’s rays, but from the effect of those rays on the companion red dwarf.
However, AR Scorpii’s system defied our understanding of white dwarfs with rotational speeds that are normally only achieved through mass transfer from red dwarfs. However, the speed of a white dwarf’s spin-down suggests a strong magnetic field, and a significant amount of mass must be moved to reach the white dwarf’s dizzying rotational speed.
One possible explanation is the changes the white dwarf undergoes as it cools and crystallizes. The AR Scorpii white dwarf may start out without a magnetic field and increase its rotation rate as it slowly steals mass from the red dwarf.
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But as the white dwarf cooled, the combination of internal density changes and convection allowed heat to escape and power the generator. These rotating, conducting, and convective fluids convert kinetic energy into magnetic energy that rotates from objects to magnetic fields.
We don’t really know what goes on inside white dwarf stars. We know that they are incredibly dense around the mass of the sun packed into an Earth-sized object, and they don’t decay anymore because electrons don’t occupy the same state below a certain critical threshold. How it works is purely hypothetical. AR Scorpii could mean that the interiors of white dwarf stars can generate generators.
But a one-star sample size made it impossible to confirm, so Pelisoli and her colleagues searched for more. They scoured survey data to find stars with similar characteristics to AR Scorpii. They then followed up to see if they matched the candidates.
“After observing dozens of candidates, we found that they exhibit very similar light changes to AR Scorpii. In subsequent campaigns with other telescopes, the system sent radio and X-ray signals in our direction about every five minutes. It turns out,” she says.
“This confirms that there are more white dwarf pulsars, as predicted by previous models.”
The newly discovered J1912-4410 fits well with many other characteristics of the dynamo model. The white dwarf pulsar must be relatively cool and close enough to its binary companion that material transfer may have occurred in the past to show that crystallization is occurring within it and to increase the white dwarf’s spin. J1912-4410 matches these characteristics perfectly.
A second study, led by astrophysicist Alex Schwoffe at the Leibniz Institute for Astrophysics in Potsdam, Germany, independently found J1912-4410 in data from the X-ray space observatory eROSITA. They too concluded that the object was a white dwarf pulsar like AR Scorpii, strongly suggesting that many more such objects exist.
And it could help astronomers solve an ongoing mystery. For example, something near the center of a galaxy flashes radio waves at a regular beat of 18.18 minutes. It could be a white dwarf pulsar because it doesn’t tick all the boxes seen in AR Scorpii and J1912-4410.
But this discovery provides new tools for understanding the strange things we detect in the Milky Way.
“We are delighted to have found the target independently in an X-ray allometric investigation performed with SRG/eROSITA,” said Schwope. “Subsequent investigations using the ESA satellite XMM-Newton revealed pulsations in the high-energy X-ray regime, thus confirming the unusual properties of the new object and firmly establishing white dwarf pulsars as a new class.”
Two papers have been published. natural astronomy and astronomy and astrophysics.
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