Abstract:
Astronomers at Curtin University in Australia have discovered an extremely rare binary star system in which one of the dead stars, a white dwarf, emits regular radio pulses every about eight minutes. Its companion star is a star similar to the sun. This is the first time that scientists have detected such periodic radio signals in a binary system composed of a white dwarf and a sun-like star, providing new clues to understanding a mysterious type of celestial body in the Milky Way.
This system, named GPM J071507−114153, is more than 5,000 light-years away from Earth and consists of two stars orbiting each other. One of them is a white dwarf, a high-density core left behind after a star has exhausted its nuclear fuel and evolved; the other is a G-type star, which has similar spectral types and basic characteristics to our sun.
This discovery is of particular interest because similar radio pulse systems previously discovered by scientists usually contain white dwarfs and smaller, cooler red dwarfs. The companion star discovered this time is a star similar to the sun, indicating that this type of mysterious celestial body may have a richer composition than previously recognized.
Researchers classify this type of object into a category called "Long-period Transients (LPT)". Such objects can emit radio pulses at intervals ranging from minutes to hours, with the signal turning on and off periodically. Since researchers at the International Center for Radio Astronomy Research (ICRAR) first discovered such objects in 2021, astronomers have confirmed about a dozen similar systems, but the mechanism through which they generate radio radiation has not yet been fully explained.
Scientists have previously discovered binary structures composed of white dwarfs and other stars in some long-period transient sources, but it has always been difficult to determine which star produced the radio signal. The key progress made in this study is that researchers were finally able to directly observe the movement of the white dwarf in the binary orbit through changes in the pulse signal.
Sam McSweeney, the first author of the paper and commissioning scientist at the Square Kilometer Array Observatory (SKAO), said that this is the first time that researchers have obtained a clear enough signal to identify the motion characteristics of a white dwarf as it orbits a companion star. Therefore, scientists can determine that the radio pulse originates from a region near the white dwarf, or possibly even directly from the white dwarf itself.
In order to verify this judgment, the research team used the Pawsey Supercomputing Research Center in Australia to process more than 100 hours of radio observation data, and analyzed the pulse signal and the orbital motion of the white dwarf star.
This discovery also has an unexpected gain: the radio pulses emitted by white dwarfs actually become a natural detection tool for studying the activities of their companion stars.
Natasha Hurley-Walker, co-author of the paper and Associate Professor at Curtin University, said that the researchers found that when the radio waves emitted by the white dwarf pass through the space around the sun-like star, the signal will be affected by the activity of the star. By analyzing these changes, the team was able to identify stellar flares on the companion star's surface.
This phenomenon is similar to flare activity on the sun. Solar flares release large amounts of energy and can trigger disturbances in the Earth's magnetosphere and auroras. This study shows that astronomers can use radio waves to detect similar activity on another star more than 5,000 light-years away.
The researchers also observed that the white dwarf passed behind a region with a strong magnetic field during its orbit. Scientists speculate that this region is likely a type of stellar prominence, a large plasma structure extending from the surface of a star that is often bound by a magnetic field and can change dramatically during periods of stellar activity.
The research team first discovered this system through Australia's Murchison Widefield Array radio telescope (MWA). The telescope is capable of scanning the entire Milky Way sky observable from Australia, helping researchers search for rare periodic objects in large amounts of radio signals.
The team then used the MeerKAT radio telescope in South Africa to determine the precise location of the target, and then used the Keck Observatory (W. M. Keck Observatory) on Mauna Kea, Hawaii to conduct optical observations and finally confirmed the existence of a G-type stellar companion in the system.
This discovery also demonstrates the scientific discovery capabilities of existing leading equipment before the new generation of large-scale radio astronomy facilities are built. MWA and MeerKAT are the pilot projects for SKA-Low and SKA-Mid, respectively, the two main components of the future Square Kilometer Array Observatory. The former is located in Australia and the latter is located in South Africa.
Researchers believe that with the gradual completion of the Square Kilometer Array Observatory, the sensitivity and observation capabilities of radio telescopes will be further improved. Astronomers are expected to discover more long-period temporary sources that were previously difficult to identify and obtain more detailed radio signal data.
The significance of this research is not only the discovery of a previously unidentified binary star combination, but also the solution of an important issue in the study of long-period transient sources: which star in the binary system the radio signal comes from. By identifying white dwarfs as the source of pulsed radiation, scientists can further study the magnetic field structure, stellar activity, and interactions between binaries of such objects.
The related paper is titled "A long-period transient with a Sun-like companion" and has been published in Nature Astronomy.
At present, long-period transient sources are still a type of mysterious celestial body that has not been fully understood in astronomy. The behavior of emitting radio pulses only once every few minutes or even hours is completely different from the characteristics of ordinary pulsars that emit multiple signals per second. This discovery provides new observational evidence to explain these abnormal signals, but the complete physical mechanism behind it still needs further study.
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