There is a mysterious phenomenon in which strong radio signals arrive periodically from space, yet their source remains completely unknown. Known as “long-period radio transients” (LPTs), these phenomena are observed as radio bursts that repeat at intervals ranging from several minutes to several hours. Only a dozen or so examples have been discovered within the Milky Way, and their physical nature has long remained a mystery.
Previous research has suggested that candidates for the source of LPTs include neutron stars known as magnetars, which rotate extremely slowly, and binary systems consisting of white dwarfs with companion stars. However, the magnetar hypothesis faces the problem of contradicting existing theoretical models.
On the other hand, while a few cases suggesting a connection to white dwarf binaries have been reported, there had been no cases in which the accretion process was directly confirmed to be actually occurring.
Against this backdrop, an international research team led by the University of Sydney in Australia conducted a sky-survey using the “Australian Square Kilometre Array Pathfinder (ASKAP)” radio telescope and identified the true nature of a mysterious object named “ASKAP J174508.9-505149.” These observational results are said to be the strongest evidence to date pointing to LPT as one of the sources of this phenomenon.
“For the first time we have pinpointed the origin of these signals,” said Kovi Rose, a doctoral student at the University of Sydney’s School of Physics and the Commonwealth Scientific and Industrial Research Organisation (CSIRO), in a press release. “We’ve been able to show that the source for one of these transients comes from a white dwarf actively pulling material from a companion star.”
A White Dwarf and a Companion Star
Rose and his research team confirmed through spectroscopic observations that ASKAP J1745-5051 exhibits hydrogen emission lines (the Balmer series) and helium emission lines (HeI and HeII). In particular, the strong HeII emission line is known as an optical feature characteristic of “magnetic cataclysmic variables.”
Cataclysmic variables is a general term for close binary systems in which a white dwarf accretes matter from a companion star. Among these, those in which the white dwarf possesses a strong magnetic field and gas accretes along magnetic field lines are called “magnetic cataclysmic variables.”
Furthermore, analysis of the radial velocities of the Balmer series emission lines revealed that the orbital period of this binary system is approximately 1.368 hours, which was confirmed to match the repetition period of the radio pulses, approximately 1.345 hours. Furthermore, based on the orbital period, the companion star’s mass was estimated to be approximately 0.096 times that of the sun, and its radius approximately 0.13 times that of the sun, indicating that it corresponds to an M6-class red dwarf.
In other words, ASKAP J1745-5051 is a binary system in which a white dwarf and a red dwarf orbit each other at an extremely close distance. A white dwarf is the high-density remnant of a star that has reached the end of its life; although it is about the size of Earth, its mass is comparable to that of the sun. Its companion, the red dwarf, is larger but less dense, with a mass of only about one-tenth that of the Sun. The two stars orbit each other in a short period of just over one hour.
A Dual Mystery Revealed by Radio Waves and X-Rays
These observations have revealed that radio bursts and X-ray emissions are generated by different mechanisms. When the white dwarf accretes gas from its companion, that gas is heated and emits X-rays. At the same time, powerful radio bursts occur in the region where the magnetic fields of the two stars interact. However, since the peaks of the radio and X-ray emissions do not coincide, it is believed that they are generated at different locations within the system.
Regarding X-rays, data from the Chinese Academy of Sciences’ “Einstein Probe” observation satellite revealed radiation with a period of approximately 1.32 hours. According to the researchers, the large amplitude of the X-ray fluctuations suggests that the accretion rate onto the white dwarf is likely changing over time.
ASKAP J1745-5051 is the third LPT detected in X-rays. It is the second LPT to exhibit regular X-ray emission, and this is the first time it has been confirmed that this regularity stems from the orbital motion of a binary system.
The radio signal itself also exhibits characteristics not previously observed in LPTs. The pulses are elliptically polarized, and the upper end of the emitted frequency fluctuated up and down in sync with a longer-period beat. It is possible that this “beat” originates from the misalignment between the white dwarf’s rotation and its orbital motion, though the rotation period could not be determined in this study.
In addition, a phenomenon known as “modulation lanes”—in which the intensity of the pulses is modulated in a striped pattern—was also observed. This is reportedly the first time this phenomenon has been detected in a binary system (a system of two celestial bodies gravitationally bound to each other) other than the Jupiter-Io system.
The Rosetta Stone of the Universe
Researchers regard ASKAP J1745-5051 as a crucial reference object for deciphering LPTs. Rose emphasizes that this discovery could function like the Rosetta stone—which was key to deciphering ancient hieroglyphs—in determining whether other LPTs are associated with neutron star pulsars or white dwarf systems.
“Some similar objects had been linked to binary systems before, but this is the first one where we can clearly see both stars and the accretion process in action,” said Tara Murphy, head of the Department of Physics at the University of Sydney, in a press release.
Star systems like ASKAP J1745-5051 could serve as natural laboratories for studying the behavior of matter under strong magnetic fields and gravitational forces that cannot be replicated on Earth. The research team plans to continue observations using telescopes across radio, optical, and X-ray wavelengths to elucidate the mechanism by which LPTs are generated.
This story originally appeared in WIRED Japan and has been translated from Japanese.
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