There is a mysterious phenomenon in which strong radio signals arrive periodically from spacebut their source remains completely unknown. Known as “long period radio transients” (LPT), these phenomena are observed as radio bursts which repeat at intervals ranging from several minutes to several hours. Only a dozen examples have been discovered within the Milky Wayand 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, although a few cases suggesting a link to white dwarf binaries have been reported, there have been no cases in which the accretion process has been directly confirmed to actually be occurring.
Against this backdrop, an international research team led by the University of Sydney in Australia conducted a sky survey using the Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope and identified the true nature of a mysterious object named ASKAP J174508.9-505149. These observation results are considered the strongest evidence to date that LPT is one of the sources of this phenomenon.
“For the first time, we have identified 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, in a press release. “We were able to show that the source of one of these transients comes from a white dwarf actively extracting 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 (Balmer series) and helium emission lines (HeI and HeII). In particular, the strong HeII emission line is known as a characteristic optical feature of “magnetic cataclysmic variables”.
Cataclysmic variables are a general term for nearby binary systems in which a white dwarf accumulates material from a companion star. Of these, those in which the white dwarf has a strong magnetic field and gas accumulates along the 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 correspond to the radio pulse repetition period, approximately 1.345 hours. Additionally, based on the orbital period, the mass of the companion star was estimated to be about 0.096 times that of the Sun and its radius to be about 0.13 times that of the Sun, indicating that it corresponds to a class M6 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 only about one-tenth that of the Sun. The two stars orbit each other over a short period of just over an hour.
A double mystery revealed by radio waves and X-rays
These observations revealed that radio bursts and X-ray emissions are generated by different mechanisms. When the white dwarf accumulates gas from its companion, this 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, because the peaks of radio and X-ray emissions do not coincide, they are thought to be generated at different locations in the system.
Regarding X-rays, data from the Chinese Academy of Sciences’ Einstein Probe observation satellite revealed radiation with a period of about 1.32 hours. According to the researchers, the large amplitude of the X-ray fluctuations suggests that the accretion rate on the white dwarf is likely changing over time.
ASKAP J1745-5051 is the third LPT detected by X-rays. It is the second LPT to exhibit regular X-ray emission, and is the first time that this regularity has been confirmed to arise from the orbital motion of a binary system.
The radio signal itself also exhibits characteristics never before observed in LPTs. The pulses are elliptically polarized and the upper end of the emitted frequency fluctuates up and down in synchronization with a longer period beat. It is possible that this “beat” comes from the misalignment between the rotation of the white dwarf and its orbital motion, although the rotation period could not be determined in this study.
Additionally, a phenomenon known as “modulation pathways” – in which the intensity of pulses is modulated in a striped pattern – has also been observed. This is apparently 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 consider ASKAP J1745-5051 a crucial reference object for deciphering LPTs. Rose points out that this discovery could work like the Rosetta Stone, which was key to deciphering ancient hieroglyphs, to determine whether other LPTs are associated with neutron star pulsars or white dwarf systems.
“Some similar objects had previously been linked to binary systems, but this is the first where we can clearly see both the stars and the accretion process in action,” said Tara Murphy, head of the physics department at the University of Sydney, in a press release.
Star systems like ASKAP J1745-5051 could serve as natural laboratories to study the behavior of matter under powerful magnetic fields and gravitational forces that cannot be replicated on Earth. The research team plans to continue observations using telescopes in radio, optical and X-ray wavelengths to elucidate the mechanism by which LPTs are generated.
This story was originally published in WIRED Japan and was translated from Japanese.
