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 Kilometer 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 Organization, 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.