Mysterious Repeating Radio Signals from Space Traced to White Dwarf Binary System
An international research team using the ASKAP radio telescope has identified the source of mysterious repeating radio signals: a white dwarf accreting material from a red dwarf companion. This marks the first time such signals have been precisely pinpointed.
The Enigma of Long-Period Radio Transients
For years, astronomers have puzzled over a phenomenon known as long-period radio transients (LPTs) – strong radio signals that repeat at intervals ranging from minutes to hours. Only about a dozen such objects have been found within the Milky Way, and their physical nature remained unknown.
Previous hypotheses suggested they could be slowly rotating neutron stars called magnetars or binary systems involving a white dwarf and a companion star. However, the magnetar hypothesis conflicted with theoretical models, and no direct confirmation of accretion in a white dwarf system had been made.
A Breakthrough Identification
Now, an international team led by the University of Sydney has used the Australian Square Kilometre Array Pathfinder (ASKAP) to pinpoint the true nature of one such object, designated ASKAP J174508.9-505149. For the first time, researchers have shown that the source is a white dwarf actively pulling material from a companion star.
Spectroscopic observations revealed hydrogen and helium emission lines, particularly a strong HeII line characteristic of magnetic cataclysmic variables – close binary systems where a white dwarf with a strong magnetic field accretes gas along magnetic field lines. The orbital period was measured at approximately 1.368 hours, closely matching the radio pulse repetition period of about 1.345 hours. The companion is a red dwarf with a mass of roughly 0.096 solar masses and a radius of 0.13 solar radii, consistent with an M6-class star.
Two Different Emission Mechanisms
The system produces both radio bursts and X-rays through distinct processes. As the white dwarf accretes gas from its companion, the gas heats up and emits X-rays. Simultaneously, powerful radio bursts occur in the region where the magnetic fields of the two stars interact. The peaks of radio and X-ray emissions do not coincide, indicating different locations within the system.
X-ray data from the Chinese Academy of Sciences’ Einstein Probe satellite revealed a period of about 1.32 hours. This makes ASKAP J1745-5051 the third LPT detected in X-rays, the second with regular X-ray emission, and the first where that regularity has been confirmed to originate from orbital motion.
The radio signal itself exhibits novel features: elliptical polarization and frequency fluctuations that oscillate in sync with a longer-period beat, possibly due to a misalignment between the white dwarf’s rotation and its orbital motion. Additionally, modulation lanes – a striped pattern of intensity modulation – were observed for the first time in a binary system other than the Jupiter-Io system.
A Rosetta Stone for LPTs
Researchers regard ASKAP J1745-5051 as a key reference for deciphering LPTs. Kovi Rose, a doctoral student at the University of Sydney, described the discovery as potentially acting like the Rosetta Stone, helping to determine whether other LPTs are associated with neutron star pulsars or white dwarf systems.
Tara Murphy, head of the University of Sydney’s Department of Physics, noted that while some similar objects had been linked to binary systems before, this is the first where both stars and the accretion process are clearly visible. Such systems serve as natural laboratories for studying matter under extreme magnetic and gravitational conditions not replicable on Earth. The team plans to continue observations across radio, optical, and X-ray wavelengths to elucidate the generation mechanism of LPTs.
