Accreting white dwarf: periodic radio pulses in a binary system ✦
Long-period radio transients (LPTs) are coherent bursts of polarized emission with periods from minutes to hours. Their origin remained unclear: slowly rotating magnetars or binary systems with white dwarfs were candidates. This work reports the discovery and classification of LPT ASKAP J174508.9-505149 as an accreting white dwarf binary. The object has an orbital period of ~1.3 hours, displays orbitally modulated X-ray emission, and radio bursts with elliptical polarization. The burst frequency drifts, possibly due to a longer beat period, and disappears for several hours. It is spectroscopically confirmed as a cataclysmic variable via optical emission lines and an ongoing X-ray outburst. The results point to a connection between at least some LPTs and white dwarf binaries.
Context
Since 2022, astronomers have been puzzling over long-period radio transients — sources of powerful polarized radio bursts repeating with periods from minutes to hours. Unlike ordinary radio pulsars, discovered by Jocelyn Bell Burnell in 1967, these objects are too slow for neutron stars, unless they are magnetars with extremely strong fields. An alternative hypothesis suggests they are white dwarfs in binary systems, but until now there was no direct evidence of accretion, characteristic of cataclysmic variables. Understanding the nature of these transients is important for the physics of compact objects and the generation of coherent emission.
Methods
For a comprehensive study of ASKAP J1745−5051, the team led by Kovi Rose used a wide arsenal of instruments. Radio observations were carried out with the Australian telescopes ASKAP and ATCA, and the South African MeerKAT, which provided detailed dynamic spectra. Optical spectroscopy on the SOAR and Magellan telescopes revealed emission lines of hydrogen (Balmer series) and helium, characteristic of magnetic cataclysmic variables. X-ray data were obtained by the Swift and Einstein Probe space observatories, and photometry by the Gaia satellite. Period analysis was performed using radio pulse timing and Doppler measurements.
Results
The orbital period of the system, measured from the Doppler shift of spectral lines, was 1.368 ± 0.053 hours — one of the shortest among such objects. This period coincides with the radio pulse period (1.34497 hours), confirming orbital modulation of the emission. The X-ray light curve also shows periodicity with a phase shift relative to the radio bursts: in some observations, the X-ray peak coincides with radio pulses, in others it is in antiphase. This behavior indicates accretion of matter from a low-mass red dwarf (spectral class M6) onto a magnetic white dwarf. The radio pulses were found to be elliptically polarized, with a narrowband structure and frequency drift, and their brightness temperature exceeds 10^12 K, which unequivocally requires a coherent emission mechanism. The optical spectrum is dominated by narrow emission lines of hydrogen and helium, including the HeII 4686 Å line — an indicator of magnetic accretion. The Hα/Hβ intensity ratio ≤ 1 indicates a high electron density (ne ≳ 10^13 cm^{-3}), typical of accreting polars. The white dwarf mass is estimated at 0.83 M☉ — close to the Chandrasekhar limit, established by Subrahmanyan Chandrasekhar.
Implications
This discovery directly links long-period radio transients with cataclysmic variables for the first time, confirming that accretion plays a key role in generating the observed emission. Unlike isolated white dwarfs or neutron stars, ASKAP J1745−5051 demonstrates how the interaction of the magnetospheres of two stars can produce powerful coherent radio emission — possibly through a relativistic electron-cyclotron maser. This forces a revision of models for other transients, such as GLEAM-X J0704 and ILT J1101, and opens a new path for studying accretion physics in extreme magnetic fields.
Future development
Future long-term optical and radio observations, especially with simultaneous X-ray coverage, will help refine the geometry of magnetic fields and the emission modulation mechanism. Detailed modeling of magnetospheric interaction in asynchronous systems like this one could explain unique pulse features — such as frequency drift and modulation bands. With the commissioning of new instruments like SKA and advances in spectropolarimetry, we can expect the discovery of a whole population of such objects, turning them from curiosities into a full-fledged class of astrophysical sources.
Impact
The results will impact the physics of accreting white dwarfs, the theory of stellar magnetospheres, and the understanding of coherent emission mechanisms in astrophysical plasma. They may also help interpret other transient classes, including fast radio bursts.
Next steps
Immediate next steps include obtaining deep spectropolarimetric data to estimate the white dwarf's magnetic field and conducting coordinated multi-wavelength campaigns with high time resolution. A search for similar systems in archival radio survey data is also needed.
Key open problems
The discovery touches on several fundamental problems: the nature of long-period radio transients, the mechanism of coherent radio emission in stellar systems (electron-cyclotron maser vs other processes), and the role of magnetic fields in accretion onto compact objects. In particular, it remains unclear how exactly the accreting plasma creates conditions for maser amplification and why the pulses are so narrowband and polarized. It also resonates with the mystery of the anomalously high radio luminosity of some cataclysmic variables.
🎯 The radio pulses of ASKAP J1745−5051 are so intense that their brightness temperature exceeds 10^12 K — that's a hundred times hotter than the center of the Sun, requiring a coherent emission mechanism akin to earthly lasers, but on cosmic scales. Moreover, the radio signal shows 'modulation bands' — interference patterns previously observed only in decametric emission from Jupiter from its moon Io!
Key numbers
- orbital period: 1.37 hours
- radio period: 1.345 hours
- X-ray luminosity (0.2–10 keV): 10^30–10^33 erg/s
- radio luminosity (1.365 GHz): 10^18–10^21 erg/s/Hz
- brightness temperature: >10^12 K