Long-period transients are a class of periodic pulsed radio source repeating on the minute to hour timescale. An increasing number of them are being identified as binary systems, specifically white dwarfs with low-mass main-sequence companions. Horvath et al. analyse the most luminous long-period transient discovered to date, ASKAP/DART J1832−0911, which was discovered contemporaneously by ASKAP and the Daocheng Radio Telescope (DART). The team present two years of radio data, and propose that ASKAP/DART J1832−0911, too, may be a white dwarf system, although in a far more compact orbit than other systems. The pulses are composed of quasi-periodic components which evolve in a systematic way over days and months. The source is highly linearly or elliptically polarised and its brightness enabled very high signal-to-noise measurement of the time-resolved Faraday rotation measure, which was found to vary across pulse phase. The linear polarisation position angle, circular polarised fraction, and spectral index also varied systematically in ways not typical of pulsars and magnetars. The team show that an ultra-compact asynchronous polar explains much of the phenomenology of ASKAP/DART J1832−0911, in particular the evolution of the pulse morphology, rotation measure variation, and periodic X-ray emission. Although they cannot conclusively prove a binary nature, their model does makes testable predictions, including a re-brightening of J1832−09 by July of 2027.

The figure above captures the wealth of information gathered to date for J1832−09. It shows the pulses detected by ASKAP in chronological order (bottom to top), with the pulse name and topocentric UTC times of pulse centres marked at left. The leftmost column is folded on P1 = 2656.24978 s; the rest are folded on P2 = 2656.25325 s. The columns from left to right are: peak-normalised mean pulse profile over the band with linear and circular polarised components; linear polarisation position angle (PA); Faraday rotation measure (RM) minus the ionospheric contribution; spectral index; peak mean intensity across the band with absolute time on the vertical axis.  The horizontal red line indicates the 2024 February 14 Chandra X-ray observation.