A sibling of AR Scorpi : SDSS J230641.47+244055.8 and the observational blueprint of white dwarf pulsars

Abstract

Radio pulsating white dwarf (WD) systems, known as WD pulsars, are binary systems where the rapidly spinning WD interacts with a low-mass companion, producing pulsed non-thermal emission observed across the entire electromagnetic spectrum. Only two such systems are known: AR Sco and eRASSU J191213.9−441044. Here, we present the discovery of a third WD pulsar, SDSS J230641.47+244055.8. The optical spectrum is dominated by molecular bands from an M-dwarf companion, with additional narrow Balmer and He I lines. The long-term optical light curve folded on its orbital period (Porb = 3.49 h) shows ~ 10 per cent scatter. High-cadence photometry reveals a short-period signal, which we interpret to be the spin period of the WD primary (Pspin 92 s). The WD spin period is slightly shorter than that of AR Sco (~ 117 s), the WD pulsar prototype. Time-resolved spectroscopy reveals emission from the irradiated companion and Na I absorption tracing its centre of mass, yielding a binary mass function of f (M) 0.2 M . The Hα emission includes a low-amplitude broad component, resembling the energetic flashes seen in AR Sco. Using spectral templates, we classify the companion to be most likely a M4.0 ± 0.5 with Teff ≈ 3300 K. Modelling the stellar contribution constrains the secondary mass (0.19 M M2 0.28 M ), distance ( 1.2 kpc), and inclination (i 45◦ − 50◦). We discuss the proposed evolutionary scenarios and summarize the observational properties of all three known WD pulsars, establishing a benchmark for identifying and classifying future members of this class.