A binary model of long period radio transients and white dwarf pulsars

Abstract

Long-period radio transients (LPTs) represent a recently uncovered class of Galactic radio sources exhibiting minute-to-hour periodicities and highly polarised pulses of second-to-minute duration. Their phenomenology does not fit exactly in any other class, although it might resemble that of radio magnetars or white dwarf (WD) radio emitting binary systems. Notably, two LPTs with confirmed multi-wavelength counterparts have been identified as WD -- M dwarf binaries. Meanwhile, systems such as AR Scorpii and J1912-44 exhibit short-period pulsations in hrs-tight orbits, with polarised radio emission proposed to be generated by the interaction of the WD magnetosphere with the low-mass companion wind. Here, we investigate the longest-lived LPT known, GPM J1839-10, demonstrating that it has a ~8.75 hr orbital period. We show that its radio pulses can be modelled in the same geometric framework as WD binary pulsars, in which radio emission is triggered when the magnetic axis of a rotating WD intersects its companion's wind in the binary orbital plane. We use a 36-year timing baseline to infer the orbital period and binary geometry from radio data alone. The model naturally predicts its intermittent emission and double-pulse structure. Crucially, we show that the beat period between the spin and the orbit matches the observed pulse substructure and polarisation signatures, providing strong support for the model. Applying it to the WD pulsar J1912-44, it successfully reproduces the emission profile and geometry as well. Our results suggest analogous emission-site geometries in these related classes of binary system -- a possibility we extend to the broader LPT / WD pulsar population.