Electron cyclotron maser emission as the driving mechanism in long-period radio transients

ABSTRACT Long-period radio transients (LPRTs) are highly polarized, coherent radio sources with periods of minutes to hours and bursts typically lasting 10–100 s. Here, we consider the apparently isolated sub-class of LPRTs and argue that electron cyclotron maser emission (ECME) explains their narrow duty cycles and polarization properties. In particular, we show that intrinsically circular ECME can emerge as predominantly linear after undergoing Faraday conversion in an overlying magnetospheric plasma layer, thus reconciling the observed high linear fractions with a circularly polarized maser. In this picture, a rotating oblique magnetosphere beams radiation into a thin, hollow emission cone whose surface lies almost perpendicular to the local magnetic field. The observed very narrow pulses arise when the line of sight skims the cone, while broader profiles and weak leading or trailing components occur when multiple azimuths along the emission ring meet the maser resonance condition. The observed isotropic-equivalent luminosities of $\sim 10^{30}{-}10^{31}$ erg s$^{-1}$ correspond to modest intrinsic powers once strong ECME beaming is taken into account. We show that such power levels can be supplied by accretion from the interstellar medium (ISM), and that detectability at kiloparsec distances favours slowly rotating neutron stars with comparatively low surface magnetic fields ($B_\ast \lesssim 10^{10}$ G) and low space velocities.