Electron Cyclotron Maser Emission as the Driving Mechanism in Long-Period Radio Transients

TL;DR

This work addresses the origin of long-period radio transients (LPRTs) by proposing electron cyclotron maser emission (ECME) from the outer magnetospheres of neutron stars or magnetic white dwarfs. The method combines ECME growth in a low-density cavity with a hollow cone beaming geometry and a polarisation-transfer treatment that includes Faraday rotation and conversion in a magnetised escape layer, allowing a spectrum of outputs from circular to linear depending on plasma conditions. The authors argue that weak interstellar medium accretion can supply both the mildly relativistic electrons and the energy reservoir to sustain ECME, favoring slowly moving, low-field neutron stars over white dwarfs, and they connect observed LPRT properties (duty cycle, polarization, phase stability, occasional X-ray emission) to this framework. They also outline concrete predictions—phase drift with frequency, correlated polarisation changes, and small beaming fractions—that can be tested with current and future radio and X-ray observations, potentially revealing a hidden population of old, accreting neutron stars in the Galaxy.

Abstract

Long-period radio transients (LPRTs) are highly polarised, coherent radio sources with periods of minutes to hours and bursts typically lasting 10-100s. We argue that electron cyclotron maser emission (ECME) explains their narrow duty cycles and polarisation properties. In this picture, a rotating oblique magnetosphere beams radiation into a thin, hollow emission cone whose surface lies almost perpendicularly to the local magnetic field. The observed very narrow pulses arise when the line of sight skims the cone. Broader profiles and weak leading or trailing components occur when multiple azimuths along the emission ring satisfy the maser resonance condition. The observed isotropic-equivalent luminosities of ~10^{30}-10^{31} erg s^{-1} correspond to modest intrinsic powers once strong ECME beaming is taken into account, which is readily achievable through accretion from the interstellar medium.

Figures (3)

• Figure 1: Resonance loss-cone geometry in $(v_\perp,v_\parallel)$. Colour shading shows a bi-Maxwellian with symmetric loss cones (interior excised). Dashed lines mark the loss-cone edge $v_\perp=|v_\parallel|\tan\alpha_{\rm L}$. The red curve is a representative cyclotron resonance ellipse for fixed $(\omega,k_\parallel)$ and harmonic $n$. Yellow circles indicate the intersections where the resonance reaches the largest positive $\partial f/\partial v_\perp$, maximising maser growth at quasi-transverse propagation ($k_\parallel\simeq0$).
• Figure 2: Schematic of the ECME emission geometry in an oblique rotator. The spin axis $\hat{\mathbf z}$ defines the rotation frame, while the magnetic axis $\hat{\mathbf m}$ is tilted relative to it by an angle $\beta$. The observer’s line of sight $\hat{\mathbf n}_{\rm obs}$ makes an inclination $i$ to the spin axis. Magnetic field lines (depicted in blue) illustrate the dipolar topology. ECME arises where the local magnetic field equals the resonance value $B_{\rm loc}$. Here a hollow emission cone (shaded), centred on $\hat{\mathbf b}$ and with opening angle $\theta_{\rm cone}$, is formed. As the star rotates, the hollow cone sweeps past the observer, producing one or two pulses per rotation depending on geometry. The right panels illustrate the resulting pulse profile and the phase variation of $\theta_{Bk}$.
• Figure 3: Propagation induced evolution of the polarisation fractions in a cold, low loss, quasi transverse escape layer. Each panel shows $U/I$, $V/I$, $Q/I$, and $L/I=\sqrt{Q^{2}+U^{2}}/I$ as functions of $s/L$ for an initially circularly polarised state ($I_{0}=1$, $Q_{0}=U_{0}=0$, $V_{0}=1$). Top panel: Density sweep with $n_{e}=\{10^{8},\,3\times10^{8},\,10^{9}\}$ cm$^{-3}$ at fixed $\nu_{\rm obs}=1$ GHz and $\theta_{Bk}=85^{\circ}$. Both $\rho_{W}$ and $\rho_{R}$ scale linearly with $n_{e}$, so higher densities produce faster $U\leftrightarrow V$ oscillations and slightly larger $Q/I$ excursions. Bottom panel: Frequency sweep with $\nu_{\rm obs}=\{0.8,\,1.0,\,1.4\}$ GHz at fixed $n_{e}=10^{8}$ cm$^{-3}$ and $\theta_{Bk}=85^{\circ}$. Increasing frequency weakens the mixing ($\rho_{W}\propto\nu^{-3}$, $\rho_{R}\propto\nu^{-2}$), yielding slower conversion, smaller $Q/I$ amplitudes, and reduced growth of $L/I$ at higher $\nu_{\rm obs}$.