Coherent Curvature Radiation Spectrum by Dynamically Fluctuating Bunches in Magnetospheres

Abstract: Coherent curvature radiation by charged bunches has been discussed as the radiation mechanism for radio pulsars and fast radio bursts. Important issues for this radiation mechanism include how the bunches form and disperse in the magnetosphere of a pulsar or magnetar. More likely, bunches form and disperse continuously and it remains unclear what the spectral features are for these fluctuating bunches. In this work, we consider that the bunches in a magnetosphere have a formation rate of $\lambda_B$, a lifetime of $\tau_B$, and a typical Lorentz factor of $\gamma$, and analyze the spectral features of coherent curvature radiation by these fluctuating bunches. We find that the emission spectrum by a single fluctuating bunch is suppressed by a factor of $\sim(\lambda_B\tau_B)^2$ compared with that of a single persistent bunch, and there is a quasi-white noise in a wider band in the frequency domain. The high-frequency cutoff of the spectrum is at $\sim\max(\omega_{\rm peak},2\gamma^2/\tau_B)$, where $\omega_{\rm peak}$ is the peak frequency of curvature radiation. If the observed spectrum is not white-noise-like, the condition of $2\gamma^2\lambda_B\gtrsim \min(\omega_{\rm peak},2\gamma^2/\tau_B)$ would be required. Besides, the radiation by multiple fluctuating bunches along a field line is the incoherent summation of the radiation by single bunches if the bunch separation is longer than the wavelength. Conversely, a coherent summation should be involved. We also discuss the effects of bunch structures and the mechanism of bunch formation and dispersion.