Coherent Curvature Radiation by Bunches in Three-Dimensional Magnetic Field Geometry: Application to Pulsars and Fast Radio Bursts [HEAP]

The extremely high brightness temperatures of pulsars and fast radio bursts (FRBs) require their radiation mechanisms to be coherent. Curvature radiation by bunches is often discussed as a favored mechanism. We develop a general radiation theory of coherent curvature radiation by bunches under three-dimensional magnetic field geometry from the first principles. We consider a bunch characterized by the bunch length, curvature radius of the trajectory family, bunch opening angle, and electron energy distribution. We find that the spectrum of coherent curvature radiation is a multi-segment broken power law, with the break frequencies depending on bunch properties and trajectory configuration. Different from previous works, we find that in the pulsar magnetosphere only fluctuating net charges with respect to the Goldreich-Julian outflow can make a contribution to coherent radiation. We apply this model to constrain the observed spectra of pulsars and FRBs. For a typical pulsar ($B_p=10^{12}~{\rm G}$ and $P=0.1~{\rm s}$), a small fluctuation of net charge. e.g., $\delta n_{\rm GJ}\sim 0.1 n_{\rm GJ}$, can provide the observable flux. For FRBs, the fluctuating net charge may be larger due to its abrupt nature. For $\delta n_{\rm GJ}\sim n_{\rm GJ}$, a neutron star with strong magnetic field and fast rotation is required to power an FRB in the spindown-powered model. The requirement is less stringent for the cosmic comb model thanks to the larger cross section and compressed charge density of the bunch made by the external astrophysical stream that combs the magnetosphere.

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Y. Yang and B. Zhang
Fri, 8 Dec 17

Comments: 25 pages, 17 figures, comments welcome