Narrow spectrum in repeating fast radio burst

Kavli Affiliate: Renxin Xu

| First 5 Authors: Wei-Yang Wang, Yuan-Pei Yang, Hong-Bo Li, Jifeng Liu, Renxin Xu

| Summary:

Fast radio bursts (FRBs) can present narrow-band spectra with a variety of
polarization properties. We study spectral properties from perspectives of
intrinsic radiation mechanisms by invoking bunching mechanism and perturbations
on charged bunches moving at curved trajectories. The narrow-band emission
likely reflects some quasi-periodic structure on the bulk of bunches, which may
be due to quasi-periodically sparking in a "gap" or quasi-monochromatic
Langmuir waves. A spike would appears at the spectrum if the perturbations can
induce a monochromatic oscillation of bunches, however, it is hardly to create
a narrow-band spectrum because a large amplitude scattering of Lorentz factor
can let the spike wider. Monochromatic Alfv’en waves as the perturbations do
not generate the narrow-band spectrum from 100 MHz to 10 GHz. Both the bunching
mechanism and perturbations scenarios share the same polarization properties
with a normally distributed bulk of bunches. We investigate absorption effects
including Landau damping and curvature self-absorption in magnetosphere, which
are significant at low frequencies. Subluminous O-mode photons can not escape
from the magnetosphere, leading to a height-dependent lower frequency cut-off,
so that the spectrum is narrow-band when the cut-off is close to the
characteristic frequency of curvature radiation. The spectral index is 5/3 at
low frequency bands due to the curvature self-absorption but not as steep as
the observations.

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