Periodic structure in the FRB 121102 spectra. (arXiv:2010.15145v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Levkov_D/0/1/0/all/0/1">D.G. Levkov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Panin_A/0/1/0/all/0/1">A.G. Panin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tkachev_I/0/1/0/all/0/1">I.I. Tkachev</a>

Reiterating publically available data, we discover a remarkable periodic
structure in the spectra of repeating Fast Radio Burst (FRB) 121102: a set of
$(95pm 16)$ MHz-equidistant peaks with seemingly frequency-independent
interpeak distance. These peaks can be explained by diffractive lensing of the
FRB wave, either by a compact gravitating object of mass $10^{-4}, M_odot$ or
by a plasma cloud with smooth profile. The periodic structure is hidden in the
sea of erratic interstellar scintillations with $(3.3pm 0.6)$ MHz
decorrelation bandwidth. In addition, we reveal a new slowly evolving spectral
pattern on GHz scale which may be attributed to wide-band scintillations or
other wide-band interference phenomena. The spectra also include a large peak
at 7.1 GHz that can be caused by propagation of the FRB signal through a plasma
lens. Using the propagation effects as landmarks, we give a convincing argument
that the FRB progenitor has a narrow-band spectrum of GHz width and its central
frequency changes from burst to burst. With this paper we advance methods for
studying periodic spectral structures and separating them from scintillations.

Reiterating publically available data, we discover a remarkable periodic
structure in the spectra of repeating Fast Radio Burst (FRB) 121102: a set of
$(95pm 16)$ MHz-equidistant peaks with seemingly frequency-independent
interpeak distance. These peaks can be explained by diffractive lensing of the
FRB wave, either by a compact gravitating object of mass $10^{-4}, M_odot$ or
by a plasma cloud with smooth profile. The periodic structure is hidden in the
sea of erratic interstellar scintillations with $(3.3pm 0.6)$ MHz
decorrelation bandwidth. In addition, we reveal a new slowly evolving spectral
pattern on GHz scale which may be attributed to wide-band scintillations or
other wide-band interference phenomena. The spectra also include a large peak
at 7.1 GHz that can be caused by propagation of the FRB signal through a plasma
lens. Using the propagation effects as landmarks, we give a convincing argument
that the FRB progenitor has a narrow-band spectrum of GHz width and its central
frequency changes from burst to burst. With this paper we advance methods for
studying periodic spectral structures and separating them from scintillations.

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