Conversion of electromagnetic and gravitational waves by a charged black hole. (arXiv:2106.09731v2 [gr-qc] UPDATED)

Conversion of electromagnetic and gravitational waves by a charged black hole. (arXiv:2106.09731v2 [gr-qc] UPDATED)
<a href="http://arxiv.org/find/gr-qc/1/au:+Hadj_M/0/1/0/all/0/1">Mohamed Ould El Hadj</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Dolan_S/0/1/0/all/0/1">Sam R. Dolan</a>

In a strong electromagnetic field, gravitational waves are converted into
electromagnetic waves of the same frequency, and vice versa. Here we calculate
the scattering and conversion cross sections for a planar wave impinging upon a
Reissner-Nordstr”om black hole in vacuum, using the partial-wave expansion and
numerical methods. We show that, at long wavelengths, the conversion cross
section matches that computed by Feynman-diagram techniques. At short
wavelengths, the essential features are captured by a geometric-optics
approximation. We demonstrate that the converted flux can exceed the scattered
flux at large scattering angles, for highly-charged black holes. In the
short-wavelength regime, the conversion effect may be understood in terms of a
phase that accumulates along a ray. We compute the scattering angle for which
the converted and scattered fluxes are equal, as a function of charge-to-mass
ratio. We show that this scattering angle approaches $90$ degrees in the
extremal limit.

In a strong electromagnetic field, gravitational waves are converted into
electromagnetic waves of the same frequency, and vice versa. Here we calculate
the scattering and conversion cross sections for a planar wave impinging upon a
Reissner-Nordstr”om black hole in vacuum, using the partial-wave expansion and
numerical methods. We show that, at long wavelengths, the conversion cross
section matches that computed by Feynman-diagram techniques. At short
wavelengths, the essential features are captured by a geometric-optics
approximation. We demonstrate that the converted flux can exceed the scattered
flux at large scattering angles, for highly-charged black holes. In the
short-wavelength regime, the conversion effect may be understood in terms of a
phase that accumulates along a ray. We compute the scattering angle for which
the converted and scattered fluxes are equal, as a function of charge-to-mass
ratio. We show that this scattering angle approaches $90$ degrees in the
extremal limit.

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