Abstract
We show that the magnetic dipole and gravitational radiation emitted by a pulsar can undergo superradiant scattering off a spinning black hole companion. We find that the relative amount of superradiant modes in the radiation depends on the pulsar's angular position relative to the black hole's equatorial plane. In particular, when the pulsar and black hole spins are aligned, superradiant modes are dominant at large angles, leading to an amplification of the pulsar's luminosity, whereas for small angles the radiation is dominantly composed of non-superradiant modes and the signal is attenuated. This results in a characteristic orbital modulation of the pulsar's luminosity, up to the percent level within our approximations, which may potentially yield a signature of superradiant scattering in astrophysical black holes and hence an important test of general relativity.
Highlights
We show that the magnetic dipole and gravitational radiation emitted by a pulsar can undergo superradiant scattering off a spinning black hole companion
PACS numbers: 04.70.-s, 04.30.Nk, 97.60.Gb arXiv:1501.07605v2 [gr-qc] 13 Aug 2015. It has been known since the 1970s that low-frequency waves can be amplified upon scattering off a rotating black hole (BH) [1,2,3,4,5]
Superradiant scattering occurs for waves of frequency ω < mΩ, where Ω denotes the angular velocity of the BH horizon and m the azimuthal number that characterizes the wave’s angular momentum
Summary
When the pulsar and black hole spins are aligned, superradiant modes are dominant at large angles, leading to an amplification of the pulsar’s luminosity, whereas for small angles the radiation is dominantly composed of non-superradiant modes and the signal is attenuated. Superradiant scattering occurs for waves of frequency ω < mΩ, where Ω denotes the angular velocity of the BH horizon and m the azimuthal number that characterizes the wave’s angular momentum Such waves carry negative energy and spin into the BH from the point of view of a distant observer, which is possible inside the ergoregion that surrounds a rotating BH. In this Letter, we show that the radiation emitted by a pulsar can undergo superradiant scattering off a rotating BH companion, constituting the first example of an astrophysical system where observational evidence for BH superradiance can be found
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