We consider planar massless scalar waves impinging upon a Kerr black hole,
for general angles of incidence. We compute the absorption cross section via
the partial wave approach, and present a gallery of results. In the
low-frequency regime, we show that the cross section approaches the horizon
area; in the high-frequency regime, we show that the cross section approaches
the geodesic capture cross section. In the aligned case, we extend the complex
angular momentum method to obtain a `sinc' approximation, which relates the
regular high-frequency oscillations in the cross section to the properties of
the polar null orbit. In the non-aligned case, we show, via a semi-analytic
approximation, that the reduction in symmetry generates a richer, less regular
absorption cross section. We separate the absorption cross section into
corotating and counterrotating contributions, showing that the absorption is
larger for counterrotating waves, as expected.Comment: 11 pages, 10 figure
We study the absorption of plane waves by Kerr black holes. We calculate the absorption cross section: the area of the black hole shadow at a finite wavelength. We present a unified picture of the absorption of all massless bosonic fields, focussing on the on-axis incidence case. We investigate the spin-helicity effect, arising from a coupling between dragging of frames and the helicity of a polarized wave. We introduce and calibrate an extended sinc approximation which provides new quantitative data on the spin-helicity effect in strong-field gravity.
We compute numerically the absorption cross section of planar massless scalar
waves impinging upon a Kerr-Newman black hole with different incidence angles.
We investigate the influence of the black hole electric charge and angular
momentum in the absorption spectrum, comparing our numerical computations with
analytical results for the limits of high and low frequency.Comment: Accepted for publication in Physical Review
Black holes are among the most intriguing objects in nature. They are believed to be fully described by General Relativity (GR), and the astrophysical black holes are expected to belong to the Kerr family, obeying the nohair theorems. Alternative theories of gravity or parameterized deviations of GR allow black hole solutions, which have additional parameters other than mass and angular momentum. We analyze a Schwarzschild-like metric, proposed by Johannsen and Psaltis, characterized by its mass and a deformation parameter. We compute the absorption cross section of massless scalar waves for different values of this deformation parameter and compare it with the corresponding scalar absorption cross section of the Schwarzschild black hole. We also present analytical approximations for the absorption cross section in the high-frequency regime. We check the consistence of our results comparing the numerical and analytical approaches, finding excellent agreement.
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