Shadow cast and Deflection angle of Kerr-Newman-Kasuya spacetime (1807.00388v2)

Abstract: We study the shadow cast or silhouette generated by a Kerr-Newman-Kasuya (KNK) spacetime (rotating dyon black hole). It is shown that in addition to the angular momentum of the black hole, the dyon charge also affects the shadow image of the KNK black hole. Moreover, we analyze the weak gravitational lensing by the KNK black hole by using the Gauss-Bonnet theorem. Finally, we find that extra dyon charge decreases both the deflection angle and shadow of the KNK black hole.

• The paper reveals that dyon charges in KNK spacetime significantly shrink and distort black hole shadows, offering clear observational signatures.
• It employs the Gauss-Bonnet theorem to derive an analytical expression for the light deflection angle, emphasizing the roles of spin and charges.
• These findings provide practical insights for astrophysical observations and can aid in testing general relativity with data from telescopes like the EHT.

Shadow Cast and Deflection Angle of Kerr-Newman-Kasuya Spacetime

The paper of black hole shadows and gravitational lensing provides critical insight into the complex interplay of light and gravity in the vicinity of massive celestial objects. In this paper, the authors investigate the shadow cast by a specific black hole solution known as the Kerr-Newman-Kasuya (KNK) spacetime, characterized by rotation, electric, and magnetic charges—a setting often referred to as a rotating dyon black hole.

The intrinsic characteristics of the KNK spacetime are explored, where the metric is defined in terms of its mass $M$, spin parameter $a$, electric charge $Q_e$, and magnetic charge $Q_m$. The KNK spacetime extends the classic models of Kerr-Newman and Kerr black holes by including magnetic monopole charges, thus offering valuable scenarios that aid in understanding light deflection and shadow images influenced not only by angular momentum but also by dyon charges.

Key Findings

1. Black Hole Shadows: The shadow of a black hole is fundamentally an observational consequence of its strong gravitational field bending light from objects behind it. In the KNK spacetime, the shape and size of the black hole shadow are sensitive to the parameters $a$, $Q_e$, and particularly $Q_m$. The presence of magnetic charges tends to decrease both the shadow size and the deflection angle of the light path. The resulting shadow is notably deformed and smaller compared to a neutral black hole like the one described by Kerr or Schwarzschild metrics.
2. Deflection Angle: Utilizing the framework of the Gauss-Bonnet theorem, the paper provides an analytical expression for the deflection angle experienced by light rays traversing the KNK spacetime. The derived deflection angle comprises contributions from each of the black hole's parameters, further showing that an increase in the magnetic charge $Q_m$ reduces the deflection angle. This result underscores the potential influence of magnetic monopole-like charges in gravitational lensing scenarios involving astrophysical black holes.
3. Theoretical and Observational Implications: Given the rotation and charge, the KNK black hole offers a unique landscape for testing the limits of general relativity and the potential existence of exotic charge distributions in the universe. The capability to observe and interpret the characteristics of such shadows, especially with projects like the Event Horizon Telescope (EHT), provides a practical pathway to detect and analyze these charged and rotating black holes. The direct observation of black hole shadows, like those of Sagittarius A* and M87, could validate or challenge the existence of entities like a dyon in observable cosmic structures.

Future Directions

The implications of the KNK black hole shadow and lensing characteristics forge several lines of inquiry:

• Astrophysical Observations: Enhanced observational capabilities might allow direct determination of black hole parameters through shadow imaging, particularly employing radio telescopes.
• Gravitational Wave Astronomy: Coupled with the detection of gravitational waves, the presence of electric and magnetic charges could yield distinct signatures, informing theories beyond general relativity.
• Fundamental Physics: This paper offers an interesting exploration on the necessity of alternative theories to encompass phenomena like magnetic monopoles, potentially fostering developments in unify gravitation and electromagnetism.

In summary, the investigation of the KNK black hole provides valuable insights into the deeper phenomenology of rotating charged spacetimes and reinforces the intricate dance of light and gravity. This understanding lays an important foundation for future research and potential experimental confirmation within the field of complex black hole geometries.