On-axis absorption and scattering of charged massive scalar waves by Kerr-Newman black-bounce spacetime

TL;DR

The paper addresses how a charged massive scalar field interacts with Kerr-Newman black-bounce spacetimes when waves propagate along the rotation axis, combining geodesic analysis with a partial-wave treatment to map parameter dependencies on absorption and scattering cross sections. It introduces and analyzes the on-axis superradiance and the electric Penrose process (EPP), showing that the regularization parameter $k$ weakly affects cross sections but suppresses superradiance, while rotation $a$, charge $Q$, and field charge $q$ modulate both absorption and interference fringes in characteristic ways. A key insight is that heavier fields ($\mu$) are more readily absorbed and produce wider interference fringes, and that on-axis incidence can enhance superradiance for slower rotation, contrasting with equatorial results. The work also outlines astrophysical implications, including potential constraints on black-hole parameters and the possibility that EPP-accelerated particles could contribute to ultra-high-energy cosmic rays detectable by current or forthcoming observatories.

Abstract

We investigate the absorption and scattering of charged massive scalar waves by the Kerr-Newman black-bounce spacetime when the waves are incident along the rotation axis. Our findings indicate that a faster (slower) rotating spacetime or a more repulsive (attractive) electric force tends to reduce (increase) the absorption cross section and results in larger (smaller) angular widths of the scattered wave oscillations. We find that the rotation parameter exerts a suppressive influence on superradiance, which contrasts with the enhancing effect of the repulsive electric force. It is worth mentioning that the regularization parameter $k$ is found to modify the absorption or scattering cross sections only weakly, but can cause a noticeable reduction of superradiance. To further clarify the role of the parameters in superradiance, we study the energy extraction efficiency in the electric Penrose process. For particles moving along the rotation axis, we find that the influence of the parameters ($a, q, k$) on this efficiency is consistent with their effects on superradiance. We also discuss potential astrophysical applications, showing that particles in this process could be accelerated to ultrahigh energies in realistic environments, and could therefore be used to constrain black hole parameters. For the effect of field mass, it is found that a heavier scalar field leads to a larger absorption cross section and a wider interference fringe of the differential scattering cross section. When superradiance happens, i.e., the absorption cross section becomes negative, it is also found that the differential scattering cross section only changes smoothly, with no apparent qualitative feature showing up.

Figures (10)

• Figure 1: $r_c$ and $b_c$ of a charged particle as functions of $\omega$ (top) and $\mu$ (bottom) in KN black-bounce spacetime with $k=0.6$ and KN BH with fixing $a=0.9$, $Q=0.4$ and $q=1$.
• Figure 2: The effective potential $V_{\text{eff}}$ of the KN black-bounce spacetime for different values of $a$ (top left), $Q$ (top right), $k$ (middle left), $\mu$ (middle right), and $q$ (bottom).
• Figure 3: The total absorption cross section of the KN black-bounce spacetime for different values of $a$ (top left), $Q$ (top right), $k$ (middle left), $\mu$ (middle right) and $q$ (bottom). The gray dashed line represents the value of the geometric cross section. The inserts are a localized total absorption cross section and geometric cross section near $\omega/\mu=2.2$.
• Figure 4: Comparison of the numerical result obtained by the partial wave method with geodesic scattering and glory scattering.
• Figure 5: The differential scattering cross section of the KN black-bounce spacetime for different values of $a$ (top left), $Q$ (top right), $k$ (middle left), $\mu$ (middle right), $q$ (bottom left) and $\omega$ (bottom right). The insets at the bottom left corner of the middle plots are used to better see the effect of the parameters $k$ and $\mu$ on the differential scattering cross section.