Energy extraction from a rotating black hole via magnetic reconnection: Bumblebee gravity

TL;DR

The paper investigates energy extraction from rotating black holes via magnetic reconnection within Kerr–Sen–like spacetimes arising from Einstein–Bumblebee gravity, focusing on how Lorentz-symmetry breaking ($l$) and Bumblebee charge ($b$) affect the process. Building on the Comisso–Asenjo framework, it analyzes energy-extraction regions and a covering factor across parameter spaces, showing that larger $l$ or $b$ shift reconnection toward the central region and generally increase extraction likelihood. The results reveal that the most favorable conditions occur near the cosmic censorship boundary when the extractable energy is fixed, and that the two Bumblebee parameters have comparable impact on extraction efficiency, especially for high-spin cases. This work provides a pathway to test Lorentz-violating gravity effects in high-energy astrophysical environments and informs the viability of magnetic reconnection-driven energy extraction in alternative theories of gravity.

Abstract

Many efforts were made in order to better understand the energy extraction via magnetic reconnection from a rotating black hole, following the work of Comisso and Asenjo in 2021. We also tried to make some progress in our previous works, in which we discussed differences between bulk plasma with different streamlines and also defined the covering factor as an internal property of an accretion system to quantify its capability on extracting energy via magnetic reconnection from its central black hole. In this study, we aim to explore this topic within the framework of a Kerr-Sen-like spacetime induced from Bumblebee gravity, which, among various alternative theories of gravity beyond pure Einstein gravity, stands out as a promising candidate for explaining certain high energy astrophysical phenomena. More specifically, we would like to analyze the influence of the rate of Lorentz symmetry breaking and the Bumblebee charge, the two additional parameters in Bumblebee gravity except for the black hole mass and spin, on the energy extraction via magnetic reconnection. By analyzing the allowed regions for energy extraction and the variations of covering factor, we find that energy extraction becomes more likely to succeed and tends to occur closer to the central region when the spacetime carries bigger rate of Lorentz symmetry breaking and Bumblebee charge. Furthermore, our results indicate that the most favorable spacetime configuration for energy extraction via magnetic reconnection, when the extractable energy of the central black hole is determined, corresponds to the scenario in which the cosmic censorship hypothesis is marginally not violated.

Figures (10)

• Figure 1: The regions in which the event horizon exists are colored from green to blue for various values of $a$ in the $l–b$ parameter plane. The solid circles denote the sets of $(l,b)$ that are chosen as example sets
• Figure 2: Radii of the event horizon (first row), photon sphere (third row), ISCO (fourth row), and the radial range of the ergoregion (second row) as functions of black hole spin for the cases of example sets.
• Figure 3: Radii of event horizon (black solid lines), ergosphere (red solid lines), photon sphere (gold dotted lines) and ISCO (azure dashed lines) as functions of black hole spin in the cases of Point 1 (left panel) and 2 (right panel), respectively.
• Figure 4: The $a–r_0$ parameter planes for the combined (left panels) and circular (right panels) streamlines. The allowed regions for energy extraction in the cases of example sets with $\sigma_0=100$ and $\xi_B=\pi/12$ are colored in azure (combined streamlines) and gold (circular streamlines).
• Figure 5: The $\xi_{\rm B}–r_0$ parameter planes for the combined (left panels) and circular (right panels) streamlines. The allowed regions for energy extraction in the cases of example sets with $\sigma_0=100$ and $a=0.9$ are colored in azure (combined streamlines) and gold (circular streamlines).