Energy extraction from a rotating Buchdahl star via magnetic reconnection

TL;DR

This paper investigates magnetic reconnection as a mechanism for extracting rotational energy from rapidly spinning Buchdahl stars, showing it can surpass traditional black hole energy extraction methods in efficiency.

Contribution

It introduces the application of the Comisso-Asenjo magnetic reconnection mechanism to Buchdahl stars, demonstrating enhanced energy extraction efficiency beyond black hole limits.

Findings

Energy extraction efficiency increases with star spin parameter.

Magnetic reconnection can outperform Blandford-Znajek mechanism.

Ergoregion exists only when spin parameter exceeds 1/√2.

Abstract

In this work, we investigate the magnetic reconnection (MR) process as a mechanism for energy extraction from a rapidly rotating Buchdahl star (BS), one of the most compact horizonless objects that can, in principle, possess a spin parameter exceeding the extremal limit of a black hole (BH). We explore the energetics of the BS by focusing on the newly proposed MR mechanism developed by Comisso and Asenjo (the Comisso-Asenjo mechanism). Within this framework, we evaluate the energy extraction efficiency and the associated power output from a rapidly rotating BS. We show that the ergoregion of the BS exists only when the spin parameter satisfies $\beta>1/\sqrt{2}$. Consequently, the extraction of rotational energy through MR becomes possible only under this condition. Furthermore, we analyze the rate of energy extraction driven by fast magnetic reconnection and compare the resulting power with that predicted by the Blandford-Znajek mechanism. Our results indicate that the energy extraction rate increases significantly when the BS spin parameter exceeds the extremal limit for a BH, highlighting that MR can be substantially more efficient than the Blandford-Znajek mechanism. We demonstrate that MR can greatly enhance energy extraction efficiency from rapidly rotating BS with a large spin, making such an object potentially more efficient engines of high-energy astrophysical processes than BH.