Energy Extraction and Particle Acceleration in String-Inspired Rotating Einstein-Maxwell-Dilaton-Axion Black Hole

TL;DR

This paper investigates energy extraction and particle acceleration in a rotating Einstein-Maxwell-Dilaton-Axion black hole, revealing how dilaton hair influences efficiency, energetics, superradiance, and collision dynamics near the horizon.

Contribution

It provides new analytic expressions and insights into how dilaton hair affects energy extraction processes and near-horizon physics in EMDA black holes.

Findings

Negative dilaton hair enhances Penrose process efficiency.

Rotational energy reservoir increases with more negative dilaton parameter.

Dilaton hair modifies superradiance conditions and collision energies.

Abstract

We study energy extraction and particle acceleration in the rotating Einstein-Maxwell-Dilaton-Axion (EMDA) black hole, focusing on the impact of dilaton hair $b\le 0$ on near-horizon energetics relative to Kerr. For the Penrose process we derive analytic expressions for the maximum efficiency and show that negative $b$ can strongly enhance the ideal gain in the extremal regime (e.g., reaching $\sim 91\%$ for $b=-0.3$). We then compute the irreducible mass $M_{\rm irr}$ and the corresponding rotationally extractable energy $\mathcal{E}_{\rm rot}\equiv M-M_{\rm irr}$, finding that $M_{\rm irr}$ decreases monotonically as $b$ becomes more negative while $\mathcal{E}_{\rm rot}$ increases, indicating a larger spin-energy reservoir; at extremality the extracted share from rotation is $\mathcal{E}_{\rm rot}/M\simeq 0.63$ for EMDA, reducing to the Kerr value $\simeq 0.29$ at $b=0$. Kinematic constraints relevant to fragment production are quantified via the Wald and Bardeen--Press--Teukolsky bounds, which are progressively relaxed for more negative $b$. For wave superradiance we obtain the flux balance and the amplification window $0<\beta<k\Omega_H$, with $\Omega_H$ expressed through $\Xi=r_H^{2}+2br_H+a^{2}$; negative $b$ modifies $\Omega_H$ and enlarges the parameter region exhibiting negative horizon flux. Finally, we analyse two-particle collisions and derive $E_{\rm cm}$, showing that the Ba\~nados--Silk--West divergence persists at the horizon when one particle is tuned to the critical angular momentum $L_c=E/\Omega_H$, while $E_{\rm cm}$ remains finite for generic angular momenta. Overall, dilaton hair in EMDA simultaneously amplifies energy-extraction channels and reshapes the near-horizon thresholds governing high-energy collisions.