Observational imprints and quasi-Periodic oscillations of magnetically charged anti-de Sitter black holes

TL;DR

This paper explores how magnetic charge in Anti-de Sitter black holes affects observable phenomena like shadows and QPOs, providing theoretical predictions and comparing them with astrophysical data to constrain magnetic charge.

Contribution

It introduces a detailed analysis of magnetic charge effects on black hole observables and constrains magnetic charge magnitude using QPO data.

Findings

Photon sphere and shadow radii decrease with magnetic charge.

Magnetic charge shifts the ISCO radius and alters orbital frequencies.

QPO data constrains magnetic charge to less than about 20% of black hole mass.

Abstract

In this work, we investigate observable signatures of a magnetically charged Anti-de Sitter black hole in string-inspired Euler-Heisenberg theory. We analyze photon trajectories, the photon sphere, and the resulting black hole shadow. We derive the photon sphere and shadow radii and show that both deviate from the Schwarzschild and Schwarzschild-AdS cases. In particular, the radii decrease monotonically as the magnetic charge parameter $Q_m$ increases, indicating that magnetic charge modifies light propagation near the black hole. We also study neutral and charged particle motion and compute the corresponding epicyclic frequencies. Using the effective potential method, we obtain the specific energy and angular momentum for stable circular orbits and determine the innermost stable circular orbit (ISCO). The presence of $Q_m$ shifts the ISCO radius and alters the orbital structure. The radial, vertical, and orbital frequencies show clear deviations from the Schwarzschild case. Finally, we confront the model with twin-peak quasi-periodic oscillation (QPO) data from stellar-mass, intermediate-mass, and supermassive black hole candidates. A two-dimensional Delta chi-square analysis in the ($r$, $Q_m$) space shows that the best fit corresponds to $Q_m=0$, although finite values remain allowed within confidence levels. At the 1 sigma level, we obtain an upper bound $Q_m/M$ less than about 0.2. These results indicate that while magnetic charge produces measurable theoretical deviations, current QPO data place only moderate constraints on its magnitude.