Magnetically charged black holes from non-linear electrodynamics and the Event Horizon Telescope (1912.08231v2)

Abstract: Non-linear electrodynamics (NLED) theories are well-motivated extensions of QED in the strong field regime, and have long been studied in the search for regular black hole (BH) solutions. We consider two well-studied and well-motivated NLED models coupled to General Relativity: the Euler-Heisenberg model and the Bronnikov model. After carefully accounting for the effective geometry induced by the NLED corrections, we determine the shadows of BHs within these two models. We then compare these to the shadow of the supermassive BH M87* recently imaged by the Event Horizon Telescope collaboration. In doing so, we are able to extract upper limits on the black hole magnetic charge, thus providing novel constraints on fundamental physics from this new extraordinary probe.

• The paper rigorously computes magnetically charged black hole shadows using Euler-Heisenberg and Bronnikov NLED models to set constraints on the magnetic charge.
• It employs the effective geometry method to reveal how non-linear electrodynamics alters photon trajectories and shadow sizes in strong gravitational fields.
• The study links theoretical NLED corrections to EHT observations, offering novel astrophysical constraints that inform future investigations in gravity and quantum mechanics.

Insights into Magnetically Charged Black Holes from Non-linear Electrodynamics

The paper addresses the intriguing concept of magnetically charged black holes within the framework of non-linear electrodynamics (NLED) coupled to General Relativity (GR). Two notable NLED models, namely the Euler-Heisenberg and Bronnikov models, serve as the basis for this paper, which aims to explore new black hole solutions and their observational implications, particularly concerning the Event Horizon Telescope (EHT) observations.

NLED, as explored in this paper, provides an extension to Quantum Electrodynamics (QED) in strong-field regimes, suggesting the possibility of resolving singularities traditionally expected in black hole solutions according to GR. The Euler-Heisenberg and Bronnikov models offer different approaches to modifying the electromagnetic field's influence inside black holes, potentially producing regular solutions without singularities.

The paper's core endeavor is the rigorous computation of black hole shadows emanating from these NLED settings, examining both regular and non-regular black holes with magnetic charge. By assessing the modification of photon trajectories influenced by NLED corrections, the authors investigate how these changes impact the observed black hole shadows. Their method incorporates the effective geometry concept, wherein non-linearities in electrodynamics cause photons to travel along geodesics different from those in a GR background.

The authors systematically compute the shadows of these NLED black holes and juxtapose them with the shadow observed for the supermassive black hole M87* by the EHT. From this comparison, they derive constraints on the magnetic charge of black holes within these NLED models. These are presented as novel astrophysical constraints on fundamental physics parameters, reflecting observational consistency with the M87* shadow. The results are succinct: while the NLED coupling strength (specifically in the EEH model) is not tightly constrained, the magnetic charge of such black holes is limited to approximately the order of the black hole's mass.

The paper further explores the implications of these constraints in both models. For the EEH model, the finding is that non-linear couplings only slightly affect shadow size, which primarily serves to bound the magnetic charge parameter $Q_m$. In contrast, in the EB model, the paper highlights the potential for black holes with characteristics akin to a Schwarzschild black hole, albeit with altered electromagnetic dynamics.

While the paper does not assert revolutionary implications, it establishes a valuable standard for using black hole shadow observations to constrain higher-order theoretical models. It offers a pragmatic application for the EHT's empirical results, linking them to questions of fundamental physics and the nature of gravity in extreme environments. From a theoretical perspective, this work could guide future investigations into more elaborate or alternative NLED models and their interplay with gravitational theories.

Future developments might expand upon this work by addressing these theories' rotational dynamics or incorporating other astrophysical data beyond shadows. Additionally, given the speculative yet promising nature of NLED models in reconciling aspects of GR and quantum mechanics, continued scrutiny and expanded models could further elucidate these areas profoundly. Nonetheless, this paper stands out as a critical step in leveraging cutting-edge observational techniques to probe aspects of theoretical physics traditionally beyond empirical scrutiny.