Electromagnetic signatures of strong-field gravity from accreting black holes

TL;DR

This paper discusses electromagnetic signatures from accreting black holes in strong gravity, emphasizing the importance of new observational techniques and models to understand their environment and evolution.

Contribution

It highlights the potential of electromagnetic observations and simplified models to probe strong gravity effects near black holes, complementing existing methods.

Findings

Electromagnetic signatures can reveal strong gravity effects near black holes.

Polarization angle changes provide geometrical insights into the black hole environment.

Simplified models help interpret complex accretion phenomena.

Abstract

Observations of galactic nuclei help us to test General Relativity. Whereas the No-hair Theorem states that classical, isolated black holes eventually settle to a stationary state that can be characterized by a small number of parameters, cosmic black holes are neither isolated nor steady. Instead, they interact with the environment and evolve on vastly different time-scales. Therefore, the astrophysically realistic models require more parameters, and their values likely change in time. New techniques are needed in order to allow us to obtain independent constraints on these additional parameters. In this context, non-electromagnetic messengers have emerged and a variety of novel electromagnetic observations is going to supplement traditional techniques in the near future. In this outline, we summarize several fruitful aspects of electromagnetic signatures from accretion disks in strong-gravity regime in the outlook of upcoming satellite missions and ground-based telescopes. As an interesting example, we mention a purely geometrical effect of polarization angle changes upon light propagation, which occurs near the black hole event horizon. Despite that only numerical simulations can capture the accretion process in a realistic manner, simplified toy-models and semi-analytical estimates are useful to understand complicated effects of strong gravity near the event horizon of a rotating black hole, and especially within the plunging region below the innermost stable circular orbit.