Papers
Topics
Authors
Recent
Gemini 2.5 Flash
Gemini 2.5 Flash
134 tokens/sec
GPT-4o
9 tokens/sec
Gemini 2.5 Pro Pro
47 tokens/sec
o3 Pro
4 tokens/sec
GPT-4.1 Pro
38 tokens/sec
DeepSeek R1 via Azure Pro
28 tokens/sec
2000 character limit reached

Black Hole Shadows, Photon Rings, and Lensing Rings (1906.00873v2)

Published 3 Jun 2019 in astro-ph.HE and gr-qc

Abstract: The presence of a bright "photon ring" surrounding a dark "black hole shadow" has been discussed as an important feature of the observational appearance of emission originating near a black hole. We clarify the meaning and relevance of these heuristics with analytic calculations and numerical toy models. The standard usage of the term "shadow" describes the appearance of a black hole illuminated from all directions, including from behind the observer. A backlit black hole casts a somewhat larger shadow. Neither shadow heuristic is particularly relevant to understanding the appearance of emission originating near the black hole, where the emission profile and gravitational redshift play the dominant roles in determining the observed size of the central dark area. A photon ring results from light rays that orbit near the black hole before escaping to infinity, where they arrive near a ring-shaped "critical curve" on the image plane. Although the brightness can become arbitrarily large near this critical curve in the case of optically thin emitting matter, we show that the enhancement is only logarithmic, and hence is of no relevance to present observations. For optically thin emission from a geometrically thin or thick disk, photons that make only a fraction of an orbit will generically give rise to a much wider "lensing ring," which is a demagnified image of the back of the disk, superimposed on top of the direct emission. The lensing ring is centered at a radius ~5% larger than the photon ring and its width is ~0.5-1M. It can be relatively brighter by a factor of 2-3 and thus could provide a significant feature in high resolution images. Nevertheless, the characteristic features of the observed image are dominated by the location and properties of the emitting matter near the black hole. We comment on the recent M87* Event Horizon Telescope observations and mass measurement.

Citations (264)

Summary

  • The paper clarifies that conventional shadow heuristics can be misleading in backlit scenarios by showing that near-black-hole emission profiles and redshift govern the observed dark region.
  • The paper distinguishes between photon rings, with logarithmic brightness amplification from multiple orbits, and more practically observable, brighter lensing rings.
  • The paper employs Schwarzschild and Kerr metric models to trace geodesics, offering insights with significant implications for interpreting Event Horizon Telescope data.

Insights into Black Hole Shadows and Ring Structures

The paper "Black Hole Shadows, Photon Rings, and Lensing Rings" by Gralla, Holz, and Wald provides a detailed analysis of the observable features associated with the illumination and emission near black holes. Through a combination of analytic calculations and numerical models, the authors scrutinize the commonly discussed concept of a "black hole shadow" surrounded by a "photon ring."

Central Arguments and Findings

The authors point out that the heuristic understanding of black hole shadows, especially in the context of backlit scenarios, is often misleading when it comes to realistic emission profiles. In a backlit situation, the initial notion is a larger shadow due to the black hole casting a dark area larger than the critical curve radius. However, for emission originating from near the black hole, it is the emission profile and gravitational redshift that predominantly influence the size of the central dark region observed.

The authors highlight the distinction between photon rings and lensing rings in high-resolution images of black holes. A photon ring is formed by light rays that orbit the black hole multiple times before reaching the observer, which results in an amplified brightness near the so-called critical curve on the image plane. However, despite the theoretical possibility of infinite brightness, the paper demonstrates that this enhancement is only logarithmic and insignificant in current observational capabilities.

In contrast, the lensing ring arises when light undergoes fewer orbits, typically due to light bending that allows the observer to view a rear image of the emission region, superimposed on the direct view. This lensing ring is found to be more practical in contributing observable features, potentially twice to three times as bright compared to adjacent regions.

Methodological Approaches

The paper utilizes Schwarzschild and Kerr metric models, combined with the tracing of geodesics for theoretical derivations. The authors illustrate that the thickness and brightness variations of the rings depend significantly on factors such as the optical thickness and geometric properties of the emission region (thin versus thick disk emission).

Implications and Future Directions

The results carry significant implications for interpreting observations from initiatives like the Event Horizon Telescope (EHT). Specifically, while the photon ring is negligible in contributing to the total observed flux, the lensing ring and direct emission dictate the observational characteristics. This revelation calls for circumspection in using simplistic shadow and photon ring heuristics in publicized interpretations of observational data.

The analysis raises questions about the broader assumptions utilized in black hole imaging, suggesting the necessity for models that accurately incorporate emission properties. Future research might focus on refining these models in a variety of astrophysical settings, taking into account different disk inclinations and accretion dynamics.

While the paper does not conclusively dismiss the relevance of shadows and rings in imaging, it urges the scientific community to ground interpretations in the robust physics of light behavior in strong gravitational fields. This deeper understanding is poised to enhance the extraction of physical characteristics of black holes from the observational data, steering future astronomical investigations in novel directions.