Photon Ring and Observational Appearance of a Hairy Black Hole (2105.11770v2)

Abstract: Recently, the image of a Schwarzschild black hole with an accretion disk has been revisited, and it showed that the "photon ring", defined as highly bent light rays that intersect the disk plane more than twice, is extremely narrow and makes a negligible contribution to the total brightness. In this paper, we investigate the observational appearance of an optically and geometrically thin accretion disk around a hairy black hole in an Einstein-Maxwell-scalar model. Intriguingly, we find that in a certain parameter regime, due to an extra maximum or an "ankle-like" structure in the effective potential for photons, the photon ring can be remarkably wide, thus making a notable contribution to the flux of the observed image. In particular, there appears a wide and bright annulus, which comprises multiple concentric bright thin rings with different luminosity, in the high resolution image.

• The paper presents a detailed analysis of photon ring formation in hairy black holes using an Einstein-Maxwell-scalar model.
• It employs backward ray tracing to simulate accretion disk images, showing how changes in the effective potential widen photon rings and alter brightness patterns.
• The research correlates theoretical predictions with simulations, highlighting the impact of black hole charge and scalar coupling on observable image features.

An Analytical Review of "Photon Ring and Observational Appearance of a Hairy Black Hole"

The paper "Photon Ring and Observational Appearance of a Hairy Black Hole" by Qingyu Gan, Peng Wang, Houwen Wu, and Haitang Yang explores the intriguing optical phenomena that arise when a hairy black hole (HBH) is encircled by an optically and geometrically thin accretion disk in an Einstein-Maxwell-scalar model. The investigation builds upon the premise that in certain parametric regimes, specific structural features in the potential for photons can lead to observable deviations in the image characteristics of black holes.

Key Insights and Results

The primary focus of the paper is on the examination of a hairy black hole solution within a specific Einstein-Maxwell-scalar model defined by an exponential scalar-electromagnetic coupling. The paper elaborates extensively on the behavior of photon rings around black holes, observing that in particular regimes, these photon rings may become substantially wider than previously noted, affecting their contribution to the overall observed luminosity of the black hole's image.

1. Effective Potential and Photon Sphere Analysis: The authors perform a detailed analysis of the effective potential for photons to identify regions where photon spheres may form. In the investigated solutions, multiple photon spheres can emerge, leading to the formation of intricate bright annular patterns comprising concentric rings in black hole images.
2. Accretion Disk Imaging: Utilizing backward ray tracing from a distant observer's viewpoint, the image of the thin accretion disk around the HBH is constructed. The photon ring enlarges in width as a result of specific features in the potential, such as an ankle-like structure, significantly influencing the observed brightness pattern. This can lead to substantial contributions from photon rings compared to lensing rings.
3. Theory and Simulation Correlation: The paper achieves consistent agreement between theoretical predictions and numerical simulations, particularly in scenarios where the photon rings are broad. Although traditional views consider photon rings to have negligible contribution due to their narrowness, this research illustrates scenarios where their contribution is indeed significant.
4. Dependence on Parameters: The behaviors of shadow sizes and photon ring widths are extensively dissected with respect to two primary variables: the HBH’s charge and the scalar coupling in the model. It is shown that increasing these parameters tends to shrink the shadow while accentuating the distinct features of the photon ring.

Implications and Future Directions

This meticulous examination through combining analytical and numerical methodologies provides new insights into alternative black hole configurations beyond those offered by classical models. The paper suggests potential avenues for future work, including more detailed assessments of specific types of photon bands within wide photon rings, exploration of dynamic scalar couplings, and potential applications to rotating black holes instead of static ones.

From a theoretical viewpoint, this work challenges the prevailing assumptions about the inscrutability of photon rings, advocating for their recognition as important contributors to black hole imaging studies. Practical implications might be explored in the context of upcoming observational projects like BlackHoleCam, potentially broadening the scope of empirical tests of general relativity.

In sum, this paper adds substantive depth to understanding the interplay between black hole charge, scalar coupling, and photon ring formations, enriching the narrative on how alternative gravitational models can mimic, differ from, or even conceal observational features attributed to Kerr-like black holes.