Shadows of Kerr black holes with scalar hair (1509.00021v2)

Abstract: Using backwards ray tracing, we study the shadows of Kerr black holes with scalar hair (KBHsSH). KBHsSH interpolate continuously between Kerr BHs and boson stars (BSs), so we start by investigating the lensing of light due to BSs. Moving from the weak to the strong gravity region, BSs - which by themselves have no shadows - are classified, according to the lensing produced, as: $(i)$ non-compact, which yield no multiple images; $(ii)$ compact, which produce an increasing number of Einstein rings and multiple images of the whole celestial sphere; $(iii)$ ultra-compact, which possess light rings, yielding an infinite number of images with (we conjecture) a self-similar structure. The shadows of KBHsSH, for Kerr-like horizons and non-compact BS-like hair, are analogous to, but distinguishable from, those of comparable Kerr BHs. But for non-Kerr-like horizons and ultra-compact BS-like hair, the shadows of KBHsSH are drastically different: novel shapes arise, sizes are considerably smaller and multiple shadows of a single BH become possible. Thus, KBHsSH provide quantitatively and qualitatively new templates for ongoing (and future) very large baseline interferometry (VLBI) observations of BH shadows, such as those of the Event Horizon Telescope.

• The paper demonstrates that scalar hair significantly alters black hole shadows by introducing deviations in size, shape, and the presence of multiple shadow components.
• The paper employs backward ray tracing to distinguish between classic Kerr shadows and those influenced by boson star-like structures, offering new insights into gravitational lensing.
• The paper quantifies shadow features using parameters such as center displacement and average radius, providing actionable metrics for improving VLBI observational templates.

Shadows of Kerr Black Holes with Scalar Hair

This paper explores the intriguing domain of black hole (BH) physics by examining the shadows of Kerr black holes with scalar hair (KBHsSH). The authors utilize backward ray tracing to scrutinize how these entities, which interpolate between Kerr black holes and boson stars (BSs), impact the observation of BH shadows. The research is set against the backdrop of increased observational capabilities provided by very large baseline interferometry (VLBI) techniques, with an objective of furnishing new templates for black hole shadow observations, particularly for instruments like the Event Horizon Telescope.

Analytical Framework

KBHsSH are considered as stationary, axisymmetric solutions to Einstein's gravity equations, coupled with a massive complex scalar field. The principal insight of the paper is the exploration of how scalar hair—significant amounts of energy or matter existing outside the classical event horizon—affects the shadow of a black hole. The shadows are predicted by computing photon geodesics in the geometries of interest.

Key Findings

1. Classification of Boson Stars: The paper first describes three classifications for boson stars based on their lensing properties:
   • Non-compact BSs: These do not produce multiple images or shadows.
   • Compact BSs: These give rise to Einstein rings and multiple images.
   • Ultra-compact BSs: These possess light rings, leading to an infinite and possibly self-similar number of images.
2. Shadows of KBHsSH: There are stark distinctions between the shadows of standard Kerr black holes and KBHsSH, especially when ultra-compact BS-like hair is present. For instance, in cases with non-Kerr-like horizons:
   • Novel Shadow Shapes: The shadows deviate from the expected silhouette of Kerr black holes, with smaller sizes and potential for multiple shadows.
   • Quantitative Analysis Parameters: The paper introduces several parameters for quantitatively analyzing the shadows: center displacement, width, height, average radius, and deviations both spherically and compared to Kerr black holes.
3. Influence of Hair on Shadows: The authors substantiate that the presence of scalar hair in KBHsSH results in distinct shadow profiles, which differ appreciably from Kerr BHs by producing smaller and potentially multi-component shadows.

Implications and Future Work

The implications of these findings are significant for astrophysical observations. By proposing new templates that are distinct from the classical Kerr BH model, this research provides groundwork for identifying discrepancies in observed BH shadows that might imply the existence of such non-classical features as scalar hair. It postulates that deviations in shadow size and shape could suggest alternative gravitational settings, nudging the stagnated notion of black holes purely described by general relativity.

Future explorations could involve mapping the parametric space more extensively via such KBHsSH solutions to refine the templates used in VLBI. More realistic astrophysical simulations could extend the paper's findings to a broader set of observational data. Moreover, integrating the effects on redshift into these calculations might present an additional layer of observational data, enhancing the shadows' analytical framework.

Conclusion

This paper enriches our understanding of black hole physics, particularly by elucidating the role of scalar hair in modifying black hole shadows. These findings encourage the incorporation of complex matter structures in modeling celestial phenomena and further propel observational astrophysics toward unraveling new cosmic truths.