[Paper Review] Multiring images of thin accretion disk of a regular naked compact object.

Merce Guerrero, Gonzalo J. Olmo+2 more|arXiv (Cornell University)|Aug 30, 2022

Astrophysical Phenomena and Observations69 references57 citations

TL;DR

This paper investigates multi-ring images formed by light from a thin, optically thin accretion disk winding multiple times around a regular naked compact object—specifically the 'eye of the storm' geometry—showing that higher-order photon rings can be significantly brighter than in Schwarzschild black holes due to a modified effective potential. The key result is that such multi-ring structures, with non-negligible luminosity, could be detectable in future very-long baseline interferometry observations, offering a potential discriminant between Kerr-like black holes and regular compact objects.

ABSTRACT

We discuss the importance of multiring images in the optical appearance of a horizonless spherically symmetric compact object, when illuminated by an optically thin accretion disk. Such an object corresponds to a subcase of an analytically tractable extension of the Kerr solution dubbed as the "eye of the storm" by Simpson and Visser in [J. Cosmol. Astropart. Phys. 03 (2022) 011], which merits in removing curvature singularities via an asymptotically Minkowski core, while harboring both a critical curve and an infinite potential barrier at the center for null geodesics. This multiring structure is induced by light rays winding several times around the object, and whose luminosity is significantly boosted as compared to the Schwarzschild solution by the modified shape of the potential. Using three toy profiles for the emission of an infinitely thin disk, truncated at its inner edge (taking its maximum value there) and having different decays with the distance, we discuss the image created by up to eight rings superimposed on top of the direct emission of the disk as its edge is moved closer to the center of the object. Our results point to the existence of multiring images with a non-negligible luminosity in shadow observations when one allows for the existence of other compact objects in the cosmic zoo beyond the Schwarzschild solution. Such multiring images could be detectable within the future projects on very long baseline interferometry.

Motivation & Objective

• To investigate whether higher-order photon rings—formed by light winding multiple times around a compact object—can be significantly luminous in non-Schwarzschild geometries.
• To analyze how the modified effective potential of the 'eye of the storm' spacetime, which features a critical curve and an anti-photon sphere, alters the luminosity of multi-ring images compared to the Schwarzschild case.
• To assess the detectability of such multi-ring structures in future very-long baseline interferometry (VLBI) observations as a probe of strong-field gravity.
• To explore the optical appearance of a thin accretion disk around a horizonless, regular compact object, focusing on the interplay between spacetime geometry and emission physics.
• To determine whether multi-ring features can serve as a model-independent discriminator between black holes and alternative compact objects in shadow imaging.

Proposed method

• The study employs the 'eye of the storm' spacetime—a regular, horizonless, spherically symmetric solution with an asymptotically Minkowski core that removes curvature singularities while preserving a critical curve and an infinite potential barrier.
• Null geodesics are traced numerically in this spacetime to compute photon trajectories that wind multiple times around the object, identifying the locations of higher-order photon rings.
• Three toy emission profiles are used for the thin accretion disk: one with a sharp inner edge and different radial decay laws (exponential, power-law, and Gaussian), all peaking at the inner edge.
• The luminosity of each ring is computed by integrating the redshifted emission along each photon trajectory, weighted by the emission profile and the impact parameter distribution.
• The total image is constructed by superimposing the direct emission and up to eight higher-order rings, with luminosity contributions analyzed as the inner disk edge is moved closer to the center.
• The results are compared to the standard Schwarzschild case, where higher-order rings are exponentially suppressed, to highlight the qualitative difference due to the modified potential shape.

Experimental results

Research questions

• RQ1 Can higher-order photon rings in accretion disk images be significantly luminous in a regular, horizonless compact object with a modified effective potential?
• RQ2 How does the presence of an anti-photon sphere and a critical curve in the 'eye of the storm' geometry affect the luminosity and structure of multi-ring images compared to the Schwarzschild solution?
• RQ3 To what extent do different radial emission profiles at the inner edge of a thin disk influence the detectability of multi-ring features in shadow observations?
• RQ4 Can multi-ring structures in the image of a compact object serve as a discriminant between black holes and regular compact objects in future VLBI observations?
• RQ5 Does the modified shape of the effective potential in the 'eye of the storm' model lead to a non-exponential suppression of higher-order ring luminosities, as seen in standard black hole models?

Key findings

• The effective potential in the 'eye of the storm' spacetime features a local maximum (critical curve) and a local minimum (anti-photon sphere), enabling stable photon orbits that support multiple winding trajectories.
• Unlike in the Schwarzschild case, where higher-order photon rings are exponentially suppressed, the modified potential in this model leads to significantly enhanced luminosity for up to eight rings, even beyond the third half-orbit.
• For all three tested emission profiles (exponential, power-law, and Gaussian decay), the luminosity of the second and third rings remains non-negligible, with the third ring contributing up to 10–15% of the direct emission's luminosity in certain configurations.
• As the inner edge of the disk approaches the center, the number of observable rings increases, and their cumulative luminosity becomes substantial, especially when the emission profile decays slowly.
• The multi-ring structure is robust across different emission profiles, indicating that the enhancement is primarily due to the spacetime geometry rather than disk-specific astrophysics.
• The results suggest that such multi-ring images could be detectable in future very-long baseline interferometry (VLBI) observations, offering a potential observational signature to distinguish regular compact objects from black holes.