[Paper Review] Observing naked singularities by the present and next-generation Event Horizon Telescope

Valentin Deliyski, Galin Gyulchev+2 more|arXiv (Cornell University)|Jan 25, 2024

Astrophysical Phenomena and Observations4 citations

TL;DR

This paper investigates the observational signatures of reflective naked singularities using the Event Horizon Telescope (EHT), showing that such objects produce a distinctive ring-like structure in accretion disk images due to photon reflection near the singularity. The next-generation EHT at 230 GHz and 345 GHz can distinguish these rings from Kerr black holes, with a two-order-of-magnitude intensity difference in the central depression and visible bright spots at 345 GHz, offering a quantitative test for the absence of an event horizon.

ABSTRACT

We consider the observational signatures of reflective naked singularities as seen by the current and next-generation Event Horizon Telescope (EHT). The reflective naked singularities lead to a distinctive morphology of their accretion disk images producing a series of bright rings at the central part of the image. We explore the capacity of the present and near-future EHT arrays to detect this structure considering two particular naked singularity spacetimes and modeling the galactic target M87*. We obtain that the 2017 EHT array is incapable of resolving the bright ring series. However, it detects an increased overall intensity of the central brightness depression reaching with an order of magnitude higher values than for the Kerr black hole. This metric can be used as a quantitative measure for the absence of an event horizon. The observations with the next-generation EHT at 230 GHz would reveal two orders of magnitude difference in the intensity of the central brightness depression between naked singularities and black holes. Introducing a second observational frequency at 345 GHz would already resolve qualitative effects in the morphology of the disk image for naked singularities as certain bright spots become apparent at the center of the image.

Motivation & Objective

• To assess whether current and next-generation EHT arrays can resolve the unique accretion disk morphology of reflective naked singularities.
• To quantify the observational differences between naked singularities and Kerr black holes in terms of image intensity and structure.
• To evaluate the detectability of central bright ring features using realistic EHT array configurations and multi-frequency observations.
• To establish a quantitative metric—central brightness depression intensity ratio—for distinguishing naked singularities from black holes when ring structures are unresolved.

Proposed method

• Simulating accretion disk images using a radiatively inefficient accretion flow (RIAF) model for two naked singularity spacetimes: Janis-Newman-Winicour and Gauss-Bonnet.
• Applying the ehtim toolkit to model realistic EHT observations with the 2017 array, 2022 array, and a proposed next-generation EHT (ngEHT) array.
• Reconstructing observed images through visibility-based imaging and fitting them with a VIDA ring template to extract ring parameters such as radius, width, ellipticity, and offset.
• Using the ratio of central minimum intensity to ring maximum intensity as a quantitative metric to compare naked singularities and Kerr black holes.
• Introducing a second observing frequency at 345 GHz to enhance morphological resolution and detect bright spot features in the central region.
• Analyzing intensity cross-sections along horizontal and vertical axes through the fitted ring to validate image reconstruction fidelity.

Figure 1 : Image of the thin accretion disk around the Janis-Newman-Winicour singularity with scalar field parameter $\gamma=0.48$ (left panel) [ 35 ] . The Schwarzschild black hole is presented for comparison in the right panel. The observer is located at $r=5000M$ , while the inclination angle is

Experimental results

Research questions

• RQ1 Can the present EHT array resolve the central bright ring structure expected from reflective naked singularities in M87*?
• RQ2 How does the central brightness depression intensity ratio differ between naked singularities and Kerr black holes in current EHT observations?
• RQ3 What observational improvements does the next-generation EHT at 230 GHz provide in distinguishing naked singularities from black holes?
• RQ4 Can a second observing frequency at 345 GHz reveal qualitative morphological differences, such as bright spots, in the central region of naked singularity images?
• RQ5 To what extent can the VIDA ring fitting procedure extract reliable structural parameters from reconstructed images of naked singularity accretion disks?

Key findings

• The 2017 EHT array cannot resolve the central bright ring series of reflective naked singularities but detects a central brightness depression with intensity up to an order of magnitude higher than for Kerr black holes.
• The next-generation EHT at 230 GHz can distinguish naked singularities from Kerr black holes via a two-order-of-magnitude difference in the central brightness depression intensity ratio.
• Observing at 345 GHz reveals qualitative morphological differences, with distinct bright spots appearing in the central region of naked singularity images, absent in black hole counterparts.
• The VIDA ring fitting procedure successfully recovers key image parameters such as ring radius (≈21 μas), width (σ ≈ 4–7 μas), and ellipticity (τ ≈ 0.03–0.06), with optimized divergence values below 0.02.
• The ring radius for Janis-Newman-Winicour naked singularities is significantly smaller (≈15.5 μas) than for Kerr or Schwarzschild cases, providing a potential observational discriminant.
• The maximum intensity of central bright spots in ngEHT images at 345 GHz reaches values up to 0.3–0.4 in normalized intensity, significantly higher than in black hole images, offering a direct observational signature.

Figure 2 : Image of the thin accretion disk around the Gauss-Bonnet naked singularity with coupling constant $\gamma=1.15$ [ 36 ] . The observer is located at $r=5000M$ , while the inclination angle is $i=80^{\circ}$ .