Sagittarius A* near-infrared flares polarization as a probe of space-time I: Non-rotating exotic compact objects

TL;DR

This study tackles whether non-rotating exotic compact objects (ECOs) can be distinguished from Schwarzschild black holes using near-infrared polarization flares from Sgr A*. It employs a toy hot-spot model and polarized ray-tracing to compare eight ECO metrics across three ECO families, evaluating fits with $χ^2_red$ and $BIC$-based Bayes factors. With current GRAVITY uncertainties, only the compact Boson star 2 is clearly detectable, while Schwarzschild is excluded in several cases; the upcoming GRAVITY+ upgrade dramatically improves detectability across models, especially with lower flux uncertainties or higher time resolution. The results demonstrate that polarization signatures and plunge-through images in ECO spacetimes can serve as a powerful probe of strong-field gravity, guiding future observations and the development of more realistic flare models.

Abstract

The center of our galaxy hosts Sagittarius~A*, a supermassive compact object of $\sim 4.3\times 10^6$ solar masses, usually associated with a black hole. Nevertheless, black holes possess a central singularity, considered unphysical, and an event horizon, which leads to loss of unitarity in a quantum description of the system. To address these theoretical inconsistencies, alternative models, collectively known as exotic compact objects, have been proposed. In this paper, we investigate the potential detectability of signatures associated with non-rotating exotic compact objects within the Sgr~A* polarized flares dataset, as observed through GRAVITY and future instruments. We examine a total of eight distinct metrics, originating from four different categories of static and spherically symmetric compact objects: Black Holes, Boson stars, Fluid spheres, and Gravastars. Our approach involves utilizing a toy model that orbits the compact object in the equatorial plane, at the Schwarzschild-Keplerian velocity. Using simulated astrometric and polarimetric data with present GRAVITY and future GRAVITY+ uncertainties, we fit the datasets across all metrics examined. We evaluated the detectability of the metric for each dataset based on the resulting $χ^2_\mathrm{red}$ and BIC-based Bayes factors. Plunge-through images of ECOs affect polarization and astrometry. With GRAVITY's present uncertainties, only a compact boson-star model is discernible. GRAVITY+'s improved sensitivity allows detection of most exotic compact object models. However, enhancing the astrophysical complexity of the hot spot model diminishes these outcomes. Presently, GRAVITY's uncertainties limit us to detecting just one exotic compact object metric. With GRAVITY+'s enhanced sensitivity, we can expect to uncover additional exotic compact object models and use Sgr~A* as a laboratory for fundamental physics.

Figures (8)

• Figure 1: Time integrated image of a hot spot orbiting the Boson star 2 (top-left), Boson star 3 (bottom-left), Fluid sphere 2 (top-right) and Fluid sphere 3 (bottom-right) models with an inclination close to face-on ($i=20^\circ$). Extracted from Rosa2025 and tamm2025.
• Figure 2: Time integrated image of a hot spot orbiting the Gravastar models with $R=3M$ (Gravastar 1) in the left panel, $R=2.5M$ (Gravastar 2) in the middle panel and $r=2.01M$ (Gravastar 3) in the right panel. Extracted from tamm2025.
• Figure 3: Astrometry of an orbiting hot spot in different metric models and simulated astrometric data with current GRAVITY uncertainties.
• Figure 4: Sketch illustrating our methodology to assess the detectability of ECO's models.
• Figure 5: Contribution of the various images orders and nature, i.e. primary only in dotted line, primary + secondary (equivalent to Schwarzschild) in dashed line and all images including the plunge-through images in solid lines.