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

The center of our galaxy hosts SagittariusA*, 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 SgrA* 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. 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, none of the metric model is discernible. GRAVITY+’s improved sensitivity allows detection of some exotic compact object models. However, enhancing the astrophysical complexity of the hot spot model diminishes these outcomes. Presently, GRAVITY’s uncertainties do not allow us to detect 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.

💡 Research Summary

The paper investigates whether the near‑infrared (NIR) flares from Sagittarius A* (Sgr A*), the supermassive compact object at the centre of our Galaxy, can be used to discriminate between a standard Kerr black hole and a variety of non‑rotating exotic compact objects (ECOs). The authors consider eight static, spherically symmetric space‑time metrics grouped into four families: the Schwarzschild black hole, two solitonic boson‑star configurations, two relativistic fluid‑sphere models, and three gravastar configurations. All ECO metrics are horizonless, allowing light rays to pass through the interior and generate additional “plunge‑through” images that are absent in a black‑hole spacetime.

To probe the observational signatures, the authors adopt a simple analytical hot‑spot model for the flares. A compact, Gaussian‑shaped emitting region orbits in the equatorial plane at the Keplerian velocity appropriate for the given metric. The magnetic field is taken to be vertical, which reproduces the QU‑loops observed by GRAVITY. Using the general‑relativistic ray‑tracing code GYOTO, they compute time‑dependent astrometric positions and Stokes Q and U parameters for each metric, producing synthetic QU‑loops and light curves.

Two observational scenarios are simulated. The first uses the current GRAVITY performance (≈50 µas astrometric precision, ~0.5 % flux uncertainty). The second assumes the upgraded GRAVITY+ capabilities, in particular a four‑fold improvement in flux sensitivity while keeping the same astrometric errors. For each synthetic dataset the authors perform a χ²‑minimisation fit across all eight metrics and evaluate model comparison statistics: reduced χ² and a Bayesian Information Criterion (BIC)‑based Bayes factor.

The results show that with present‑day GRAVITY uncertainties none of the alternative metrics can be statistically distinguished from the Kerr model; reduced χ² values are close to unity for all fits and BIC differences remain below the conventional threshold of 2. However, under the GRAVITY+ scenario the enhanced flux precision makes the subtle polarization signatures of the plunge‑through images detectable for several ECOs. In particular, the more compact boson‑star configuration and the fluid‑sphere/gravastar models with radii ≤ 2.5 M yield Bayes factors exceeding 2, indicating a significant preference over the Kerr fit. The authors caution that this discriminating power diminishes rapidly when the hot‑spot model is made more realistic—e.g., by adding non‑vertical magnetic fields, eccentric or inclined orbits, multiple emitting regions, or time‑varying emissivity—because these astrophysical complexities can mimic or mask the metric‑dependent polarization effects.

In summary, the study demonstrates that NIR flare polarimetry is a promising probe of strong‑gravity space‑time structure. While current GRAVITY data are insufficient to rule out the Kerr hypothesis, the forthcoming GRAVITY+ instrument could, in principle, detect the imprint of horizonless ECOs and thereby test the nature of Sgr A*. Realising this potential will require both higher‑precision measurements and more sophisticated flare modelling to control astrophysical systematics.