Testing Van der Waals black holes using black hole photon rings

Abstract

The Van der Waals black hole (BH) is a non-Kerr model in Anti-de Sitter (AdS) spacetime that exhibits characteristic thermodynamic phase transitions. Whether the physical validity of parameters such as the BH molecular volume $b$ and pressure $P$ is feasible still requires observational verification. As a sensitive probe of strong gravitational fields, the BH photon ring directly carries spacetime geometric information, making it an ideal object for model testing. In accordance with the spacetime metric associated with this BH, this study obtains the null geodesic equations for photons and conserved quantities, determines the photon sphere conditions and radii via effective potential analysis, investigates the photon ring classification and radiative properties, and employs measurements from the Event Horizon Telescope (EHT) to place constraints on parameters and simulate the optical appearance. The results show that the molecular volume parameter $b$ significantly regulates the radius of the photon sphere radius, photon ring structure, and the luminosity. Within a specific interval, the model exhibits a high degree of agreement with the EHT measurements for M87* and Sgr A*. This interval not only satisfies the energy conditions (weak, strong, and dominant) but also corresponds to a stable Van der Waals-type phase transition region. Furthermore, the photon ring can distinguish between the Van der Waals BH and the Schwarzschild BH: due to the small-black-hole-large-black-hole (SBH-LBH) phase transition and microstructural discontinuities, the photon ring of the Van der Waals BH is particularly sensitive to the parameter $b$, whereas the Schwarzschild BH lacks these features. This work uses the photon ring to probe BH molecular models, linking its microstructure, spacetime geometry and observational signatures, and provides a new way to derive BH microphysics from observations.