Observational Constraints on Horizonless Compact Objects from Thermal Emission in AGNs

Abstract

The Swift Burst Alert Telescope Active Galactic Nuclei Spectroscopic Survey (BASS DR2) provides one of the largest and most complete samples of bright, local (z < 0.1) active galactic nuclei (AGNs) with high-quality spectroscopic measurements. These sources are typically interpreted within the framework of Kerr black holes; however, a wide range of horizonless compact-object scenarios-including naked singularities, black hole mimickers, and other exotic compact objects-have been proposed as alternatives. Largely independent of the physics of the particular horizonless model invoked, an accretion powered, thermally radiating photosphere is expected to develop on astronomically short timescales. In this paper, we investigate whether the presence of such a photosphere is consistent with the observed properties of an appropriate subsample of BASS AGNs. We find that in no instances is such a spectral feature present, and can exclude its existence in significant fraction of objects. For the remaining sources, modest improvements in observational sensitivity and spectral coverage could enable robust exclusion. Our results extend previous horizon-scale tests performed for M87* and Sgr A* to a large AGN population, providing strong, population-level evidence against horizonless alternatives. We conclude by outlining future observational strategies that can further tighten these constraints and significantly increase the number of objects for which horizons are required.