Foiling Black Hole Foils: Revealing Horizon Alternatives with Baryonic Atmospheres

Abstract

Event horizons are a defining feature of black holes. Consequently, there have been many efforts to probe their existence in astrophysical black hole candidates, spanning ten orders of magnitude in mass. Nevertheless, horizons remain an obstacle to unifying general relativity and quantum mechanics, most sharply presented by the information paradox. This has motivated a proliferation of horizonless alternatives (black hole foils) that avoid event horizons and are therefore presumably benign. We explore the fate of accreting material in the context of spherically symmetric horizonless compact objects in the steady state regime. We find that for typical accreting astronomical targets, foils can develop an optically thick, convectively stable baryonic atmosphere, immediately implying the creation of an emitting photosphere at modest redshift even when the surface redshift is extreme. The emergent photosphere luminosity is driven toward the accretion powered equilibrium value and is only weakly sensitive to the foil redshift. Moreover, local gas-surface interaction provides a microphysical lower bound on the effective base temperature, insulating the atmosphere from arbitrarily cold foils. The unknown properties of the foil enter only through local boundary conditions controlling baryon processing and thermal coupling at the surface, making the solutions broadly applicable to horizonless alternatives that do not invoke significant additional nonlocal interactions. Thus, under minimal assumptions (GR exterior and local surface interactions), spherically symmetric horizonless foils in accreting systems are generically observationally exposed: the absence of a thermal photosphere directly constrains or rules out broad classes of such models.