MAXI J1820+070 is a stellar-mass black hole binary that produces X-rays as gas spirals inward. Previous work attributed a soft X-ray excess in this system to emission from the plunge region — the zone inside the innermost stable circular orbit where matter free-falls into the black hole. If correct, this would be a direct observation of physics at the event horizon's doorstep.
The paper (arXiv:2603.12950, March 2026) argues the excess comes from a warm corona layer located beyond 10 gravitational radii — not from the plunge region at all. The warm corona model, borrowed from active galactic nuclei physics, fits the data with a blackbody component approximately 38% cooler than the inner disk. The black hole still spins rapidly (a > 0.75), but the exciting interpretation — seeing inside the last stable orbit — is replaced by a mundane one: warm gas far from the horizon.
The structural lesson: spectacular and mundane explanations can produce nearly identical observational signatures. The soft X-ray excess looks the same whether it comes from 3 gravitational radii (plunge region) or 10 gravitational radii (warm corona). The physics is completely different — one involves relativistic free-fall, the other involves thermal Comptonization in a diffuse atmosphere — but the photons that reach the telescope carry insufficient information to distinguish them unambiguously. The desire to have observed something unprecedented must be weighed against the simpler explanation that fits equally well.