A black hole is generally described as an object surrounded by a boundary called an event horizon. Once this limit is crossed, neither matter nor light can return to the outside. Some theories nevertheless propose extremely compact objects that would resemble black holes, but without a genuine horizon.
The difference could leave an observable trace. Matter continuously falls toward the massive objects located at the centers of many active galaxies. If the central object has a surface, this matter must eventually hit it. The energy from the impact would then strongly heat the surface and produce recognizable thermal radiation.
What the passage through a black hole's event horizon would look like.
NASA illustration
With an event horizon, the scenario changes. Matter that crosses it disappears from our view without hitting a solid surface. Researchers therefore specifically looked for the luminous signature that a surface heated by an impact of matter would produce.
The team relied on BASS DR2, a survey of nearby active galactic nuclei observed notably in X-rays. This dataset provides detailed spectral measurements. The authors selected the objects that made it possible to compare their observed emissions with those expected in the presence of a surface.
Result: none of the objects studied displays the sought-after thermal signature. For a significant portion of the sample, the observations already make it possible to rule out a radiating surface such as the one predicted by many horizonless models. For the others, the current data are simply not yet precise enough.
This test is particularly valuable because it depends relatively little on the exact nature of the alternative object. Whenever a surface continuously receives matter, it should heat up and radiate. The absence of this light therefore narrows the range of several families of models proposed as alternatives to classical black holes.