During its 2021 outburst, the black hole in the GX 339-4 system behaved like an actress changing costumes between scenes. In the hard state, it wore two coronas at once: a hot, rarefied one, and over it, a warm, dense one. In the soft state, only the hot corona remained.

Astronomers observed the black hole simultaneously with three X-ray telescopes: NuSTAR, NICER, and Insight-HXMT. They measured how the disk around the black hole shines. It turned out that if you consider only one hot corona, the disk in the hard state appears ten times smaller than in the soft state. This contradicts the established view that the disk should be larger in the hard state.

But when they added the second, warm corona, the disk sizes fell into place. In the hard state, the disk turned out to be larger than in the soft state, as theory predicts. The warm corona acts like a translucent veil that hides part of the light from the disk.

So in the hard state, the black hole has a double corona: hot on the outside and warm on the inside. In the soft state, the warm corona disappears, leaving only the hot one. This is like a wardrobe change: for going out, you put on both a coat and a jacket, but at home you wear just the jacket.

This double corona explains why astronomers were previously confused about the disk size. Without the warm corona, the disk appeared smaller than it actually was. With it, everything falls into place. Therefore, when studying hard states of black holes, one must always take the warm corona into account.