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.
as written by the authors arXiv:2603.08917
We present a broadband spectral analysis of the black hole X-ray binary GX 339-4 during its 2021 outburst, covering both hard and soft spectral states. Using simultaneous observations from NuSTAR, NICER, and Insight-HXMT, we investigate the evolution of the accretion disk with a focus on the disk normalization derived from the diskbb component, which serves as a proxy for the apparent inner disk radius. In the standard single Comptonization model, the disk normalization in the hard state is more than an order of magnitude lower than in the soft state ($$\sim$$0.3$$\times$$10$$^3$$ vs. $$\sim$$3.0$$\times$$10$$^3$$). This result contradicts the widely accepted view that the disk radius is smaller in the soft state than in the hard state. By incorporating an additional warm Comptonization component, the disk normalization in the hard state increases to values ($$\gtrsim 10^4$$) exceeding those in the soft state ($$\sim 10^3$$), yielding results consistent with a physically truncated, cooler accretion disk. The results of this work support the presence of a dual-corona geometry in the hard state, comprising both a hot, optically thin corona and a warm, optically thick corona, while the soft state spectrum is well described by a single hot Comptonization component alone. Our findings emphasize the importance of including a warm corona in hard-state spectra, as it leads to a more physically consistent picture of the accretion geometry across spectral states.
The temperature of the hot corona around a black hole can reach a billion degrees — hundreds of times hotter than the center of the Sun.
In the movie Interstellar, the black hole Gargantua is shown with a bright accretion disk, but with no hint of the double corona that astronomers observed in GX 339-4.