Black Hole Corona: The Wind That Polarizes Light express
The black hole Cygnus X-1 is one of the most studied in our galaxy. It pairs with a normal star, pulling matter from it. The gas spirals into a glowing disk, like water down a drain, and above the disk hangs a scorching corona — a cloud of electrons at temperatures of hundreds of millions of degrees. It is in this corona that X-ray photons gain extra energy and become polarized, meaning their oscillations align in one direction.
New observations from several space telescopes show that the corona is not stationary. It streams outward at about 0.3 times the speed of light — roughly 90,000 kilometers per second. This wind accelerates electrons, and they scatter light unevenly: photons moving against the wind gain more energy than those moving with it. This explains why the polarization barely changes with photon energy, even though it would normally be expected to rotate significantly.
Comparison with calculations shows that the black hole rotates slowly. If it spun fast, its gravity would cause the polarization plane to rotate by different angles for different energies. Observations show no such rotation. So the hole itself plays little role in creating polarization: it's all the work of its corona-wind.
Thus Cygnus X-1 again confirms: even in a quiescent state, a black hole is not asleep but continuously ejects matter. And this ejection can be seen not only in brightness but also in how light oscillates.
Original abstract
as written by the authors arXiv:2603.10870
We aim to identify the physical mechanism responsible for the observed X-ray emission and polarization of Cyg X-1 in the soft state. We performed a detailed spectral analysis of X-ray data obtained with NICER, NuSTAR, and INTEGRAL during observations simultaneous with IXPE on 2023 June 20, supplemented at higher energies with archival CGRO data. We developed a new model, retBB, which describes thermal disk emission and its returning reflection, and applied it together with accurate models of Comptonization and relativistically broadened reflection. Using the resulting spectral solution, we computed the expected polarization signal and compared it with the IXPE measurements. Our spectropolarimetric modeling shows that the observed polarization is in agreement with being produced by Comptonization in a corona undergoing a semi-relativistic outflow at a velocity of ~0.3c. Our spectral solutions admit either low or high black-hole spin values, depending on the adopted model setup. However, the observed polarization strongly favors a low spin. At high spin, the polarization angle would inevitably rotate significantly across the energy band, which is inconsistent with the observations. Apart from this rotation of the polarization angle, general relativistic effects do not play a significant role in producing the observed polarization. In particular, we find that, at most, returning disk radiation contributes only a minor amount.
Cygnus X-1 was the first black hole discovered via X-ray emission. In 1974, Stephen Hawking bet Kip Thorne that it wasn't a black hole, and lost.
In Dan Simmons's novel 'Hyperion', a black hole serves as a portal, but Cygnus X-1 has no traversable event horizon — only a wind from its corona.