International collaboration between Japan and Sweden has helped clarify how gravity affects the shape of matter around a black hole in the Cygnus X-1 binary. Their findings, published in Nature Astronomy this month, will help scientists further understand the physics of strong gravity and the evolution of black holes and galaxies.
Near the center of the constellation Cygnus is a star orbiting the first black hole found in the universe. Together, they form a binary system known as Cygnus X-1. This black hole is also one of the brightest X-ray sources in the sky. However, the geometry of the matter that generates this light was uncertain. The research team revealed this information thanks to a new X-ray polarimetry technique.
Taking a picture of a black hole is not easy. First, a black hole cannot be seen because light cannot leave it. Instead of observing the black hole itself, scientists can observe light emanating from matter next to it. In the case of Cygnus X-1, this light will be emitted by a star near the black hole.
Most of the light we see vibrates in many directions. Polarization filters the light so that it vibrates in one direction. This is similar to how snow goggles with polarized lenses help skiers see where they are going down the mountain because the filter disperses the snow reflecting off the snow.
"It's the same with hard X-rays near a black hole," says co-author Hiromitsu Takahashi. “But this filter is getting hard X-rays and gamma rays from the black hole. No glasses will save you from these rays, so we need another special device to measure this scattering of light."
The team needed to figure out where the light comes from and where it scatters. For both measurements, a PoGO + X-ray balloon polarimeter was used. Two competing models describe what matter looks like next to a black hole in a binary system like Cygnus X-1: the lamp post and the extended model. In the lamp post model, the corona is compact and closely related to the black hole. The photons bend towards the accretion disk, which results in more light reflection. In the extended model, the corona is larger and spreads around the black hole. The light reflected by the disc is weaker. Since light did not bend much in the black hole's strong gravity, the team concluded that the black hole follows the expanded corona model.
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Ilya Khel