Researchers have discovered how the strong gravitational field coming from a black hole influences the shape of matter that encircles it.

An international team of researchers from Japan and Sweden has found that light bouncing off the accretion disk around a black hole is dispersed by an extended corona, a mysterious source of highly charged particles surrounding a black hole.

The team says their findings provide new insight into the nature of strong gravity and pave the way toward a better understanding of the evolution of black holes.

First Black Hole Discovered

In a new paper published in the journal Nature Astronomy, the team looked into the black hole binary system Cygnus X-1, a pair consisting of a black hole and a star in the Cygnus constellation 6,070 light-years from Earth.

Cygnus X-1, which comprises the first black hole to be discovered by man, is one of the brightest sources of X-ray in the sky. However, scientists have long been puzzled about the nature of the matter that creates this X-ray emission.

Black holes are regions in space defined by an extremely strong gravitational field. The pull of black holes is so strong that even light cannot escape once it falls past the event horizon.

This is why it is practically impossible to study black holes using visible light telescopes. Instead, experts study light coming from the matter that encircles the black hole. In the case of Cygnus X-1, the researchers examined the light coming from the star moving in orbit around the black hole.

X-Rays From Black Holes Penetrate Polarized Filters

Light travels in multiple directions. To force the light to travel in just one direction, scientists use polarization filters that cut off light from other sources.

This is the same method used for polarized ski goggles. The polarization filters on the lens cuts off light that bounces from the snow to minimize glare. It also works the same when it comes to hard X-rays coming from black holes.

"However, hard X-rays and gamma rays coming from near the black hole penetrate this filter," says Hiromitsu Takahashi, co-author and assistant professor at Hiroshima University. "There are no such 'goggles' for these rays, so we need another special kind of treatment to direct and measure this scattering of light."

The Shape Of Matter Around Black Hole

By using a new technique called X-ray polarimetry, the researchers were able to distinguish the shape of the matter that generates the abundant amounts of X-rays in Cygnus X-1.

The researchers launched an X-ray polarimeter on the stratospheric balloon PoGO+ to help them identify the source of light coming from the binary system.

This allowed them to determine the hard X-rays that bounced off the accretion disk of the black hole and identify the shape of matter surrounding the black hole.

There are two prevailing models for the shape of matter around a black hole. The first is the lamp-post model, where the black hole has a dense corona bound tightly to it. In this scenario, the particles of light are bent toward the disk, which creates more light bouncing off it.

The second model is the extended model, where the corona is diffused around the black hole. This means the light bouncing off the accretion disk is far weaker than in the lamp-post model.

The researchers found that light did not bend that much in the vicinity of the black hole, suggesting that the extended model applies.

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