- Researchers used X-ray polarimetry to understand the effect of gravity on matter near the black hole.
- They discovered that the corona is large and spread unevenly around the black hole, in a binary system named Cygnus X-1.
Black holes are known for their strong gravitational force: so strong that even light can’t escape. Near the center of the Cygnus northern constellation, there is a star orbiting a black hole. Together they create a binary system called Cygnus X-1.
This is the first strong black hole ever discovered by NASA”s Chandra X-Ray Observatory. The binary system emits a large amount of X-rays that are captured by Earth’s satellites. However, the geometry of objects that give rise to this X-ray doesn’t seem certain.
Recently, a team of researchers from Sweden and Japan unveiled this information, using a new method known as X-ray polarimetry. They explained how gravity affects the shape of matter near a black hole in Cygnus X-1.
Radiation Coming From Objects Close to the Black Hole
Capturing an image of black hole isn’t easy as it sounds. Since light can’t escape the intense gravity of the black hole, it isn’t yet possible to observe one. Therefore, instead of observing the black hole directly, researchers can detect light coming from nearby matter. In the binary system Cygnus X-1, this matter comes from a star orbiting the black hole.
X-ray image of Cygnus X-1 | Credit: NASA
Although light vibrates in several directions, polarization (a property that specifies the geometrical orientation of the oscillations of a wave) filters it and makes it vibrate in a single direction. This is how polarized lenses in snow goggles work: they cut light reflecting off the snow.
Same happens with hard X-rays (highest energy X-rays) near the black hole. However, gamma rays and hard X-rays emitted by near-black-hole-objects can easily penetrate such filters. Thus, we require a special technique to analyze this light scattering phenomenon.
To find out where the light is coming from and where it’s getting scattered, researchers attached an X-ray polarimeter on a balloon, named PoGO+. This helped them figure out the part of hard X-rays that is being reflected off the accretion disk (a structure formed by diffused material in orbital motion around a gigantic central body) and analyze the shape of the matter.
Reference: Nature astrology | doi:10.1038/s41550-018-0489-x | Hiroshima University
There are 2 competing models that show how matter around a black hole can appear in the Cygnus X-1 and other binary systems.
Lamp-post model: The corona is compact, dense and tightly bound to the black hole. In this case, all photons are bent towards the accretion disk, which results in more reflected light.
Extended model: The corona is big and spread unevenly around the black hole. Thus, light reflected by the disk is quite weak.
In Cygnus X-1, photons didn’t bend much towards the accretion disk, thus researchers deduced that the black hole fits the extended model.
This information will help scientists find more unknown facts about black holes. One of the key characteristics is its spin, which can alter the space-time near the black hole. Moreover, it could help us understand the evolution of black holes. Either the speed of black hole’s spin has decreased gradually since the inception of the universe, or it has increased due to the fact the black hole is consuming a vast amount of matter.
Cygnus is just one example: researchers plan to analyze more black holes via X-ray polarimetry. This could eventually uncover the mysteries of black holes as well as the evolution of galaxies.