An article published in the journal “Nature Astronomy” describes unprecedented observations of matter around a black hole. A team of researchers used the data detected using an X-ray polarimeter on board the PoGO+ satellite to obtain information on the part of hard X-rays that are reflected from the accretion disk around the black hole of the Cygnus X-1 system and identify the shape of the matter that composes it.
Approximately 6,100 light years from Earth, Cygnus X-1 is a binary system formed by a very massive star cataloged as HDE 226868 and a black hole. According to a research published in the journal “The Astrophysical Journal” in November 2011, the star has a mass 19.2 ± 1.9 times the Sun’s while the black hole has a mass 14.8 ± 1.0 times the Sun’s. According to a research published in the journal “Science” in May 2003, the black hole was created from a progenitor star that had an initial mass greater than 40 times the Sun’s whose core collapsed directly without exploding in a supernova.
The Cygnus X-1 system is an X-ray source discovered in 1964. Subsequent studies led to the discovery of a blue supergiant star and an invisible companion. The collected data led to the hypothesis that it was a black hole, confirmed over time by observations made with increasingly sophisticated instruments.
There was still an uncertainty concerning the geometry of the matter that surrounds the black hole and emits the X-rays and a team of researchers tried to solve the mystery using a new technique called X-ray polarimetry. They exploited the fact that part of the hard X-rays and gamma rays are reflected by the accretion disk surrounding the black hole and can be filtered by a polarimeter.
The instrument was launched aboard the PoGO+ satellite on a huge balloon and detected the part of reflected X-rays, identifying the shape of the material that composes the accretion disk. There were two alternative models describing the shape of that matter in a binary system such as Cygnus X-1, illustrated in the bottom image (Courtesy Fumiya Imazato, Hiroshima University. All rights reserved): in the lamp-post model, the corona is compact and bound closely to the black hole while in the extended model the corona is larger and spread around the area surrounding the black hole.
X-rays are reflected differently by the matter depending on its shape, with greater intensity in the lamp-post model and more weakly in the extended model. The detections indicated that the correct model is the extended one. This is a step forward in the research but the observations will continue to get new information on the matter that forms the accretion disk.
In binary systems such as Cygnus X-1 the black hole steals gas from its companion as shown in the top image but the consequences on the system may be different depending on the characteristics of the pair. X-ray polarimetry can be useful to better understand the evolution of black holes and their influence on the surrounding environment, including the entire galaxy that hosts them.