The Cygnus X-1 system is one of the brightest sources of X-rays in the Milky Way, consisting of a black hole called Cygnus X-1 and its giant companion, a star packing 41 times the mass of our Sun. Researchers recently measured the polarization of those X-rays to better understand the geometry of the superheated plasma in the Cygnus X-1 system.
Though nothing is visible beyond any black hole’s event horizon, superheated matter usually surrounds a black hole. At millions of degrees, the matter emits X-rays that offer information about the environment they came from.
“Previous X-ray observations of black holes only measured the arrival direction, arrival time and energy of the X-rays from hot plasma spiraling toward the black holes,” said Henric Krawczynski, a physicist at Washington University in St. Louis and the paper’s lead author, in a university release. “IXPE also measures their linear polarization, which carries information about how the X-rays were emitted — and if, and where, they scatter off material close to the black hole.”
Some of the data came from the Imaging X-Ray Polarimetry Explorer (IXPE) mission. IXPE is a satellite in orbit around Earth that launched in December 2021. The mission is planned to last two years; besides black holes, IXPE is looking at X-ray sources like neutron stars, pulsars, nebulae, and the remains of supernovae.
The researchers combined the IXPE observations with data from NASA’s NICER and NuSTAR X-ray observatories to get a better idea of the whereabouts of the plasma around Cygnus X-1. Their work is published today in the journal Science.
They found the plasma formed a disk that is perpendicular to the jets of material that shoot out from either side of the black hole. Black hole jets are thought to be formed by strong magnetic fields, which direct some of the superheated material out of either side of the object in vast streams of energised particles accelerated to nearly the speed of light.
If Sagittarius A*, the supermassive black hole at the centre of our galaxy, has these jets, we wouldn’t be able to see them. That’s because the black hole is pointed directly at us.
The directions of the jets and the polarization of the system’s X-rays are aligned, which suggested to the research team that whatever is at play in the bright region near the black hole is related to the formation of the jets.
Furthermore, the X-rays’ polarization made the team suspect that the black hole’s plane may be out of alignment with the Cygnus X-1 system’s plane. Previous evidence has indicated that this can happen when the black hole is formed by the violent death of its progenitor star.
The explosion of material from the star’s death can give the newly formed black hole a natal kick, which knocks it out of alignment with its surrounding objects. The small black hole in MAXI J1820+720, for example, is out of alignment with the disk of material surrounding it by about 40 degrees.
IXPE (and its fellow X-ray observatories) will continue to reveal new aspects of black hole formation and evolution in time, giving us a fuller-fledged picture of the diversity of these extremely dense, enigmatic objects.