The system , which sits about 6,100 light-years away in the constellation Cygnus, contains a young blue supergiant star (HDE 226868) about 20 times the mass of our Sun, and a black hole about 15 times the mass of our Sun. It’s called a high-mass X-ray binary because the companion star (the supergiant) is massive and the black hole gives off X-rays. The two orbit with a distance between them of only 0.2 astronomical units (AU; 1 AU = 93 million miles [150 million km]), about half the distance that Mercury orbits from our Sun. (Earth orbits at 1 AU, for reference.) The X-rays we receive from the system vary every 5.6 days, which accounts for one orbit of the supergiant and the black hole, as well as every 300 days, which astronomers believe is due to precession of the jets. Because the jets are not shooting out straight up and down (from our viewpoint), that slight tilt changes as the black hole moves, causing the longer variation.As the black hole sucks in matter from its companion, that matter swirls into an accretion disk, making its way inward until it eventually falls past the event horizon and disappears from view. The accretion disk is extremely hot – so hot that it shines in energetic light: X-rays and even gamma rays. Astronomers’ current picture of the accretion system around a black hole includes both the flattened disk, as well as a spherical “corona” of hot gas, which gives off hard (high-energy) X-rays, near the black hole. On top of that, Cyg X-1 also has jets, which shoot out perpendicular to the disk from near the event horizon. Astronomers don’t know exactly what causes the jets, though they suspect magnetic fields come into play.There are two leading models of the accretion disk and corona that astronomers use to explain the X-rays we see from Cyg X-1 and other similar systems. The first, called the lamp-post model, envisions the corona as a compact region of gas tightly bound to the black hole, but not surrounding it. In the second model, the extended model, the corona is a larger region of gas that encompasses the black hole entirely. The two models produce different observations — X-rays from the disk and corona are bent or scattered differently, depending on the geometry of the system. To differentiate between the two models, the researchers here used a technique called X-ray polarimetry: They measured the orientation of incoming X-rays — e.g., they looked at the direction the light was vibrating — to build up a picture of how those X-rays had been scattered when they left the black hole system.