The X-rays appear thanks to energetic charged particles that are accelerated by a black hole's magnetic field. The particles form into jets that vault into space from the black hole's north and south poles, streaming for distances that can top 1 million light-years.
The region of a jet with the most intense X-ray emissions lies at the end nearest the black hole. These X-rays can in effect be reflected by the swirling disk of material falling into the supermassive black hole. Meanwhile, the black hole's enormous gravity tugs on the very fabric of space-time itself as the object spins, distorting the disk of infalling material. The largest amount of distortion appears in the region nearest the black hole's event horizon – the point of no return for infalling material. This distortion shows up in the spectra of the disk material, carried by the X-rays that the material reflects. The brightest, most distorted spectra provide a measure of the black hole's spin.
Between the two telescopes, the researchers were able to measure iron's X-ray spectra from the black hole's vicinity with higher precision, in more detail, and over a wider range of X-ray energies than previous instruments could. This not only allowed them to zero in on emissions closest to the black hole, but it also allowed them to rule out competing explanations for the spectra they recorded.