Astronomers developed a new computer simulation that shows how accretion disk of Black Hole glows. You can watch the video here https://www.youtube.com/watch?v=-OtUVDRL_wM The temperature of gas inside the accretion disk reaches to 20 million degrees and as the gas spirals and moves toward the center it shines in low-energy or soft, X-rays. You can read the news here https://www.nasa.gov/image-feature/accretion-disk-of-black-hole-glows-in-new-simulation [The image is taken from www.nasa.gov]