The study teams observed X-ray emissions from the "broad iron K line" near the event horizon of several super-massive black holes of galaxies called MCG-6-30-15 and MCG-5-23-16. The normally narrow K-line is broadened by the doppler shift (red shift or blue shift) of the X-ray light emitted by matter being affected by the gravity of the black hole.
Researchers observed X-ray emissions from a specific iron line near the event horizon of several supermassive black holes in galaxies MCG-6-30-15 and MCG-5-23-16. The narrow iron K-line was broadened due to the Doppler shift caused by matter being affected by the black hole's gravity.