The K-line is a spectral peak in astronomical spectrometry used, along with the L-line, to observe and describe the light spectrum of stars.
The K-line is associated with iron (Fe) and is described as being from emissions at ~6.4keV (thousands of electron volts).
"The observations include clocking the speed of a black hole's spin rate and measuring the angle at which matter pours into the void, as well as evidence for a wall of X-ray light pulled back and flattened by gravity.
"[1] 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 teams were led by Andrew Fabian of Cambridge University, England, and James Reeves of NASA's Goddard Space Flight Center, Greenbelt, Maryland, United States.