Profile of X-ray lines

The Fe K$\alpha$ line at 6.4keV is a common feature in the X-ray spectra of AGN. It is thought to arise from fluorescence of the X-ray continuum off cold material in the surroundings, possible associated with the accretion disk around the SMBH.

This line is produced very close to the SMBH, and it exhibits Doppler motions approaching relativistic speeds, $\sim 10^5{\hbox{\rm km s$^{-1}$}}\approx 0.3c$. Now crucially, the line-profile displays asymmetries consistent with a gravitational redshift. The photons lose energy as they climb out of the deep potential well near the black hole. In doing so, their wavelengths become longer - a `gravitational redshift'.

For the AGN galaxy MCG-6-30-15, the best fitting accretion disk has an inner radius of 6 Schwarzschild radii, i.e. very close to the event horizon. A similar analysis has now been performed for other AGN as well. The shape of the line probably provides the best evidence to date for the existence of a SMBH. However, other mechanisms for generating the line profile are possible, but implausible. With better data, detailed modelling of the profile even has the potential to determine the spin of the SMBH.