The super-massive objects in galactic nuclei are thought to be the Kerr black holes predicted by General Relativity , although a definite proof of their actual nature is still lacking .
The most massive objects in AGN ( M \sim 10 ^ { 9 } M _ { \odot } ) seem to have a high radiative efficiency ( \eta \sim 0.4 ) and a moderate mass accretion rate ( L _ { bol } / L _ { Edd } \sim 0.3 ) .
The high radiative efficiency could suggest they are very rapidly-rotating black holes .
The moderate luminosity could indicate that their accretion disk is geometrically thin .
If so , these objects could be excellent candidates to test the Kerr black hole hypothesis .
An accurate measurement of the radiative efficiency of an individual AGN may probe the geometry of the space-time around the black hole candidate with a precision comparable to the one achievable with future space-based gravitational-wave detectors like LISA .
A robust evidence of the existence of a black hole candidate with \eta > 0.32 and accreting from a thin disk may be interpreted as an indication of new physics .
For the time being , there are several issues to address before using AGN to test the Kerr paradigm , but the approach seems to be promising and capable of providing interesting results before the advent of gravitational wave astronomy .