Broad iron lines in active Galactic nuclei

An intrinsically narrow line emitted by an accretion disk around a black ho le appears broadened and skewed as a result of the Doppler effect and gravi tational redshift. The fluorescent iron line in the X-ray band at 6.4-6.9 k eV is the strongest such line and is seen in the X-ray spectrum of many act ive galactic nuclei and, in particular, Seyfert galaxies. It is an importan t diagnostic with which to study the geometry and other properties of the a ccretion flow very close to the central black hole. The broad iron line ind icates the presence of a standard thin accretion disk in those objects, oft en seen at low inclination. The broad iron line has opened up strong gravit ational effects around black holes to observational study with wide-reachin g consequences for both astrophysics and physics.