Extragalactic
Active Galactic Nuclei
Prior to EXOSAT, AGN were not thought to show rapid variability. EXOSAT "long look" observations showed varibility down to timescales of ~ 1000 seconds.
This placed stringent constraints on the size of the emitting region, which could not be much larger than the inner solar system (~ 500 000 000 km).
Iron Lines
Last Update: 1 September 2019