Echo Mapping of AGN Emission Regions
  Echo mapping exploits light travel time delays, revealed by multi-wavelength variability studies, to map the geometry, kinematics, and physical conditions of reprocessing sites in photo-ionized gas flows. In AGNs, the ultra-violet to near infra-red light arises in part from reprocessing of EUV and X-ray light from a compact and erratically variable source in the nucleus. The observed time delays, 0.1-2 days for the continuum, 1-100 days for the broad emission lines, probe regions only micro-arcseconds away from the nucleus. The continuum time delays map the temperature-radius profiles of the AGN accretion discs. The emission-line delays reveal radially stratified ionization zones, identify the nature of the gas motions, and estimate the masses of the central black holes. By using light travel time to measure the sizes of AGN accretion discs and photo-ionized zones, echo mapping offers two independent ways to measure redshift-independent distances to AGNs. 
Keith Horne