Echo Mapping of X-Ray Binaries and Active Galactic Nuclei
Echo Mapping exploits light travel time delays to resolve structures on micro-arcsecond scales in Active Galactic Nuclei (AGN) and X-ray Binaries (XRB). In these systems radiation from a compact variable source drives responses in the surrounding medium. We outline maximum entropy techniques used to find smooth delay maps that fit incomplete and noisy measurements of the driving and responding lightcurves. In AGNs, erratically variable ionizing flux from the nucleus drives emission-line responses in surrounding photo-ionized gas. The observed time delays reveal radial ionization structure on 1–100 light day scales, constrain the gas kinematics, and provide rough virial masses. In XRBs, time delays up to of order l0s arise as X-rays from the accreting neutron star or black hole are reprocessed to optical line and continuum radiation in the surrounding accretion flow and on the inward face the binary companion star. A wealth of information on the structure of the flow is coded in the way the delay distribution changes with phase as the binary rotates.
KeywordsBlack Hole Accretion Disk Active Galactic Nucleus Companion Star Roche Lobe
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