Investigations in Theoretical Astrophysics

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ABSTRACT AST- 9423209 Narayan, Ramesh This grant will pursue two topics. The first is Gravitational Lensing, the phenomenon by which light rays from a distant source are deflected by the gravitational influence of intervening galaxies. In some cases, this results in an observer seeing multiple images of a source. It is possible to deduce the gravitational distribution of the intervening mass from the observed images, and it is proposed to develop a new algorithm for this, based on the Maximum Entropy Method, a well-known image processing technique. Furthermore, when a source is multiply-images, light takes different lengths of time to reach the observer by different paths. By combining a model of the lens with the measured time difference between correlated variations in the image, the distance to the source can be estimated, which gives a value for Ho, the Hubble constant. The second topic is Accretion Flows, viscous spiraling in of gas onto a compact gravitating mass, such as a neutron star or a black hole. The PI proposes to model the inner regions close to the accreting mass, whence most of the observed radiation derives. The neutron star and black hole cases will be treated differently, since gas accreting onto a neutron star ultimately slows down and settles on the stellar surface, whereas flow around a black hole accelerates to supersonic radial speeds before falling through the horizon. ***