Line Emission from an Accretion Disk around a RotatingBlack Hole : Toward a Measurement of Frame

Line emission from an accretion disk and a corotating hotspot about a rotating black hole are considered for possible signatures of the frame dragging eeect. We explicitly compare integrated line prooles from a geometrically thin disk about a Schwarzschild and an extreme Kerr black hole and show that the line proole diierences are small if the inner radius of the disk is near or above the Schwarzschild stable-orbit limit of radius 6GM=c 2. However, if the inner disk radius extends below this limit as is possible in the extreme Kerr spacetime, then diierences can become signiicant, especially if the disk emissivity is stronger near the inner regions. We also consider lightcurves and frequency shifts in line emission as a function of time for corotat-ing hotspots in extreme Kerr and Schwarzschild geometries. The frequency shift proole is an excellent measure of orbital parameters and might possibly be used to detect frame dragging even at radii approaching 6GM=c 2 if the inclination angle of the orbital plane is large. The lightcurve from a hotspot shows diierences as well; although, these too are pronounced only at large inclination angles. This work is based on a parallel ray-tracing code which generated accretion disk images by generating the geodesics for those null rays which intersect the disk plane. The typical resolution of the images was 10001000 pixels. A set of images or templates generated at this resolution are suitable for tting real line-emission data which can be expected from the next generation of X-ray satellites.