Ray tracing near a black hole

We describe a new routine that aims to enhance capabilities to model emission line profiles from an accretion disc near a black hole. The new routine, "ky", cooperates with XSPEC. It is supposed to be more accurate and versatile than laor (Laor 1991), and faster than kerrspec (Martocchia et al. 2000). Among other features, ky allows to fit various parameters such as black hole angular momentum, observer inclination, an accretion disc size, etc. It also allows to change the grid resolution and, hence, to control accuracy and computational speed. There is an option to explore time variable sources.

The current routine implements tabulated Kerr metric in equatorial plane. A set of data files has been prepared that contains information about light rays originating in a thin disc and reaching a distant observer. The files can be used to compute observed spectra of a Keplerian accretion disc around a rotating (Kerr) black hole. This page provides some technical details about ray-tracing computations that have been performed to create the output data.

A conference poster (Computational fluid dynamics in astrophysics, September 2003, University of Leicester, UK) illustrates basic features and capabilities of the code. Further, a more detailed description of the new XSPEC subroutine is available.

First, we had to create data tables that characterize light-ray propeties (so that geodesics do not have to be integrated each time a spectrum is computed). Some assumptions about the source emissivity must be adopted at this point: e.g., a planar disc near a black hole. Given a set of data tables, one can use stand-alone codes (without any link to XSPEC) to compute predicted spectra. This approach is convenient to test the data tables.

References


December 2003