In the vicinity of a rotating black hole: a fast numerical code for computing observational effects

Abstract

We describe a numerical code which has been developed to calculate images of various effects occurring in the close vicinity of a rotating (Kerr) black hole, as seen by a distant observer. Null geodesics are first integrated numerically, fitted by Chebyshev polynomials, and their shape stored in the computer. In this way, for a given set of parameters describing the configuration of the black hole and position of the observer, we obtain a 'catalogue' of photon paths which is then used by subsequent codes adapted to specific problems. Several astrophysically meaningful examples are given, in particular photometric light curves of a bright spot located on the surface of an accretion disk and orbiting the black hole. This approach enables us to speed up appropriate application codes by approximately two orders of magnitude.