Variation in the primary and reprocessed radiation from an orbiting spot around a black hole

We study light curves and spectra [equivalent widths (EWs) of the iron line and some other spectral characteristics] which arise by reflection on the surface of an accretion disc, following its illumination by a primary off-axis source – an X-ray ‘flare’, assumed to be a point-like source just above the accretion disc resulting in a spot with radius Δr/r≲ 1. We consider General Relativity effects (energy shifts, light bending, time-delays) near a rotating black hole, and we find them all important, including the light bending and delay amplification due to the spot motion. For some sets of parameters, the observed reflected flux exceeds the observed flux from the primary component. We show that the orbit-induced variations in the EW with respect to its mean value can be as high as 30 per cent for an observer's inclination of 30°, and much more at higher inclinations. We calculate the ratio of the reflected flux to the primary flux and the hardness ratio which we find to vary significantly with the spot phase mainly for small orbital radii. This offers the chance to estimate the lower limit of the black hole spin if the flare arises close to the black hole.