  Spectral features can arise by reflection of coronal X-rays on a black hole
accretion disc. The resulting profile bears various imprints of strong
gravitational field acting on the light emitting gas. The observed shape of the
reflection line is formed by integrating contributions over a range of radii
across the accretion disc plane, where the individual photons experience
different level of energy shifts, boosting, and amplification by relativistic
effects. These have to be convolved with the intrinsic emissivity of the line,
which is a function of radius and the emission angle in the local frame. We
study if the currently discussed instruments on-board X-ray satellites will be
able to reveal the departure of the line radial emissivity from a simple smooth
power-law function, which is often assumed in data fitting and interpretation.
Such a departure can be a result of excess emission occurring at a certain
distance. This could be used to study variations with radius of the line
production or to constrain the position of the inner edge of the accretion
disc. By simulating artificial data from a bright active galactic nucleus of a
type-1 Seyfert galaxy (inclination ~ 30 deg, X-ray flux 1-2 mCrab in keV energy
band) we show that the required sensitivity and energy resolution could be
reached with Large Area Detector of the proposed LOFT mission. Galactic black
holes will provide another category of potentially suitable targets if the
relativistic spectral features are indeed produced by reflection from their
accretion discs.
