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In many active galaxies, the X-ray reflection features from the innermostregions of the accretion disc are relativistically distorted. This distortionallows us to measure parameters of the black hole such as its spin. The ratioin flux between the direct and the reflected radiation, the so-calledreflection fraction, is determined directly from the geometry and location ofprimary source of radiation. We calculate the reflection fraction in the lamppost geometry in order to determine its maximal possible value for a givenvalue of black hole spin. We show that high reflection fractions in excess of 2are only possible for rapidly rotating black holes, suggesting that the highspin sources produce the strongest relativistic reflection features. Usingsimulations we show that taking this constraint into account does significantlyimprove the determination of the spin values. We make software routines for themost popular X-ray data analysis packages available that incorporate theseadditional constraints.

The role of the reflection fraction in constraining black hole spin